XXXVIII. THE LOCOMOTIVE…

XXXVIII. THE LOCOMOTIVE

In Fig. 3326 is shown a modern freight locomotive, the construction being as follows:

For generating the steam we have the boiler, which at the front end is firmly bolted to the engine cylinders, which are in turn bolted to the frames, while at the back end the boiler is suspended by the links B (one at each end of the fire box on each side of the engine).

The starting bar is shown in position to start the engine, and it is seen that the rod a and bell crank b are in such a position as to open the valve T, and thus admit steam from the dome to the pipe e, whence it passes through pipes f, g and r into the steam chest i, the slide valve V distributing the steam to the cylinder. The exhaust occurs through the exhaust port d, whence it passes up the exhaust pipe and out at the smoke stack.

The boiler is fed with water as follows:

The feed pipe from the tender supplies water to the injector, which is forced by the injector through the feed pipe to boiler and into the latter.

In the figure the parts are shown in position for the engine to go ahead, hence the reversing gear is in the extreme forward notch of the sector, and the valve gear is in full gear for the forward motion.

The lever m is for opening and closing the cylinder cocks, which are necessary to let the water of condensation out of the cylinder when the engine is first started and the cold cylinder condenses the steam.

To supply steam to the injectors (of which there are two, one on each side of the engine) and to the steam cylinder of the pump, there is a steam pipe leading from the dome to the steam drum, the pipe K supplying steam to the injector, and pipe J supplying steam to the steam cylinder of the air pump. The pipe for supplying oil to the slide valve and cylinder is furnished with a sight feed oil cup, the oil being carried by steam from the steam drum.

This pipe passes beneath the lagging until it reaches the smoke box, which is done to keep it warm and prevent the oil from freezing, while the steam pressure enables the oil to feed against the steam pressure in the steam chest.

The slide valve is balanced by means of strips let into its back, and bearing against a plate fixed to the steam chest cover.

The frame on the side of the engine shown in the engraving is shown broken away from the yoke A to the fire box, so as to expose the link motion to full view, the shaded portion of the frame being that on the other side of the engine.

The yoke or brace A carries one end of the guide bars. The safety valve S may be raised to see that it is in working order, or to regulate the steam pressure, by the lever O, which has a ratchet tongue engaging with the notches at l.

In addition to the safety valve with spring balance, however, a pop safety valve is employed on the part of the dome that is shown broken away, the construction of this pop valve being shown in the outside view, Fig. 3326_a_, and a sectional view, Fig. 3326_b_, the casing being removed from the latter. In the valve seat B is a recess a, and upon the circumference of the valve is a threaded ring C´. When the valve lifts, the steam is somewhat confined in the annular recess of the valve, and the extra valve area thus receiving pressure causes the valve to lift promptly and the steam to escape freely. The degree of this action is governed as follows:

The sleeve C´ is threaded upon the upper part of the valve, so that by screwing it up or down upon the valve the amount of opening between the annular recess a a, and the lower edge of the sleeve C´ C´, is increased or diminished at will; the less this opening, the more promptly the valve will rise after lifting from its seat.

To secure the sleeve or ring in its adjusted position, the ends of the screws L, L seat in notches cut in the upper edge of the sleeve. In many engines pop valves alone are used, and in some cases levers are provided by means of which the pop valve can be raised from its seat to test if it is in working order.

Referring again to Fig. 3326, H is the handle for operating the injector, and w a rod for opening the injector overflow.

We now come to the automatic air brake; steam for the steam cylinder of which, is received from the steam drum through the pipe J, passing through the pump governor, or regulator G. The exhaust pipe for the steam cylinder of the air pump passes into the smoke box. The air cylinder receives its supply of air through the small holes at k, k, and delivers it through the pipe C into the air reservoir or tank, from which it passes through the tank pipe up to the threeway cock or engineer's brake valve, whose handle is shown at M. The brakes are kept free from the wheels and out of action so long as there is air pressure in the air reservoir and in the train pipe, hence the normal position of the handle M is such as to let the air pass from the air reservoir up the pipe x and into the train pipe. When the brakes are to be applied, handle M is moved so that there is an open connection made between the train pipe and the pipe to open air, which releases the air pressure and then puts on the brakes not only on each car, but also on the engine, because the engine brake cylinders receive their air pressure from the pipe shown leading to the train pipe. From the tank pipe x a pipe h leads to the top of the pump governor G, whose action is to shut off the steam from the steam cylinder of the air pump whenever the pressure in the air reservoir or tank exceeds 70 lbs. per square inch. A small pipe leads up from pipe h to the air pressure gauge.

For regulating the draught of the fire there is a damper door at each end of the ash pan, and to increase the draught, a pipe leads from the steam drum into the smoke box, where it passes up alongside of the exhaust pipe, its end being shown at Z. This is called the blower, and its pipe is on the other side of the engine. The plate shown at P, P in the smoke box checks the draught in the upper tubes, and therefore distributes it more through the lower ones.

There are two sand valves, both of which are operated by one rod, the construction being shown in Fig. 3327, which is a plan showing the bottom of the sand box broken away to expose the gear for moving the valves. The two valves v, v for the sand pipes are on raised seats e, e, and are fast on the same shafts as the segments s, s, but the valves are obviously above, while the segments are beneath the bottom of the sand box. The gear wheel W is pivoted to the under side of the bottom of the sand box, and the arm L is fixed to the wheel. At t are pieces of wire, which, being fast in the spindle, revolve with it and stir up the sand when the valves are moved. As shown in the figure, the two sand pipes a, a are open, but suppose the rod is moved endways and L will revolve W, which will move s, s and the valves v, v, causing the latter to move over and cover the pipes a, a, and shut off the sand from the pipes.

Fig. 3328 represents an American passenger locomotive with a steam reversing gear, or in other words, a reversing gear that is operated by steam.

The link motion is substantially the same as that shown in Fig. 3326 for a freight locomotive, the eccentric rods in this case being straight, as there is no wheel axle in the way.

The injector for feeding the boiler is the same as that shown on the freight locomotive.

The ash pan is provided with two dampers, one at each end, and the front one is operated by the bell crank a c.

The sand boxes are here fastened to the frame, both sand valves being operated by the lever m, which at its lower end connects to a rod, u, which at its back end connects to an arm, p, on a shaft that extends across the fire box and connects to a rod corresponding to rod u, but situated on the other side of the engine and connecting with the other sand valve.

The steam pump for the automatic air brake is on the other side of the engine, and the air reservoirs, of which there are two, are horizontal and situated beneath the front end of the boiler. The air pipe to the triple valve here connects to the front pipe of the three beneath the triple valve, the middle pipe being that which is open to the atmosphere, which is the usual construction. The engine brake receives its air from a pipe on the other side of the engine which feeds the pipes G, V, for the brake cylinder shown in the figure. When the engine is running backwards, the train brakes are operated through the medium of the "pipe to air brake and to front end of engine" which is shown broken off.

The construction of the steam reversing gear is shown in Fig. 3328_a_. A is a steam cylinder and B a cylinder filled with oil or other liquid. Each of these cylinders has a piston, the two being connected together by their piston-rods C C´. These rods are also connected to a lever D E F, which works on a fulcrum E. The lower end of the lever is connected to the reverse rod F G, the front end of which is attached to the vertical arm of the lifting or reverse shaft. It will readily be seen that if the piston in B is free to move and steam is then admitted to either end of the steam cylinder A, the two pistons will be moved in a corresponding direction, and with them the lever D E F, and the other parts of the reversing gear. A valve, H, is provided, by which communication is opened between the cylinder A and the steam inlet pipe. Another valve, I, is placed between H and the cylinder A, by which the steam may be admitted either into the front or back end of the cylinder. It will be apparent, though, that if the piston in A is thus moved, and the reverse gear placed in any required position, some provision must be made to hold it there securely. This is accomplished by the oil cylinder and piston B. To it a valve, J, is provided, by which communication between the front and back ends of the cylinder may be opened or closed. It is evident that if the piston B is in any given position, and both ends of the cylinder are filled with liquid, the former will be held securely in that position if the liquid in one end cannot flow into the other. If, however, communication is opened between the two ends, then, if a pressure is exerted on the piston B, it will cause the liquid to flow from one end of the cylinder to the other, and thus permit B to move in whichever direction the pressure is exerted.

R is the reverse lever, made in the form of a bell crank, the short end of which works in a slot c, in the upper end of a shaft or spindle d. This shaft is inclosed by a tubular shaft S, to which the fulcrum of R is fastened. The tubular shaft has an arm b. The reverse lever has two movements, the one to raise the end up, and the other to turn on the axis of the tubular shaft. The arm b on the latter is connected by a rod, f, with the valves J and H. The lower end of the shaft d is connected with a bell crank, f´, which, in turn, is connected by a rod, k l, with the valve I. Therefore, by turning the lever R so as to partly revolve the shaft S, the valves J and H may be opened or closed, and by moving the lever R up or down, the valve I is moved to admit steam to the front or back end of A. To reverse the engine, therefore, the lever R is turned so as to open the valves J and H. This opens communication between the opposite ends of B, and H admits steam to I. Now, by reversing the end of the reverse lever R, the valve I is moved so as to admit steam to either end of A, the pressure in which will move the reverse gear to the desired position. When this is done, the valves J and H are closed. This prevents the fluid in B from flowing from one end of the cylinder to the other, and thus securely locks the piston B in the position it may happen to be in, and at the same time the valve H shuts off steam from the cylinder A.

The bar K is graduated, as shown in the plan of R, K, to indicate to the locomotive runner the position of the reversing gear.

This apparatus enables the reversing gear to be handled with the utmost facility, and with almost no exertion on the part of the engineer. The engine can be reversed almost instantly, and it can be graduated with the most minute precision.

THE LINK MOTION AND REVERSING GEAR.

The link motion of an American locomotive is shown in Figs. 3329 and 3330. In Fig. 3329 it is shown in full gear for the forward gear, or in other words, so as to place the engine in full power for going ahead.

The meaning of the term full power is that, with the link motion in full gear, the steam follows the piston throughout very nearly the full stroke.

In Fig. 3331 the link motion is shown in mid gear, in which position the engine is at its least power, the cut off occurring at its earliest point, and in Fig. 3332 it is shown in full gear for the backward motion.

Referring to Fig. 3329 for the full gear forward, the reversing gear proper consists of the reversing lever, the segment, the reach rod, the tumbling shaft, and its counterbalance rod and spring; while the link motion proper consists of the eccentrics and their rods, the link, the link block or die, the suspension link S, the rock shaft and the rod P P. These, however, are terms applied for shop purposes, so as to distribute the work in sections to different men, it being obvious that a complete link motion includes the reversing gear, the eccentrics, the link and its block, the rock shaft, the rod P P, and the valve and its spindle or stem. This mechanism, as a whole, may also be called, and is sometimes called, the valve gear, because it is the mechanism or gear that operates the slide valve.

The link motion may be moved from full gear forward to full gear backward or to any intermediate position, whether the engine is running or at rest, but is, when the engine is running, harder to move from full gear forward toward back gear, and easier to move from full gear backward toward mid and forward gears, which occurs because of the friction of the eccentrics in the straps, and it follows that this will be the case to a greater extent in proportion as the revolutions of the eccentrics are increased.

If in a properly constructed link motion we move the link from full gear forward to mid gear when the engine is standing still, and watch the valve, we shall find that the lead or opening at f gradually increases; and if we then move it from mid gear to full gear backward, the lead will gradually decrease and finally become the same as it was in full gear forward. The construction of the parts is as follows:

Referring to Fig. 3329 (full gear forward), the segment is fixed in position and the reversing lever is pivoted at its lower end. r r is a bell crank, which is pivoted to the reversing lever and to which the latch rod is pivoted at its upper end. The spring acts on the end of r r, and thus forces the tongue of the latch into the notches on the sector as soon as the tongue comes fair with the notch and r r is released from the hand pressure. As the reversing lever is moved over from full gear forward, the reach rod moves the tumbling shaft, whose lower arm i (through the medium of the suspension link S) lifts the link and brings the centre of the saddle pin nearer to the centre of the pin in the link block, which reduces the amount of motion given to the lower arm (B, Fig. 3331) of the rock shaft, and therefore reduces the amount of valve travel, thus causing the point of cut-off to occur earlier in the piston stroke.

The weight of the eccentric rods, the link, suspension link S, and the tumbling shaft arm i, is counterbalanced by the counterbalance spring in the box s s, whose rod attaches to the lug g on the tumbling shaft. To regulate the proper amount of counterbalancing, the nuts at m are provided, these nuts regulating the amount of tension on the spring s s.

The forward eccentric E is that which operates the valve when the link motion is in full gear forward, as in Fig. 3329, and the backward eccentric is that which moves the valve when the link motion is in back gear, as in Fig. 3332.

This occurs because it is the eccentric rod that is in line or nearest in line with the link block that has the most effect in moving the valve. When the link is in full gear, the motion of the valve is almost the same as though there was no link motion and the eccentric rod was attached direct to the rod P P, the difference being so slight as to have no practical importance. This will be seen by supposing that we were to loosen the backward eccentric F upon the shaft and revolve it around the shaft by hand, in which case it would swing the lower end of the link backward and forward with the centre of the link block as a pivot or centre of motion, the forward eccentric rod rising and falling a trifle only, and therefore moving the rock shaft to a very slight amount.

Let it now be noted that the suspension link not only sustains the weight of the link and eccentric rods, but also compels the centre of the saddle pin to swing (as the link is moved by the eccentrics) in an arc of a circle of which the centre is the upper end of the suspension link. Suppose, therefore, that the backward eccentric rod was to break, or was taken off and the engine could still run forward, but no motion would be given to the valve, if the link was placed in mid gear, because in that case the forward eccentric rod would simply swing the link on the centre of the link block as a pivot. Now, suppose the forward eccentric rod was to break or be taken off, and the engine may be made to go ahead by setting the backward eccentric fair with the forward eccentric and connecting its rod to the upper end of the link.

Similarly, if the engine was running with the smoke stack toward the train and the link motion in backward gear, and the backward eccentric rod was to break, we may take it off, shift the forward eccentric so that it comes fair or stands in line with the backward eccentric and connect its rod to the lower end of the eccentric and with the link motion in backward gear, the engine would still haul the train.

If the reach rod was to break, the tumbling shaft could be held in position by loosening the cap bolts of the tumbling shaft journal and putting between the cap and the tumbling shaft journal a piece of metal, which, on bolting up the cap screws again, would firmly grip the shaft and prevent it from moving.

SETTING THE SLIDE VALVES OF A LOCOMOTIVE.--The principles of designing, and the action of D valves, such as are used upon locomotives, have been so thoroughly explained with reference to stationary engines, that there is no need to repeat them in connection with the locomotive, and we may proceed to explain how to set the valves of a locomotive. In doing this, there are two distinct operations, one of which is to place the crank alternately exactly on its respective dead centres, and the other is to set the position of the eccentrics, and get the eccentric rod of the proper length. These two operations comprise all that require to be done to set the valves, under ordinary and workmanlike conditions; hence we may proceed at once to explain the operation.

The first thing to be done is to put the crank pin on a dead centre, and it does not matter which one.

In Fig. 3333 it is supposed that the piston is to be at the head end of the cylinder when the crank is on its corresponding dead centre.

The first thing to do is to put the reversing gear in full gear forward, so as to set the forward eccentric, and see if its rod is of proper length.

The next thing to do is to move the wheel so that the crank pin is nearly on the dead centre, and then take a tram (such as shown in the figure), pointed at each end, and mark on the splash plate, or any other convenient place, a centre punch dot in which the point b of the tram can rest. Next, from the centre of the axle as a centre, mark arcs or portions of circles a, a. This being done, point b of the tram is rested in the centre punch dot before referred to, and with the other end a line c is marked, a straight edge is then rested against the ends e e of the cross head, and a line d is marked on the guide bar, this line being exactly even or fair with the end e e of the cross head.

We then move the wheel in the direction of the arrow, and as soon as we begin to do so, the cross head will move to the left and away from the line d on the guide bar. But as soon as the crank pin has passed its dead centre, the cross head will begin to move to the right, and as soon as the end e e comes again exactly in line with the line d marked on the guide bar, we must stop moving the wheel, and again resting the point b of the tram in the centre punch mark before mentioned, we move its other end so as to mark a second line, which will be the line or arc f.

The next thing to do is to mark a fine centre punch dot, where c and f cross the arc or line a, and then find the point g midway between f and c, and mark a fine centre punch mark there. This being done, we must move the wheel back into the position it occupies in the figure, and then slowly move it in the direction of the arrow, until with the end b of the tram resting in the centre punch dot, the other end of the tram will fall dead into the centre punch dot at g, at which time the crank pin will be exactly on the dead centre.

During this part of the process we have nothing to do with anything except getting the crank pin on the dead centre, but there is one point that requires further explanation, as follows:

In this operation we have first put the crank on one side of the dead centre and then put it to the same amount on the other side of the dead centre, both being improper positions; but by finding the mean or mid position between the two, we have found the proper position. In doing so, however, we have moved the wheel, the wheel has moved the connecting rod, and the connecting rod has moved the piston. But in the actual running of the engine, this order of things will be reversed; for the steam will move the piston, the piston will move the connecting rod, and the connecting rod will move the crank and therefore the wheel.

The difference between the two operations is this: Suppose there is lost motion or play between the connecting rod brasses and the crank pin, or between the connecting rod brasses and the cross head pin, and then if we move the wheel in the direction denoted by the arrow, we take up this lost motion, so that if the tram was fair with the centre punch at g and steam was admitted to the piston, then there would be no lost motion to take up, and as soon as the piston moved the crank pin would move. But if we moved the wheel in the opposite direction to that denoted by the arrow, then we are placing any lost motion there may be in the opposite direction, and if steam were turned on, the piston and connecting rod might move before the crank and wheel moved.

In which direction the wheel should be moved while placing the crank on the dead centre depends upon the condition of the engine, as will be explained presently, the assumption being at present that the engine is in thorough good order, in which case the wheel should (while placing the crank on the dead centre) be moved in the direction of the arrow in the figure.

The object is under all conditions to bring the working surfaces to bear (while setting the valve) in the same way as they will bear when the engine is actually at work.

Having placed the crank on the dead centre, and thus completed the first operation in valve setting, we may turn our attention to the second, viz., correcting the lengths of the eccentric rods and setting the valve lead. Almost all writers who have dealt with this part of the subject have fallen into a very serious error, inasmuch as they began the operation by what they call squaring the valve. This means so adjusting the length of the eccentric rod that the valve will travel an equal distance each way from its mid position, so that if the engine wheel is revolved and the extreme positions of the valve marked by a line, these lines will measure equally from the edges of the steam ports, or, what is the same thing, from the centre of the cylinder exhaust port. This procedure is entirely erroneous, because, on account of the angularity[57] of the eccentric rod, the valve cannot, if equal lead is to be given to the valve, travel equally beyond the two steam ports, and if the eccentric rods are so adjusted for length as to square the valve, they are made wrong.

[57] See page 376, Vol. II., for the meaning of angularity.

The valve lead, and the lead only, it is that determines the length of the eccentric rods. Suppose that, as is generally the case, the lead is to be equal, or, in other words, that there is to be as much valve lead when the piston is at one end of the cylinder as there is when it is at the other, and if we make the eccentric rods of such a length that the valve travels equally on each side of the steam port, there will be less lead at the head end port than there is at the crank end port. The proper method, therefore, is (as soon as the crank is on the dead centre and the link in full gear, as in Fig. 3334) to set the eccentric so as to give the desired amount of lead, and then give the wheel a half revolution, the lower end of the tram falling into the centre punch dot at s, when the crank pin will be on its other dead centre and ready for the lead to be measured again. If the lead is equal at each end, one eccentric rod is of the right length, and all we have to do is to set the eccentric so that the right amount of lead is given.

We now turn our attention to the backward eccentric and its rod, putting the reversing lever in full gear for the backward motion, and putting the crank on the respective dead centres, and testing the lead for both ports as before, and when the required amount of valve lead is given the valve setting is complete.

In some practice the wheel is blocked up on the pedestal guides while setting the valves, but a more correct method is to let the engine rest on the rails and push it back and forth with a crowbar to revolve the wheels when putting the crank pin on the dead centre. The best thing to measure the lead with is a wooden or leaden wedge having but a slight degree of taper, as say 3/16 or 1/4 inch in a length of four inches. We have in this example of valve setting supposed the parts to be of the proper dimensions, as they would be in a new engine or in an engine that had been running and merely had a new valve or a new eccentric put in.

But suppose the notches were not cut in the sector, and we have then to mark them off while setting the valves. All the difference that this makes to the operation is that we must clamp the reversing lever to the sector while setting the valve, taking care to so clamp it that there is the same space between the top end of the link block and the end of the link slot in the full forward gear as there is between the bottom end of the link block and the end of the link slot when the engine is in full backward gear. In this connection it is, however, to be remarked that when the link is in full gear, either forward or backward, and the crank is on the dead centre, the link block is not at the end of its motion toward the end of the link slot; hence it is a good plan to move the wheels around and to so regulate the length of the reach rod and the position of the reversing lever on the sector, that when the link block is at the highest point in the link slot for the forward gear and at the lowest point in the link slot for the backward gear, it comes an equal distance from the end of the link slot.

The setting for an Allen valve is the same as that for an ordinary one, but in determining the amount of the lead it is to be borne in mind that it is virtually twice as much as it measures at the port because there are two openings for the steam. This will be seen from Fig. 3335, in which the valve is open to the amount of the lead at f. But the steam also enters at e, and passes through the port in the valve and into steam port a.

We have now to call attention to the fact, that the eccentric rods, when properly connected, are, in an American locomotive, crossed when the piston is at the crank end of the cylinder. In Fig. 3334, the piston is at the head end of the cylinder, and the rods are open. In Fig. 3336, however, the crank pin is supposed to be at B, and the eccentric rods are crossed, F being the forward and E the backward eccentric.

THE INJECTOR.

The injector shown in the general view of a freight locomotive, Fig. 3326, is that constructed by William Sellers & Co., and there are two, one on each side of the engine. The details of its construction are as follows:

Fig. 3337 is a side elevation, Fig. 3338 a section on a vertical plane, Fig. 3339 a section on a horizontal plane, Fig. 3340 an end view of the injector at the right-hand side of the engine, and Fig. 3341 a plan of the injector on the left-hand side of the engine.

This injector is self-contained, or in other words, it has both steam and check valves, so that it can be connected directly without other fittings, although, of course, it is generally desirable to place another stop valve in the steam pipe, and a check valve in the delivery pipe, so that the injector can be taken to pieces or disconnected at any time. Another important feature of this injector is, that it is operated by a single handle, and that the waste valve is only open at the instant of starting.

Referring to Fig. 3338, A is the receiving tube, which can be closed to the admission of steam by the valve X. A hollow spindle passing through the receiving tube into the combining tube is secured to the rod B, and the valve X is fitted to this spindle in such a way that the latter can be moved a slight distance (until the stop shown in the figure engages with valve X) without raising the valve X from its seat. A second valve W, secured to the rod B, has its seat in the upper side of the valve X, so that it can be opened (thus admitting the steam to the centre of the spindle) without raising the valve X from its seat, if the rod B is not drawn out any farther, after the stop on the hollow spindle comes in contact with the valve X. D is the delivery tube, O an overflow opening into space C, V the check valve in delivery pipe, and Z the waste valve. The upper end of the combining tube has a piston N N attached to it, capable of moving freely in a cylindrical portion of the shell M, M, and the lower end of the combining tube slides in a cylindrical guide formed in the upper end of the delivery tube.

The rod B is connected to a cross head which is fitted over the guide rod J, and a lever H is secured to the cross head. A rod W, attached to a lever on the top end of the screw waste valve, passes through an eye that is secured to the lever H; and stops T, Q control the motion of this rod, so that the waste valve is closed when the lever H has its extreme outward throw, and is opened when the lever is thrown in so as to close the steam valve X, while the lever can be moved between the positions of the stops T, Q without affecting the waste valve. A latch F is thrown into action with teeth cut in the upper side of the guide rod J, when the lever H is drawn out to its full extent and then moved back; and this click is raised out of action as soon as it has been moved in far enough to pass the last tooth on the rod J. An air vessel is arranged in the body of the instrument, as shown in the figure, for the purpose of securing a continuous jet when the injector and its connection are exposed to shocks, especially such as occur in the use of the instrument on locomotives.

The manipulation required to start the injector is exceedingly simple--much more so in practice, indeed, than it can be rendered in description. Moving the lever H until contact takes place between valve X and stop on hollow spindle, which can be felt by the hand upon the lever, steam is admitted to the centre of the spindle, and, expanding as it passes into the delivery tube D and waste orifice P, lifts the water through the supply pipe into the combining tube around the hollow spindle, acting after the manner of an ejector or steam siphon. As soon as solid water issues through the waste orifice P, the handle H may be drawn out to its full extent, opening the steam valve X and closing the waste valve, when the action of the injector will be continuous as long as steam and water are supplied to it.

To regulate the amount of water delivered, it is necessary only to move in the lever H until the click engages any of the teeth on the rod J, thus diminishing the steam supply, as the water supply is self-regulating. If too much water is delivered, some of it will escape through O into C, and, pressing on the piston N N, will move the combining tube away from the delivery tube, thus throttling the water supply; and, if sufficient water is not admitted, a partial vacuum will be formed in C, and the unbalanced pressure on the upper side of the piston N N will move the combining tube toward the delivery tube, thus enlarging the orifice for the admission of water. From this it is evident that the injector, once started, will continue to work without any further adjustment, delivering all its water to the boiler, the waste valve being kept shut. By placing the hand on the starting lever, it is easy to tell whether or not the injector is working; and, if desired, the waste valve can be opened momentarily by pushing the rod W, a knob on the end being provided for the purpose.

THE WESTINGHOUSE AUTOMATIC AIR BRAKE.

Figs. 3342, 3343 and 3344 represent the Westinghouse automatic air brake applied to an engine and tender, and in the following figures details of the construction of various parts are shown.

The pump governor, which is shown at G in Fig. 3326, of a modern freight locomotive, is shown in section in Fig. 3345.

The pump governor is employed for the purpose of cutting off the supply of steam to the pump when the air pressure in the train pipe exceeds a certain limit, say 70 lbs. per square inch.

Its operation is as follows:

When valve 10 is (by means of hand wheel 8) screwed home to its seat the steam is entirely shut off from the steam cylinder, but by operating wheel 8 to unscrew spindle 9, valve 10 is permitted to open and let the steam pass through A and B to the steam cylinder which operates, forcing air into the reservoir and thence into the train pipe. A pipe from the train pipe connects to the upper end of the pump governor, hence air from the train pipe passes around the stem 14 to the upper side of the thin diaphragm 18, which is held in its position by the spring 12 with a force sufficient to enable it to resist, without moving, a pressure of 70 lbs. per square inch. But when the pressure exceeds 70 lbs. per square inch it forces the diaphragm down, pushing down valve 13 and allowing the steam in A to pass up through valve 13 and out of the exhaust pipe 6. The steam pressure in A being thus reduced, that in B acts on the under side of the valve, causing it to rise and seat itself and thus cut off the supply of steam to the pump.

When the pressure in the train pipe is diminished by the brakes being applied, the diaphragm is restored to the position it occupies in the figure by the action of the spring 16. Then valve 13 is seated by the spring 12, and the steam pressure accumulates on the upper end of valve 10, forcing it down and letting the steam from A into B and thence into the steam cylinder, starting it into action, which continues until the pressure in the train pipe exceeds 70 lbs. per square inch.

The use of this governor not only prevents the carrying of an excessive air pressure in the train pipe, which may result in entirely preventing the wheels from revolving and causing a flat place to wear on the wheel tire, but it also causes the accumulation of a surplus of air pressure in the main reservoir, while the brakes are applied, which insures the release of the brakes without delay. It also obviates the unnecessary working of the pump when the desired air pressure has been obtained.

A sectional view of the steam and air cylinders is shown in Fig. 3346, the construction being as follows:

Steam is distributed to the steam cylinder by means of a piston valve, composed of three pistons, marked 16, 14, and 20 respectively, the steam entering between pistons 16 and 14, and, in the positions in which the parts occupy in the figure, steam can pass through the bushing 18 and beneath the steam piston 7, propelling it upwards until the bottom of the hole in its piston rod strikes the end of rod 12, and raises it and valve 13. The chamber 23, in which valve 13 works, receives steam through a suitable port from the steam space between valves 14 and 16; and the steam from chamber 23, it is that (in the positions the parts occupy in the figure), acting on the area of the large valve piston 20, holds the valve down against the pressure on the bottom face of piston 14 of the valve. As soon, however, as the piston rod 7 strikes and raises rod 12 and valve 13, the steam is exhausted from the top of piston 20 of the valve, and the steam beneath piston 14 of the valve raises the valve, opening the lower port in the sleeve 18 for the exhaust, and piston 14 admits steam to the upper side of the steam piston 7. The construction of the air cylinder differs somewhat from that shown in the freight locomotive, Fig. 3326, this air pump corresponding with that shown on the engine and tender, Fig. 3342. A detail list of the parts may be given as follows:

No.

2. Steam cylinder head (with reversing cylinder, piston, and valve bushes). 3. Steam cylinder (with main valve and bushes). 4. Centre piece. 5. Air cylinder (with lower discharge valve). 6. Air cylinder head. 7. Steam piston and rod. 8. Air piston. 9. Piston rings. 10. Steam piston plate. 11. Steam piston bolt. 12. Reversing rod. 13. Reversing valve. 14. Piston valve. 15. Piston valve rings. 16. Piston valve rings. 17. Upper valve bushing. 18. Lower valve bushing. 19. Reversing piston casing. 20. Reversing piston. 21. Piston rings. 22. Reversing cylinder cap. 23. Reversing valve bush. 24. Reversing valve cap. 25. Piston rod nut. 26. Piston packing gland. 27. Piston packing nuts. 28. Packing glands. 29. Right Chamber cap. 30. Left chamber cap. 31. Air valve bushing. 32. Air valve. 33. Air valve. 34. Air valve. 35. Delivery union. 36. Exhaust steam outlet. 40. Steam cylinder gasket. 42. Top air-pump gasket. 43. Bottom air-pump gasket. 44. Waste water pipe. 46. Exhaust union stud. 49. Air exhaust union stud.

A side view of the driving wheel brakes is shown in Fig. 3347 and an end view in Fig. 3348. The brakes are, it is seen, suspended by links so that their weight tends to keep them from the wheels. The brake piston rod carries at its end two links which attach to the arms attached to the brakes. The ends of these arms being curved roll together, the arrangement forming in effect a rolling toggle joint. The construction of the piston of the driving wheel brake is shown in Fig. 3349. The piston is made air tight by leather packing indicated by 11, held out by a spring 12. The piston rod packing, 7, is leather held in place by the cap 6 and the spring 8. The air for operating the brake enters below the piston.

LOCOMOTIVE RUNNING.

The engineer's duty in running a locomotive is more arduous and requires more watchfulness than any other engine running, because of the peculiarities attending it. In the first place, the jolts and jars to which the engine is subject are liable to cause nuts, pins, etc., to come loose, and some of the parts to become disconnected and cause a breakdown of the engine.

This renders necessary a careful examination of the engine, which should be made both before and after each trip.

In the second place, we have that the amount of load the engine has to pull varies with every varying grade in the railroad track, and the variation is so great that on some descending grades the engine may require no steam whatever, while on ascending grades the utmost power attainable from the engine may be required. In firing, feeding the pumps, oiling the parts, and determining the depth of water in the boiler, the grade and the length of each grade has an important bearing, and so has the weather, since it is clear that between the heat of summer and the blizzards of winter there is a wide range of the conditions under which the engine runs.

In former times, from the less perfect construction of locomotives, the engineer's duties were greatly enhanced from breakdowns, which are comparatively rare with modern locomotives, and there is promise that from improved construction and safeguards they will become less frequent in the future.

It is as important for the locomotive engineer to be familiar with the track as it is to be with the engine, and there is no field of engine driving or running in which more scope is permitted to the engineer to exercise judgment and skill in his management, so as to effect economy in fuel consumption.

The quality and size of the coal is another element that requires attention and observation on the part of the engineer, in order that his train may keep its time and the fuel consumption be kept down.

GETTING THE ENGINE READY.

The first thing to be done in getting ready for a trip is to see that there is sufficient water in the boiler, so that if there is not, there is time to supply the deficiency.

If the boiler is cold it may be that the condensation of the steam in cooling may have left a partial vacuum in the boiler, and it will be necessary in that case to open the top gauge cock and let in air so that the water will come to its proper level in the gauge glass. Similarly, in filling the boiler, it may be necessary to open a gauge cock to let the air out.

The lower cock of the gauge glass should be opened to let the steam blow through if there is pressure on the boiler, or to let a little water out if there is not.

The safety valve should next be examined and moved to see that it works properly and does not stick to its seat.

Before laying the fire the fire bars and ash pan should be cleared of ashes and clinkers, and the grate bars tried with the shaking levers to see that the grates will shake properly. It should be seen that the tubes, etc., are clear of ashes.

In laying a new fire an ample supply of lighting material should be used, disposing it so that the fire will light evenly and not in spots, and a good layer of wood should be evenly distributed over the bars, the thinnest pieces being at the bottom as they will light easiest, and it is necessary to light the fire at the bottom, so that the heat from the wood that is first lighted shall pass through that to be lighted.

The wood should be kept burning without coal until the lower stratum has ceased to blaze and covers the bars, while there is an even layer of blazing wood above it.

The quantity of coal to be fed at a time, and the depth of fire to be kept, depends upon the size of the coal, because the larger the coal the less it obstructs the draught, and the thicker the layer required in order to prevent currents of air from passing through without entering into combination with the gases from the coal.

If the coal is mixed, containing large lumps, they should be broken.

The first layer of coal should be enough to cover the fire to a depth of about two inches, which will permit of a good draught. This will get well alight while the wood is still serviceable, and a second layer may be applied of another two inches. The third feeding should be given with a view to have a greater depth of fire at the sides than at the middle of the fire box, because the cool sides of the box prevent perfect combustion, and currents of cold air are more apt to find their way through the sides than in the middle of the fire box.

Banking a fire consists of piling it up at the back half of the fire box and covering it up with green coal, so that it may keep alight and keep the boiler hot without increasing the steam pressure.

The air passing through the uncovered half of the fire bars prevents rapid combustion and a dead fire is maintained.

In starting up a banked fire, the first thing to do is to clean it of ashes, clinker, etc., shaking up the bars to see that they will work properly. The fire is then spread evenly over the bars, and wood fed to enliven the fire and promote the draught.

The blower or blast pipe is then set going, and coal gradually fed a little at a time, evenly distributed, covering those parts the most where the fire burns through the most brightly.

A steady fire is better than one that is forced, because the combustion is more perfect and less clinker is formed, hence less cleaning will be necessary, and the fire door will not be kept open so long to let in cold air. This is important because a steady temperature in the fire box promotes its durability, as well as giving a uniform boiler pressure. The strains placed upon a fire box by a fierce fire suddenly cooled by a heavy charge of coal or of cold air from an open fire door are highly destructive.

Furthermore, the greatest economy of fuel is attained by keeping the boiler pressure up, and using the steam expansively by hooking up the links to shorten the point of cut-off.

A safety valve steadily blowing off steam, whether the engine is running or not, is a sign of bad firing and wastefulness.

It is the fireman's duty to attend to the fire, but nevertheless a careful engineer will be as much interested in proper firing as in his own duties, and as the engineer has more experience than the fireman, he is warranted in exercising an ordinary supervision on the firing, which will be welcome to an earnest or ambitious fireman.

The engineer should examine, with a wrench in hand, the bolts and nuts about the trucks and axle boxes, as these are apt to become loose and come off on the road. A proper construction would remedy this defect almost entirely, and by a proper construction is meant the more frequent employment of split pins, cotters, and other similar safety appliances now omitted for the sake of economy of manufacture.

Nothing in the future of the locomotive is more certain than improvement in this respect, and nothing is more urgently needed, as any engineer will become satisfied if he will gather up along a mile of ordinary railroad the nuts and washers that lay along the track.

The eccentric straps and the pins in the link motion require an examination, which may be done while oiling the parts of the engine.

The oiling requires careful attention; first the cups themselves sometimes become loose, an argument in favor of having, wherever possible, the cups solid on the parts, as done in European practice.

Oil holes are apt to get choked by gumming, which is that the oil in time forms into a brown gummy substance that fills the oil hole. Perfect lubrication does not imply wasteful lubrication by any means, but a wasteful use of oil is probably less expensive than insufficient lubrication.

A thorough engineer will use no more oil than is necessary; he will leave nothing to conjecture or chance, but know from personal inspection that his engine is in complete working order, and to this end the lubrication of the working parts is a vital element.

After having oiled the eccentric straps, the link motion and the reversing gear beneath the engine, the reversing lever and the parts above the frame must be oiled, and the reversing lever moved back and forth several times, from end to end of the sector or quadrant, so as to distribute the oil throughout the joints and working surfaces.

The axle boxes require careful attention in oiling. In English practice, tallow is packed in the corners of the cavities of the top of the box, so that if the box should begin to heat the tallow will melt, and afford extra lubrication with a heavier lubricant than usual, which will often stop the heating.

The connecting and coupling rods then require attention, the cups being filled and the lubrication adjusted.

When steam is up the gauge glass should be blown through again, and it will be found that the water stands higher in the glass than it did before the boiler was under pressure.

The packing of the piston and of the pump glands, if the engine has pumps, should be known to be properly set up, bearing in mind that a leaky pump gland lets air into the pump and impairs its action.

The sand box should contain dry sand, as wet sand will not feed properly.

If steam is raising too rapidly, close the lower damper to reduce the consumption of fuel and save blowing off steam through the safety valve, which should always be avoided as much as possible.

Before starting the engine, open the cylinder cocks and keep them open until the sound discloses that dry steam, and not steam and water, is issuing.

Open the throttle enough to start the engine easily and not with a jump, and be prepared to shut off steam instantly if a blow in any part of the engine should indicate an obstruction to its working.

In starting a train, the reversing lever is put in the end forward notch and the cylinder cocks opened. Then the throttle is opened a little at first, so as to avoid starting with a violent shock that might break the couplings.

If in starting (or in ascending gradients) the wheels are forced to slip, the sand lever should be operated, a slight sprinkling of sand serving better than a heavy one. If the sand is damp, it will fall in lumps and not distribute evenly as it should do, while at the same time a great deal more sand will be found necessary.

When the train is fairly under way, the aim should be to maintain full boiler pressure, so as to keep up the required speed with the links hooked up to work the steam as expansively as possible, bearing in mind that the higher up the links are hooked the more expansively the steam is used, and that therefore less steam is used to do the work and the boiler pressure can be kept up easier.

To understand this clearly, let it be supposed that the steam pressure in the boiler is 90 lbs. per square inch, and that the piston area is 400 inches, and the total pressure impelling the piston will be 36,000 lbs.; if this follows the piston for 22 inches, the power becomes 792,000 inch lbs. per stroke.

Now suppose the pressure is 150 lbs. per square inch, and this multiplied by the piston area (400) gives 60,000 lbs. impelling the piston, and this would require to follow the piston but 13.2 inches in order to give 792,000 inch lbs. In the one case we have 22 inches, and in the other 13.2 inches of the cylinder to fill with steam. Of course it will take more fuel under the heat of firing to keep the pressure up to the 150 lbs.; but on the other hand, when the steam is cut off in the cylinder there will be 160 lbs. per square inch in it, and all the work that this does in expanding is gained during the rest of the stroke, so that the required amount of power would be obtained by cutting off earlier than at 13 inches.

The water should, under ordinary conditions, be kept at an uniform level in the boiler. Steam can of course be made quicker with a small than with a large quantity of water, but the smaller the quantity of water the more the steam pressure is liable to fluctuate, and the closer the firing must be attended to.

Furthermore, the more water there is in the boiler, the greater the safety, because the longer the boiler can go without feeding, and, if the pumps or injectors, as the case may be, should act imperfectly, there is more time to get them working properly.

In testing the water level, the gauge glass alone is not to be entirely depended upon, hence the gauge cocks should be opened. The water should not be allowed to go below the middle gauge cock.

It is obvious that when the water is below a certain gauge cock, the gauge glass only can give any information as to how far it is below it, hence it must be used for this purpose.

When using it, it should be blown through by opening its lower cock, and if there is any doubt about its showing the proper water level it should be blown through two or three times, watching the level of the water in the glass at each trial.

A constant boiler feeding is the best, as it is more conducive to a uniform boiler pressure and temperature.

The fire should be fed in small charges, the fire door being kept open as little as possible, because a high temperature in the firebox is necessary to perfect combustion. If heavy charges of coal are given at once, then for some time the fire box will be cooled, and then, as the fire burns through, a fierce heat will be generated. This alternate heating and cooling is very destructive to the fire box and the tubes, as it causes an expansion and contraction that rack the joints and seams.

There are several ways of firing, each having its advocate. Upon the following points, however, there is no dispute. First, a slow combustion is the most perfect, because it produces less clinker, which saves fuel and also saves a large amount of fire cleaning and therefore of admission of cold air to the fire box. A high temperature is necessary to combustion, and the temperature of the fuel is most difficult to keep up at the sides of the fire box.

By light and frequent firing the bright fire will never be covered up, hence the temperature will be maintained. This favors an even distribution over a large surface of the fire of each shovelful of coal. But if at any point the draught is lifting the fire, and small bright pieces of fire are lifting up, it is an evidence that the fire is thinnest there or else that the bars are cleanest there. In either case, an extra amount of coal is required at that spot.

Some engineers will charge one side of the fire box lightly and then the other, this being done so as to keep up the temperature in the fire box. Others will fire first the front and then the back of the box, which answers the same purpose, but in no case should the charge be heavy.

A fireman may become so accustomed to the road and his engine, that under some conditions he may fire when he reaches certain points on the road, regulating it like clockwork.

On a trip from Philadelphia to Reading, on an engine having a Wooten fire box (whose special feature is a large fire box, which enables slow combustion), the firing was conducted as follows:

The fire was was not fed or touched until just before reaching Bridgeport, 18 miles from Philadelphia, when a thin layer of coal Was spread evenly on the fire. Eleven miles were then made without opening the fire door, the next firing taking place just before reaching Phoenixville.

Ten miles were run before the next firing, which occurred just before arriving at Pottstown.

The next firing occurred at Bordenboro', three miles from Pottstown. The remaining 8 miles were made without firing. The steam pressure did not vary more than 10 lbs. per square inch during the trip.

On a trip from New York to Philadelphia by the lightning express train the firing was conducted as follows:

The coal was anthracite and in lumps from 5 to 7 inches in diameter; at one end it reached up to the level of the fire door, while at the tube plate end of the fire box it was about 6 inches deep.

The grate bars were constructed to shake in three sections, and shaking the bars to clear out the fire caused it to feed forward of itself, and the combustion of the coal caused it to break up into lumps about 2 inches in diameter at the tube plate, where the fire was much brighter than at the fire door end. The steam pressure varied about 10 lbs. during the trip.

We now come to the best times to fire, to feed, and to oil the valves, and this depends on the level of the road.

On a level road these matters could be attended to with regularity, but as the engine has most work to do in ascending inclines, it is necessary to prepare for such emergencies: First, by having a good fire prepared, so that the fire door may be kept closed as much as possible while the engine is ascending; second, by having plenty of water in the boiler, so as to keep steam, without feeding any more than possible when the engine is calling for more steam, by reason of the reversing lever being put over towards full gear.

The speed is kept well up before reaching the incline, and the reversing lever moved forward a notch or two at a time to maintain the speed, while at the same time moving the sand lever to feed the sand as soon as the engine speed shows signs of reducing.

ACCIDENTS ON THE ROAD.

The accidents to which the locomotive is most liable when running upon the road, and the course to be pursued by the engineer to enable him to take the engine to the depot or complete the trip, are as follows:

KNOCKING OUT THE FRONT CYLINDER HEAD OR COVER.--This arises from various causes, such as a breakage of a connecting rod strap, or of a piston rod or cross head. It is the practice of some locomotive builders to cut in the cylinder cover flange a small groove close to the part that fits the cylinder bore, so that the cover shall break out in the form of a disc, leaving the cover, flange, bolts, and nuts intact, and diminishing the liability to break the cylinder itself as well as the cover.

The provisional remedy for this accident is to take off the connecting rod (on the side of the broken cover) and also the valve motion, either at the rock shaft arm or by taking down the eccentric rod straps. Then place the valve in the centre of its travel so that it shall cover and enclose both the cylinder steam ports and leave the exhaust port open. Then block the cross head firmly on the forward centre, and go ahead with the other cylinder.

HEATING OF PISTON RODS.--This the engineer can often discover by sight, or by smelling it from the cab. The remedy is to stop the engine and slack back the gland until the steam from the engine cylinder leaks freely through the packing. Then apply a little extra lubrication or water while running slowly.

BREAKING OF A PISTON ROD.--If the piston rod breaks, but does not knock out the cylinder head or cover, pursue the same course as directed for breaking the cylinder cover, taking the additional precaution to block the piston, which may be done by fitting pieces of wood between the guide bars, making the pieces long enough to fit between the cross head and guide yoke.

The cylinder or waste water cocks on the side of the accident must also be opened, to prevent any leakage of steam past the slide valve from getting into the cylinder and driving the piston against the cylinder cover, and breaking the cylinder cover or even the cylinder itself.

If the piston rod breaks from the cross head, it is safest to remove it from the cylinder, though this is unnecessary, if it be securely blocked against the cylinder head so that it cannot move, though steam may leak in on either side of it.

BREAKING A CRANK PIN.--This is a somewhat frequent accident, but seldom takes place on both sides of the engine at once.

The remedy is to take off all the parallel or coupling rods, and if it is the crank pin on the driving wheels which breaks, take off the connecting rod also, and securely block the cross head, disconnecting the valve motion as before directed, and opening the cylinder waste water cocks. In the case of this accident occurring, it is absolutely essential to take off the parallel rods on both sides of the engine, or otherwise the crank pins on the other side are apt to break.

THROWING OFF A WHEEL TIRE.--In this case the best plan is to block the tireless wheel entirely clear of the track, which may be done by putting a block of wood into the oil cellar of the driving box, and then tow the engine to the repair shop; for if the engine is run to the shop, and the wheel touches the rail, it will impair its diameter for the proper size of tire.

THROWING OFF A DRIVING WHEEL.--This is not a common accident, but nevertheless it sometimes occurs; they break usually just outside of the driving axle box. In this case take out the driving box and fit in its place a block of wood affording journal bearing for the axle. Let this block rest on the pedestal cap, holding the axle up in the centre of the pedestal. Then secure the piston and disconnect the valve gearing and open the cylinder cocks as before, and the engine can be run slowly to the repair shop without danger of further accident, or, if convenient, it can be towed by another engine.

BREAKING A SPRING OR SPRING HANGER.--Lift the engine with the jacks until the driving wheel axle box is about in the centre of the pedestal, and put any convenient piece of iron across the top of the driving axle box and between it and the engine frame, thus taking the weight of the engine on the frame instead of on the spring. Place also a block of iron between the end of the equalizing bar and the top of the engine frame, so as to prevent the movement of the equalizing bar, and to allow the spring at the other end of the equalizing bar to operate without moving the said bar. Every engineer should carry in his tool box pieces of metal suitable for this purpose, because this is a frequent accident. It does not, however, materially affect the working of the engine, and should not delay a train more than a few minutes.

BURSTED FLUES AND TUBES.--These are usually plugged by tapering a piece of pine wood and driving it into the bursted tube by means of an iron bar. Taper iron plugs are often carried, and then driven into the end of the tube after the wooden one has been driven in. To enable this job to be done, it is necessary to thickly cover the fire with green coal, which operates to cool the tubes and prevent the loss of the water in the boiler. Sometimes careful engineers prepare for use pine plugs turned slightly taper, and a little slack, for the inside of the tube. In case of leak, this plug is inserted in the flue, and driven along it until it covers the fracture, the expansion due to its saturation causing it to become locked in the tube.

SLIPPING OF ECCENTRICS.--Place the reverse lever in the forward notch of the sector. Place the crank on its forward dead centre, as near as can be judged by the eye, and loosen the set screw of the forward eccentric, that is to say, the eccentric which connects to the upper end of the link. Move that eccentric round upon the axle until the slide valve leaves the steam port at the front end of the cylinder open to the amount of required valve lead. In moving the eccentric round upon the shaft, move it in the direction in which it will rotate when the engine is running forward, so as to allow for and take up any lost motion there may be in the eccentric straps, in the eccentric rod eyes and bolts, and in the other working parts of the valve gear; for if the eccentric was moved backward, all such lost motion would operate to vitiate the set of the valve. The eccentric being placed as directed fastens its set screw securely.

If the backward eccentric is the loose one, throw the reverse lever to the backward notch of the sector, lifting the link up so that the eccentric connected to the lower end of the link may be approximately adjusted by moving it around upon the axle in the direction in which it will rotate when the engine is running backward, until the back cylinder port is open to the amount of the valve lead. Another very ready plan of temporarily adjusting the eccentrics is as follows: Place the reverse lever in the end notch forward, and place the engine crank or driving crank pin as near on a dead centre as the eye will direct, and open both the cylinder waste water cocks. Then disconnect the slide valve spindle from the rocker arm, and move the slide valve spindle until the opening of the cylinder steam port corresponding to the end of the cylinder at which the piston stands will be shown by steam blowing through the waste water cock at that end of the cylinder; the throttle valve being opened but a trifle, to allow a small steam supply to enter the steam chest and cylinder, for if much steam is admitted, it may pass through a leak in the piston and blow through both the waste water cylinder cocks.

The position of the valve being thus determined, the eccentric must be moved upon the driving axle until the valve spindle will connect with the rocker arm without being moved, or moving the valve at all.

HOT AXLE BOXES.--If not convenient to reduce the speed of the engine, or if that and free lubrication do not cool the box, a plentiful supply of cold water should be administered, it being well to have at hand a small hose pipe, by means of which water from the tender tank can be used. If the brasses have Babbitt metal in them and it should melt, it is better, if possible, to cool the axle box while the engine is moving, which will injure the journal less than if the journal is stopped to cool the box, because in the latter case the brass or box is apt to become soldered to the journal of the axle, and when the engine is again started, the cutting or abrasion will recommence with extreme violence.

BREAKING A LIFTING LINK OR THE SADDLE PIN THAT CONNECTS THE SLOT LINK TO THE REVERSE SHAFT.--Cut a piece of wood and tie it into the slot of the link, over the link block or die, making it of a length to keep the link in the position for hauling the train. Then fasten another piece of wood in the link slot beneath the sliding block or die, thus securing that die in the proper position for the engine to go ahead. In this case, the engineer must be careful in stopping, as he cannot reverse the engine on the crippled side.

Secondary accidents are almost sure to occur if a disconnected piston is not securely blocked in the cylinder, or from blocking the piston aright and attempting to let the slide valve run, or from attempting to run with the parallel rods on one side only disconnected. There are numerous accidents, which only common sense and a familiarity with the locomotive can provide a temporary remedy for, but those here enumerated are by far the most common.

ADJUSTING THE PARTS OF A LOCOMOTIVE.

When the wedges of the axle boxes are to be adjusted for fit to the pedestal shoes, the engine should be moved until the coupling rods on one side of the engine are in line with the piston rod, because in this position the rod will, to a certain extent, act as a guide in keeping the axles parallel to each other, and at a right angle to the line of engine centres.

Bear in mind that the distance from the centre to centre of axle boxes must be the same as the distance from centre to centre of the crank pins, and that when the coupling or side rods are in line with the piston rod, they act to resist the axle boxes from being set up too close together.

The importance of a proper adjustment of the axle boxes, coupling boxes, and connecting rods cannot be overestimated, and it is necessary therefore to explain it thoroughly. In Fig. 3350, then, s, s represent two wheel axles, whose boxes are between their wedges. At S, S´ are the screws for setting up the shoes or wedges W and W´ respectively. The axles are shown on the line of centres C, C of the engine, the piston being at the head end of the cylinder, and the crank pins on the line of centres as denoted by the small black circles. The wedges W and W´ are shorter than the leg of the pedestal, so that they may be set up by the set screws S and S´, and take up the wear.

In some engines the wedges V and V´ are also shorter for the same purpose. Now it is clear that setting up the screws S and S´ will move the axles s, s´ to the left, and this will alter the clearance between the piston when it is at the end of the stroke and the cylinder cover.

It is clear that the distance between the centres of the two axles must be the same as the distance between the centres of the two crank pins, or else the frame will be subjected to a great strain, tending to break the crank pins and the side rods.

In order to keep the clearance equal and to know when it is equal, it is necessary, at some time when the cylinder cover at the head end is off, to disconnect the connecting rod and push the piston clear up against the left hand cylinder cover, and from the cross head as a guide, make on the side of the guide a line L´. Then put on the cylinder cover at the head end and push the piston up against it and mark a line L. Then when the connecting rod is put on again, the wheels may be moved around if the engine is jacked up, or, if not, the engine may be moved along the rails with a pinch bar, and the clearance will be equal when the cross head (at the ends of the stroke) comes within an equal distance of the respective lines L´, L when the crank is on the dead centres, and it is well to adjust the wedges W W´ so that the cross head does travel within an equal distance, and mark on the guide bar two more lines, one at each end of the bar.

These lines are a permanent guide in setting up the shoes or wedges, and lining up the connecting rods, and coupling or side rods, because it is clear that from the method employed in marking them the distance between the end of the cross head, when at the end of its stroke, and the line L, and that between the face of the piston and the cylinder cover, will be equal.

A proper adjustment, therefore, should be made as follows: The piston should be at the end of its stroke, the crank pins being on the line of centres.

Screw S should be operated to set up the wedge W, taking up the wear of the sides of the box, and bringing the edge of the cross head the proper distance from the line L. The connecting rod brasses should then be set up to fit the pins, and the screw S´ operated to set up wedge W´ to have easy contact with the side of its axle box. If, however, there has been so much wear on the axle boxes that they are still too loose between the wedges, both wedges may be set up to take up this wear, since it is more important to have the axle boxes a proper fit between the wedges than it is to maintain an exactly equal amount of clearance at each end of the cylinder.

The engine will then be in proper tram on this side, or, in other words, the distance from the centre to centre of the crank pins will be the same as that from centre to centre of the axles.

On the other side of the engine the process is the same, the engine being moved until the crank pins are on the line of centres C C and the wedges set up according to the lines.

Spotted a typo or a problem in this chapter? Tell us

XXXIX. THE MECHANICAL POWERS. LEVERS, PULLEYS, GEAR WHEELS, ETC

Power is distinguishable from force or pressure in that the term power means force or pressure in motion, and since this motion cannot occur without the expenditure of the force or pressure, power may, with propriety, be termed the expenditure of force or pressure.

If we suppose a piston to stand in a vertical cylinder sustaining a weight upon its surface and compressing the air within the cylinder, so long as there is no motion no work is done, as the term "work" is understood in a mechanical sense, and the weight merely produces a pressure. If, however, the weight be removed, the compressed air will force the piston upward, performing a certain quantity of work which may best be measured by the amount of power exercised or expended.

The mechanical value of a given amount of power cannot be either increased, diminished or destroyed by means of any mechanical device or appliance whatsoever through which it may be transmitted.

It may be concentrated, as it were, by decreasing the amount of its motion. It may be distended, as it were, by increasing the distance through which it moves, or it may be expended in giving or producing motion, but in either case the amount of duty or work done is the exact equivalent of the amount of power applied.

A gain or increase in speed is not, therefore, a loss of power, but merely a variation in the mode of using or utilizing such power.

For instance, 1 lb. moving through a distance of 12 inches in a given time represents an amount of power which may be employed either as 1 lb. moving a foot, 2 lbs. moving six inches, or 1/2 lb. moving through 24 inches, in the same space of time, the amount of the power or duty remaining the same in each case, the method of utilization merely having differed.

It is an inexorable law of nature that power is concentrated in proportion as the amount of its motion is diminished, or distended in precise proportion as such motion is increased.

Suppose, for example, that in Fig. 3351 L is a lever having its fulcrum at F, which is 4 inches from end A, and 8 inches from end B, and (leaving the weight of the lever out of the question) if we place an 8 lb. weight on a it will just balance 4 lbs. at B.

If the lever is moved, the amount of motion will be twice as much at end B as it is at end A.

If we apply the power at A, the lever has become a means of converting 8 lbs. moving a certain distance into 4 lbs. moving twice that distance, and nothing has been either gained or lost.

If we apply the power at B, the lever has merely been used as a means of converting 4 lbs. moving a certain distance into 8 lbs. moving one half that distance, and nothing has been gained or lost.

Suppose that end A was moved an inch, and the power at that end will be 8 inch pounds or 8 lbs. moving an inch, whereas at the end B the power is 4 lbs. moving 2 inches; we have, therefore, reduced the weight in the same proportion that we have increased the distance moved through.

Suppose now that the lever is moved to the position denoted by the dotted line M M, and the leverages will be altered; that at end A becoming that denoted by the distance from F to the vertical C, and that for end B being denoted by the distance from F to the vertical D.

This occurs because we are dealing with gravity, which always acts in a vertical line.

A crow bar is an excellent example of the application of the lever. In Fig. 3352, for example, we have a 1 lb. weight on the long end of the lever, and as we are dealing with a weight, the effective length of the long end of the lever is from the fulcrum f to w, which is divided into 10 equal divisions. The short end of the lever is from f to p, which is equal to one division, hence the 1 lb. is balanced by the 10 lbs.

A simple method of distending power is by means of pulleys or gear wheels. Suppose, for example, that in Fig. 3353, we have a weight of 12 lbs. suspended from a shaft or drum, whose radius a is 10 inches, and that on the same shaft there is a pulley, whose radius b is 20 inches, and the two weights will balance each other.

In this case the falling of either weight would not effect the leverage, because the distance of both weights would remain the same from the centre of the shaft. The leverage of the 12 lbs. is denoted by the line a, and that of the 6 lbs. by b.

So far as the transmission of power is concerned, therefore, pulleys are in effect revolving levers, which may be employed to concentrate or to distend power, but do not vary its amount.

Suppose we have two shafts, on the first of which are two pulleys, B and C, Fig. 3354, while upon the second there are two pulleys D and E. A belt H, connecting C to D. Let the pulleys have the following dimensions:

If we take the first pair of wheels B and C, we have that the velocity will vary in the same ratio or degree as their diameters vary, notwithstanding that their revolutions are equal.

Radius. Diameter. Circumference. B = 5-1/8 inches. 10-1/4 inches. 32.2 inches. C = 10-1/4 " 20-1/2 " 64.4 " D = 7-5/8 " 15-1/4 " 47.9 " E = 15-1/4 " 30-1/2 " 95.8 "

The velocity is the space moved through in a unit of time, and as it is the circumference of the pulley that is considered, the velocity of the circumference is that taken; thus, if we make a mark on the circumferences of the two pulleys, B and C, Fig. 3354, the velocity of that on C will be twice that upon B, or in the same proportion as the diameters.

Let there be suspended from the circumference of B 10 lbs. weight, and let us see the degree to which this power will be distended by this arrangement of pulleys, supposing the weight to rotate B, and making no allowance for the friction of the shaft.

Suppose the weight to have fallen 32.2 inches, and we have 10 lbs. moving through 32.2 inches, this power it will have transmitted to pulley B.

To find what this becomes at the perimeter of C, we must reduce the number of lbs. in the same proportion that the perimeter of C moves faster than does that of B; hence we divide the circumference of one into the other, and with the sum so obtained divide the amount of the weight; thus, 64.4 (circumference of C) ÷ 32.2 (circumference of B), = 2; and 10 lbs. ÷ 2 = 5 lbs., which, as the circumference of C is twice that of B, will move twice as fast as the 10 lbs. at B, hence for C we have 5 lbs. moving through 64.4 inches.

Now C communicates this to D by means of the belt H, hence we have at D the same 5 lbs. moving through 64.4 inches.

Now E moves twice as fast as D, because its circumference is twice as great, and both are fast upon the same shaft, hence the 5 lbs. at D becomes 2-1/2 lbs. at E, but moves through a distance equal to twice 64.4, which is 128.8 inches. To recapitulate, then, we have as follows:

The weight gives 10 lbs. moving through 32.2 inches. Pulley B " 10 " " " 32.2 " " C " 5 " " " 64.4 " " D " 5 " " " 64.4 " " E " 2-1/2 " " " 128.8 "

That the amount of power is equal in each case, may be shown as follows:

For C, 5 lbs. moving through 64.4 inches is an equal amount of power to 10 lbs. moving through 32.2 inches, because if we suppose the first pair of pulleys to be revolving levers, whose fulcrum is the centre of the shaft, it will be plain that one end of the lever being twice as long as the other, its motion will be twice as great, and the 5 at 10-1/4 inches just balances 10, at 5-1/8 inches from the fulcrum, as in the common lever.

In the case of D we have the same figures both for weight and motion as we have at C, because D simply receives the weight or force and the motion of C. In the case of E, we have the motion of the weight multiplied four times; for the distance E moves is 128.8 inches, which, divided by 4, gives 32.2 inches, which is the amount of motion of the weight, hence the 10 lbs. of the weight is decreased four times, thus 10 lbs. ÷ 4 = 2-1/2 lbs., hence the 2-1/2 lbs. moving through 128.8 inches is the same amount of power as 10 lbs. moving 32.2 inches, and we may concentrate or convert the one into the other, by dividing 128.8 by 4, and multiplying the 2-1/2 lbs. by 4, giving 10 lbs. moving 32.2 inches.

If, therefore, we make no allowance for friction, nothing has been lost and nothing gained.

Thus far, we have taken no account of the time in which the work was done, more than as one wheel is caused to move by the other, and all of them by the motion of the weight, they must all have begun and also have to move at the same time. Suppose, then, that the time occupied by the weight in falling the 32.2 inches was one minute, and the amount of power obtained may be found by multiplying the lbs. of the weight by the distance it moved through in the minute, thus 10 lbs. moving 32.2 inches in a minute gives 32.2 inch lbs. per minute, being the amount of power developed by the 10 lb. weight in falling the 32.2 inches.

We may now convert the power at each pulley perimeter or circumference into inch pounds by multiplying the respective lbs. by the distance moved through in inches, as per the following table:

Distance moved. Lbs. Inches. Inch lbs. of power. Weight at B 10 × 32.2 = 322 " " C 5 × 64.4 = 322 " " D 5 × 64.4 = 322 " " E 2-1/2 × 128.8 = 322

If we require to find the power in foot lbs. per minute, we divide by 12 (because there are 12 inches in a foot), thus 322 inch lbs. ÷ 12 = 26.83 foot lbs. per minute.

Now suppose that B was moved by a belt, with a pull of 10 lbs. at its perimeter, and made 100 revolutions in a minute instead of one, then the pull at the perimeters of C, D, and E would remain the same, but the motion would be 100 times as great, and the work done would therefore be increased one hundred fold. It will be apparent, then, that the time is as important an element as the weight.

The velocity and power of gear wheels are calculated at the pitch circle.

Now suppose the gear A in Fig. 3355 has 30, gear B 60, gear C 10 and gear E 80 teeth, and that 5 lbs. be applied at the pitch circle of A; to find what this 5 lbs. would become at the pitch circle of E, we multiply it by the number of teeth in B and divide it by the number of teeth in C, thus:

Lbs. At pitch of circle A 5 Number of teeth in B 60 --- Number of teeth in C 10 ) 300 --- 30 --

Answer, 30 lbs. at the pitch circle of E.

Now suppose that on the shaft of A there is a pulley 20 inches in diameter, and that on this pulley there is a belt exerting a pull of 5 lbs., while on the shaft of E there is a pulley 16 inches in diameter, and to find how much this latter pulley would pull its belt, we proceed as follows:

2 ) 20 = Diameter of pulley on A. -- 10 = Radius of pulley on A. 5 = Pull on pulley A. -- Number of teeth in A = 30 ) 50 = Pull at centre of shaft ----- of A. 1.666 60 = Number of teeth on B. ------ Number of teeth on C = 10 ) 99.960 = Pull at axis of shaft ------ of B. 9.996 80 = Number of teeth on E. ------- Radius of pulley on shaft of E 8 ) 799.680 Pull at axis of shaft E. ------- 99.96 Pull at perimeter of last pulley.

We have in this case treated each pulley as a lever whose length equalled the radius of the pulley, while in the case of the wheel we have multiplied by the number of teeth when the power was transmitted from the circumference to the shaft, and divided by the number of teeth (the number of teeth representing the circumference) when the power was transmitted from the shaft to the teeth.

We thus find that power is composed of three things, first, the amount of impelling force; second, the distance that force moves through; and third, the time it takes to move that distance.

If we take a number of pulleys, say four, and arrange them one after another so that they drive by the friction of their circumferences, then the amount of power transmitted by each will be equal and the velocities will be equal, whereas, if we arrange them as in Fig. 3354, the power will be equal for each, but the velocities or space moved through in a given time will vary.

What is known as the unit of power is the foot lb., being the amount of power exerted in raising or lifting one lb. one foot, and from what has already been said, it will be perceived that this is the same amount of power as 12 lbs. moving a distance of one inch.

Watt determined that the power of a horse was equal to that necessary to raise 33,000 lbs. one foot high in a minute, and this is accepted, in English speaking countries, as being a horse power.

An engine or machine has as much horse power as it has capacity to lift 33,000 lbs. a foot high in a minute.

CALCULATING THE HORSE POWER OF AN ENGINE.

The horse-power of an engine may be calculated as follows:

Rule.--Multiply the area of the piston by the average steam pressure upon the piston throughout the stroke, and by the length of the stroke in inches, which gives the number of inch pounds received by the piston from the steam during one stroke.

As there are two piston strokes to one revolution of the engine, we multiply by two, and thus get the number of inch pounds received by the piston in one revolution.

By multiplying this by the number of revolutions the engine makes in a minute, we get the number of inch pounds of power received by the piston in a minute.

By dividing this by 12, we get the number of foot pounds the piston receives per minute, and dividing this by 33,000 lbs. we get the horse-power of the engine.

It has already been stated that Watt determined that a horse was capable of exerting a power equal to the raising of 33,000 lbs. one foot high in a minute, hence, having foot pounds of the engine per minute, dividing them by 33,000 gives the horse power.

This gives the amount of power received by the piston, but it is evident that the engine cannot exert so much power, because part of it is expended in overcoming the friction of the moving parts of the engine.

The amount of the piston power expended in overcoming the friction depends upon the fit of the parts, upon the lubrication and the amount of the load.

Thus, the friction of the cross head guides, of the cross head pin, of the crank pin and of the crank shaft bearings will increase with the amount of resistance offered to the piston motion.

The average pressure on the piston is a difficult thing to find, however, for several reasons.

First, because the pressure in the cylinder may, during the live steam period, vary from that in the steam chest because of the ports being too small or from the passages being choked from a defective casting.

Second, because the steam is wire drawn during the time that the slide valve is closing the port to effect the cut off.

Third, because the live steam in the port and passage at the time the cut off occurs gives out some power during the period of expansion.

Fourth, because there is some condensation of the steam in the cylinder after the point of cut off, and there is no means of finding by calculation how much loss there may be from this cause.

During the live steam period there is also loss from condensation in the cylinder, but this is made up for by steam from the steam chest.

Fifth, the loss from condensation after the cut off has occurred will vary with the speed of the engine, and is greater in proportion as the piston speed is less, because there is more time for the condensation to occur in.

Sixth, there is some pressure on the piston between the time that the exhaust begins and the piston ends its stroke.

Seventh, because the compression absorbs some of the piston power.

Assuming the average pressure on the piston to be known, however, we may calculate the horse power as follows:

Example.--What is the horse power of an engine whose piston is 20 inches in diameter, and stroke 30, the revolutions per minute being 120, and the average pressure on the piston 60 lbs. per square inch?

Diameter of piston 20 Diameter of piston 20 --- Diameter of piston squared 400 --- .7854 400 ------------ Area of piston = 314.1600 ([<--] these two ciphers neglected.) 60 average steam pressure. -------- lbs. pressure on piston 18849.60 ([<--] this cipher neglected.) 30 length of stroke in inches. -------- 565488.0 inch lbs. per stroke. 2 two piston strokes per revolution. ------- 12 ) 1130976 inch lbs. per revolution. ------- 94248 foot lbs. per revolution. 120 revolutions per minute. ------- 1884960 94248 -------- 11309760 foot lbs. per minute.

33000 ) 11309760 ( 342.72 = horse power of engine. 99000 140976 132000 ------ 89760 66000 ----- 237600 231000 ------ 66000 66000 -----

In working out the calculation, the ciphers that are decimals and are on the right hand are neglected or taken no account of, because they represent no value and may therefore be discarded.

Thus the area of the piston is 314.1600 inches, the two right hand ciphers having no value. Again the lbs. pressure on the piston is 18849.60 lbs., and the right hand cipher, having no value, is discarded. The inch lbs. per stroke is 565488.0, and the decimal cipher, representing nothing, is discarded when multiplying by the 2.

We have in this case taken no account of the fact that the piston rod prevents the steam from acting against a part of the piston area during one stroke; hence for correct results we must subtract from the area of the piston one half the area of the piston rod.

The horse power thus obtained is that which the engine receives from the steam, and is more than the engine is capable of exerting to drive machinery, because a part of this power is consumed in overcoming the friction of the working parts of the engine.

TESTING THE HORSE POWER OF AN ENGINE.

The useful horse power of a stationary engine may be readily and accurately obtained by means of a pair of scales, and a brake, as shown in Fig. 3356, which is constructed and used as follows:

On the crank shaft of the engine is a pulley enveloped by a friction brake, which consists of an iron band, to which wooden blocks are fastened.

The ends of the iron band do not meet, but are secured together by a bolt as shown.

By screwing up the bolt the wood blocks are brought to press against the circumference of the wheel.

This forms a friction brake that would revolve with the wheel, were it not for two arms that are secured to the brake, and rest at the other end upon a block placed upon a pair of scales.

The principle of action of this device is that the amount of friction between the brake and the wheel is weighed upon the scales, and this amount, multiplied by the velocity of the wheel at its circumference and divided by 33,000, is the horse-power of the engine.

It is necessary, in arranging this brake, to have its end rest upon the scale at the same height from the floor as the centre of the crank shaft, so that the line marked 5' 3" (5 feet 3 inches), which represents the length of the lever, shall stand parallel with the surface of the platform of the scale.

To test the horse-power, we proceed as follows:

Suppose the pressure of the end of the lever on the scale is found by the weight on the scale beam to be 540 lbs., the diameter on which the brake blocks act being 3 feet, the length of the leverage being 5 feet 3 inches, as marked, and the engine making 150 revolutions per minute, and the calculation is as follows:

540 lbs. on scale. 5.25 leverage in feet. ----- 2700 1080 2700 ------ radius of pulley in feet 1.5 ) 2835.00 ( 1890 lbs. at pulley perimeter. 15 --- 133 120 --- 135 135 ---- ...0 ====

Then

3.1416 3 diameter of pulley in feet. ------ 9.4248 circumference of pulley in feet. 150 revolutions per minute. ------- 4712400 94248 --------- 1413.7200 velocity of pulley perimeter. 1890 pounds at pulley perimeter. --------- 12723480 1130976 141372 ---------- 2671930.80 foot lbs. per minute. ==========

Then

33000 ) 2671930.80 ( 80.9 264000 ------ 319308 297000 ------ 223080 ======

Answer, 80-9/10 horse power.

In this calculation we have nothing to do with the size of the cylinder or the steam pressure, because the scale beam tells us how many lbs. the brake exerts on the scale, and we treat the brake and brake pulley as levers. Thus by multiplying the lbs. on the scale by the leverage of the brake arm we get the number of lbs. exerted at the centre of the crank shaft, and by dividing this by the radius of the brake pulley we get the number of lbs. on the circumference, or, what is the same thing, the perimeter of the brake pulley.

By multiplying the circumference of the pulley in feet by the revolutions per minute, we get the speed at which the pounds travel, and by multiplying this speed by the number of lbs. we get the foot lbs. per minute, which, divided by 33,000, gives us the effective horse power of the engine.

This effective horse power is correct, because in loading the engine by the brake the crank pin, the cross head guides, etc., are all placed under the same friction as they would be if it was a circular saw, or some other piece of machinery or machine that the engine was driving.

SAFETY VALVE CALCULATIONS.

Among the most frequent questions asked in an engineer's examination are those relating to the safety valves of boilers.

These questions may be easily answered from a study of the following:

The safety valve is a device for relieving the boiler of steam after it has reached a certain pressure.

This it accomplishes by letting the steam escape after it has reached the required pressure.

At what pressure the safety valve will blow off depends upon the position of the weight on the safety valve lever.

The calculations referring to this part of the subject are, finding how much weight will be required to be placed at a given point on the lever, in order, with a given sized valve, to blow off at a given pressure.

Finding the position on the lever of a given amount of weight, in order to blow off at a certain pressure.

Finding, with a given sized valve and a given weight, how to mark off the lever and where the notches must be cut for given pressures.

In each of these calculations there are three elements: first, the area of the valve and the steam pressure, which constitute the effect of acting to lift the valve; second, the amount of the weight and its position upon the lever, which acts to keep the valve closed; and third, the weight of the lever and of the valve, which act to keep the valve closed.

In Fig. 3357 we have a drawing of a safety valve shown in section, and if there was no weight upon the lever, the pressure of steam the valve would hold in the boiler would be that due to the weight of the valve and of the lever upon the valve.

To find out how much this would be, we would have to put the valve itself and the pin a on a pair of scales and weigh them.

Then put a piece of string through the hole at a in the lever, and see how much it weighed when suspended from that point.

Suppose the valve and pin to weigh 2 lbs. and the lever (suspended by the string) 10, and the total will be 12 lbs.

Next we find the area of the valve, and suppose this to be 8 square inches; then we may find how much pressure the valve would keep in the boiler, by dividing the area of the valve into the weight holding the valve down, thus:

Lbs. Weight of valve and pin, 2 " " lever, 10 -- Area of valve, 8 ) 12 --- Pressure the valve would hold, 1.5 lbs.

The area of the valve is that part of its face receiving the steam pressure when the valve is seated, so that if the smallest part of the valve diameter is equal to the diameter of the seat bore, the diameter from which the valve area is to be calculated will be that denoted by D in the figure, and cannot in any case be less than this. But if the smallest end of the valve cone is of larger diameter than the smallest end of the seat cone (which should not, but might be the case), then it is the smallest diameter of valve cone that must be taken in calculating the area, because that is the area the steam will press against.

Now suppose we rest a 20 lb. weight on the top of the valve that is on the point denoted by I, and there will be 32 lbs. holding the valve down, thus, weight of valve 2 lbs., of lever 10, and weight added, 20 lbs., and to find how much pressure this would hold in the boiler, we divide it by the valve area, thus:

Weight on valve. Valve area = 8 ) 32 -- 4 = pressure valve will hold.

But suppose we put the weight on the lever, in the position shown in the figure, which is six times as far from the fulcrum F of the lever as the valve is, and its effect on the valve will be six times as great as it would if placed directly upon the valve, so that leaving the weight of the valve and of the lever out of the question (as is commonly done in engineers' examinations), we may find out what pressure the valve will hold, as follows:

Rule.--Divide the length of the lever by the distance from the centre of the valve to the centre of the fulcrum. Multiply by the amount of the weight in lbs., and divide by the area of the valve.

Example.--The area of the valve is 8 inches, the distance from the centre of the fulcrum to the centre of the valve is 4 inches, and the distance from the fulcrum to the point of suspension of the weight 24 inches, the weight is 40 lbs., what pressure will the valve hold?

Length of lever. From fulcrum to valve, 4 ) 24 -- 6 40 amount of weight. --- Area of valve, 8 ) 240 --- 30

Lbs. per square inch the valve will hold = 30.

The philosophy of this is clear enough when we consider that as the weight is six times as far from the fulcrum as the valve is, and each 1 lb. of weight will press with a force of 6 lbs. on the valve, hence the 40 lbs. will press 240 lbs. on the valve, and as the valve has 8 square inches, the 240 becomes 30 lbs. for each inch of area.

Example.--The area of a safety valve is 8 inches, the distance from the fulcrum to the valve is 4 inches, and the weight is 40 lbs., how far must the weight be from the fulcrum to hold in the boiler a pressure of 30 lbs. per square inch?

In lbs. From fulcrum to valve, 4 ) 40 amount of weight. -- 10

Area of valve, 8 square inches. Pressure required, 30 --- 10 ) 240 --- 24

Answer = 24 inches from the fulcrum.

Example.--The diameter of a safety valve is 4 inches, the distance from the centre of the fulcrum to the valve is 3 inches, a 50 lb. weight is 30 inches from the fulcrum, what pressure will the valve hold?

3 diameter of valve. 3 -- 9 .7854 ----- 36 45 72 63 ------ 7.0686 = area of valve. ======

3 ) 30 -- 10

50 = weight 10 = leverage of weight. ------- Area of valve, 7.068 ) 500.000 ( 70.7 = lbs. pressure per sq. in. 49476 ------- 52400 49476 ----- 2924

HEAT.

The heat unit, or the unit whereby heat is measured, is the quantity of heat that is necessary to raise 1 lb. of water from its freezing temperature (which is 32° Fahrenheit) 1°, and this unit is sometimes termed a thermal unit.

The reason that some specific temperature, as 32° Fahrenheit, is taken, is because the quantity of heat required to heat a given quantity of water 1° increases with the temperature of the water; thus, it takes more heat to raise 1 lb. of water from 240° to 245° than it does to raise it from 235 to 240, although the temperature has been raised 5° in each case.

The whole quantity of heat in water or steam is not, however, sensible to the thermometer, or, in other words, is not shown by that instrument. The heat not so shown or indicated is termed latent heat.

Water obtains latent heat while passing from a solid to a liquid state, as from ice into water, and while passing from a liquid to a gaseous state, as while passing from water into steam, and the existence of latent heat in steam may be shown as follows:

If we take a body of water at a temperature above freezing, and insert therein a thermometer, the decrease in the temperature as the water becomes frozen will be shown by the thermometer. If, then, its temperature being say at zero, heat be continuously imparted to the ice, the thermometer will mark the rise in temperature until the ice begins to melt, when it will remain stationary at 32° so long as any ice remains unmelted, and it is obvious that all the heat that entered the water from the time the ice began to melt until it was all melted became latent, and neither sensible to the sense of feeling nor to the thermometer. Similarly, if the water, after the ice is all melted, be heated in the open air, the thermometer will mark the rise of temperature until the water boils, after which it will show no further rise of temperature, although the water still receives heat. The heat that enters the water from boiling until it is evaporated away is the latent heat of steam. The latent heat of water is 143° Fahrenheit, and that of steam when exposed to the pressure of the atmosphere, or under an atmospheric pressure of 15 lbs. (nearly), is 960°, which may be shown as follows:

If a given quantity of water, as say 1 lb., has imparted to it a continuously uniform degree of heat sufficient to cause it to boil in one hour, then it will take about 5-1/3 more hours to evaporate it all away, hence we find the latent heat by taking the difference in the amount of heat received by the water, and that shown by the thermometer thus:

Degrees. Temperature by thermometer at boiling point 212 Less the temperature of the water at first 32 --- Heat that entered the water in the first hour 180 Hours that the water was subsequently heated 5-1/3 ------- 900 One-third of 180 = 60 --- Heat that entered the water during the 5-1/3 hours 960 degrees.

This, however, is not quite correct, as it would take slightly more than 5-1/3 hours to boil the water away, and the heat that entered the water after it commenced to boil would be about 966 degrees.

If the steam that arose from the water while it was boiling were preserved without increasing the pressure under which it boiled, and without losing any of its heat, it will have a temperature the same as that of the water from which it was boiled, which is a temperature of 212°, so that neither the steam nor the water account, by the thermometer, for the 966° of heat that entered the water after it boiled, hence the 966° became latent, constituting the latent heat of the steam when boiled from and at a temperature of 212°.

The total heat of steam is the sensible heat, or that shown by the thermometer, added to the latent heat; hence the heat necessary to evaporate water into steam at a temperature of 212° (which corresponds to a pressure of 14.7 lbs. per square inch) is 212° + 966°, which is 1178°, and these, therefore, are the number of degrees that must be imparted by the coal to the water, in order to form steam at a temperature of 212°.

WATER.

Water is at its greatest density when at a temperature of 39.1° Fahrenheit, that is to say, it occupies its least space and weighs the most per given quantity (as per cubic inch) when at that temperature.

At a lower temperature water expands, its freezing point being 32° Fahrenheit, below which it forms ice. The weight of a cubic foot of water when at its maximum density (39.1) is 62.382 lbs. Water also expands as its temperature is increased above 39.1°; thus, while it is heated from 39.1 to 212°, its volume increases from 1 to 1.04332. The expansion for each degree of heat added to its temperature increases from 0 at 40° Fahrenheit to .0043 at 212°.

The rate of expansion of water at a temperature above 212° is unknown.

STEAM.

At every temperature above freezing point water passes from the liquid into a gaseous state, the gas being termed steam. While water is below its boiling point its evaporation occurs at its surface only; but when its mass is heated to boiling point, and additional heat is imparted to it, evaporation occurs from the water lying against the surface from which it receives the heat, and an ebullition is caused by the vaporized water passing through the mass, the ebullition being what is known as boiling.

The temperature at which water boils depends upon the pressure acting upon its surface, the boiling point being at a lower temperature in proportion as the pressure is reduced; thus water at the top of a mountain, where the pressure of the atmosphere is less than at the sea level, would boil at a lower temperature than 212°, which is the boiling point when the atmospheric pressure is 14.7 lbs., which it is assumed to be at the sea level. Conversely, the boiling point is raised in proportion as the pressure upon its surface is raised, whether that pressure consists of air or of steam. As, however, the pressure is increased, the boiling point is at a higher temperature. So long as the steam is in contact with the water both are at the same temperature, as denoted by the thermometer (although they do not contain the same quantity of heat, as will be show presently), and the steam is termed saturated steam.

The pressure of saturated steam cannot be either increased or diminished without either increasing or diminishing its temperature, hence there is a definite relation of pressure to temperature, which enables the pressure to be known from the temperature, or conversely, the temperature to be known from the pressure. But if the steam be separated from the water and heated, it may be what is termed superheated, which is that it may be surcharged with heat or contain more heat than saturated steam at the same pressure. Such additional heat, however, is latent.

The pressure of steam is the lbs. of force it exerts upon a given area, as upon a square inch. In non-condensing engines the effective pressure of the steam is its pressure above that of the atmosphere, because the exhaust side of the piston being exposed to the atmosphere receives the atmospheric pressure, which must be overcome by a corresponding pressure of steam on the steam side of the piston, and this pressure is not, therefore, available for producing work or power in the engine.

In condensing engines, however, the exhaust side of the piston is (as nearly as practicable), relieved of the atmospheric pressure, and assuming a perfect vacuum to be formed, the whole of the steam pressure is exerted to propel the piston, in which case the steam pressure is termed the absolute pressure.

In considering the weight or density or the expansion of steam, its absolute and not its effective pressure must obviously be taken.

What is termed dry steam is saturated steam that does not contain what may be termed entrained water, which is water held in suspension in the steam, which may be caused by the surface of the water through which the steam is allowed to rise being too small in proportion to the volume of steam formed, in which case the rapid passage of the steam through the water causes it to carry up water with it and hold it in suspension, this action being termed foaming or priming.

Suppose, for example, that a boiler be filled with water up to the bottom of the steam dome, then all the steam formed would require to find exit from the water within the area of the dome, and the violence of the ebullition would cause foaming. Obviously, then, to obtain dry steam there must be provided a sufficient area of water surface for the steam to pass through.

But water so entrained is evaporated into steam, if the steam is wire drawn, that is, allowed to expand and reduce in pressure.

THE EXPANSION OF STEAM.

A cubic inch of water, when evaporated into steam at a pressure of 14.7 lbs. per square inch, occupies as steam a space or volume of 1644 cubic inches, and its weight will be equal to that of the water from which it was evaporated.

If additional heat be imparted (after its evaporation into steam), such additional heat becomes latent and does not cause an increase of sensible temperature or of pressure.

The weight of a given volume of steam, therefore, bears a definite and constant relation to the pressure and sensible temperature of the steam, so that the pressure or the sensible temperature being known, the weight of a given volume, as say a cubic foot, may be known therefrom. Or the weight of a cubic foot of steam being known, its sensible temperature and pressure may be known therefrom.

This would not be the case if steam expanded by heat. Suppose, for example, we have a cubic foot of steam at any absolute pressure, as say 15 pounds per square inch, a cubic foot weighing .0387 of a lb., and its sensible temperature will be 213°. Now it is evident that the weight will remain the same whatever the amount of heat that may be imparted to the steam. Now if the steam were maintained within the cubic foot of space, and was capable of expansion by the absorption of additional heat, its pressure would increase and its weight remaining the same, there would be no definite relation between the weight and the temperature and pressure.

But if the cubic foot of steam were allowed to expand so as to occupy more space, then additional heat is necessary to prevent its condensation.

The relation between the temperature, pressure, and weight of steam is not quite proportional to the volume, because steam is not a perfect gas, and does not, therefore, strictly follow Mariotte's law.

A perfect gas is one that during expansion or compression follows the law laid down by Boyle and Mariotte, this law being that, if maintained at a constant temperature, the volume is inversely proportional to the pressure.

For example, the quantity of gas that, if confined in a cubic inch of space, would give a pressure of 80 lbs. per square inch, would give a pressure twice as great (or 160 lbs. per inch of area), if confined in one-half the space, that is, if compressed into one-half of a cubic inch. Conversely, if the cubic inch was allowed to expand until its pressure was 40 lbs., it would occupy 2 cubic inches of space, assuming, of course, that the temperature remains the same. Since, however, if a gas be compressed, its temperature is increased by reason of the friction of the particles moving one upon the other, the law of Mariotte may be better explained as follows:

Suppose we have three vessels, A, B, and C, filled with a fluid which is a perfect gas, the temperatures being equal. Let the pressure be: A 40, B 80, and C 160 lbs. per square inch, then 2 cubic inches of the fluid in B will weigh the same as 4 cubic inches in A, because that in B is at twice the pressure of that in A, and the 2 cubic inches in B will weigh the same as 1 cubic inch in C, because its pressure is one-half that of C, or, what is the same thing, whatever number of cubic inches of the fluid in C it takes to weigh a pound, it will take twice as many in B, and four times as many in A to weigh one pound.

But steam is not a perfect gas, as is evidenced by the fact that its volume does not increase in a ratio inverse to its pressure. For example, if a cubic inch of water be evaporated into steam at a pressure of 14.7 lbs. per square inch, its volume will be 1644 cubic inches, and its temperature 212° Fahrenheit.

But if the cubic inch of water be evaporated into steam at twice the pressure, which is 29.4 lbs per square inch, its volume will be 838 inches.

The volume then is not inversely as the pressure, although the actual quantity and weight remain the same, as is proven by the fact that if the steam at either pressure were condensed it would pass back into the cubic inch of water from which it was generated.

This may be accounted for in the difference in the boiling point of the water in the two pressures, or in other words, by the difference in the temperatures; thus the boiling point of the water at a pressure of 14.7 lbs. is 212°, while that for the pressure of 29.4 is increased about 38.4 degrees, and the steam is at the higher pressure expanded by these 38.4 degrees of heat, which adds to its pressure, although not affecting its actual quantity or weight.

The amount of this expansion may be estimated as follows:

Taking the 1644 cubic inches, and supposing the steam to be a perfect gas, we divide it by 2 to obtain half the volume, 1644 ÷ 2 = 822.

If then we subtract this 822, which is the volume of the steam if it acted as a perfect gas from the 838 it actually occupies, we get 16 (838-822 = 16), which is the number of cubic inches of expansion due to the increase in the boiling temperature.

THE CONVERSION OF HEAT INTO WORK.

When steam performs work a certain portion of the heat it contains is converted into work, the steam simply being a medium of conveying the heat into the cylinder in which the motion of the piston converts this proportion of heat into work. It has been proven that a given quantity of heat will pass into a given quantity of work, and conversely that a given quantity of work is convertible into a given quantity of heat, and it has also been proven that so much heat is convertible into so much work, independent of the temperature of the heat during its conversion into work, power, or energy, all three of these words being used to imply pressure, force, or weight in motion.

The accepted measurement of the conversion of heat into work is known as Joule's equivalent; Joule having determined that the amount of power exerted in raising 772 lbs. one foot is the equivalent of the amount of heat that is required to raise the temperature of 1 lb. of water when at or near its freezing point (that is, at a temperature of 32°) one degree.

This is called the mechanical equivalent of heat, being merely the quantity of heat necessary to do a certain amount of work, but having no relation to the time in which that work was done.

The conversion of heat into work and of work into heat may be demonstrated as follows: Suppose a cylinder to be so situated that heat can neither be transferred to it or from it, and that saturated steam be admitted under the piston so as to fill one-half of the cylinder at a pressure of 50 lbs.

Suppose then that we raise the piston from an independent application of power, the steam simply expanding to fill the space given by the piston, but not exerting its force to move the piston.

Now suppose the experiment is repeated, permitting the force of the steam to lift the piston, and the temperature of the steam will be less in the second than it was in the first, proving that in the second experiment a certain portion of the heat in the steam was converted into the work of raising the piston.

If we desire to reconvert the work into heat, we may force the piston back again to its original position, and its temperature will be restored to what it was before we allowed it to raise the piston. It is here, of course, assumed that there is no friction in moving the piston in the cylinder.

The apparent or external work performed by steam in expanding and moving a piston against a given resistance is measurable by multiplying the amount of the resistance against which the piston moved by the distance it moved through, thus:

Suppose a piston weighs 100 lbs. and had resting upon it a weight of 50 lbs., and that it be raised by the expansive action of steam a distance of a foot, then, since the total resistance it moved against would be (supposing it to move frictionless in the cylinder) 150 lbs., and since the amount of motion was 1 foot, the external or apparent amount of work performed by the steam will be 150 foot lbs., or 150 lbs. moved 1 foot.

But in expanding, the steam has performed a certain amount of what is called internal work, that is to say, its particles or atoms have done work in expanding, and this work has been done at the expense of some of the heat in the steam, so that the loss of heat due to the motion of the piston is the amount of heat converted into work in moving the piston against the piston resistance, added to that converted into the internal work due to the expansion of the steam.

It is because of this internal work that the steam in expanding does not strictly follow Mariotte's law.

The mechanical theory of heat is, that the atoms of which bodies are composed are at absolute rest when at a temperature of 461.2° below the zero of Fahrenheit, which is supposed to be absolute cold, and at any degree of temperature above this the atoms are in motion; the extent and force of their motion determines what we know as the temperature of the body.

Atoms are capable of transmitting their motion to adjoining atoms of the same or of other bodies, losing, of course, the amount of motion they transmit, and it is in this way that heat is conveyed from one to another part of the same body, or from one body to another, this being known as the heat of conduction.

But heat may be conveyed by means of what is known as radiation, and also by convection.

Thus, the air surrounding a heated body becomes heated, and by reason of its expansion it then becomes lighter and rises, a fresh supply of cooler air taking its place, becoming in turn heated, and again giving place to cooler air; the heat thus conveyed away by the fluid or air is conveyed by what is termed convection.

Heat also passes from a body in straight lines or rays, which do not heat the air through which they pass to their own temperature, but do impart that temperature to a solid body, as iron or water; the heat that passes from a body in this manner is termed radiant heat, or the heat of radiation.

In the cylinder of a steam engine, therefore, the heat contained in the steam is disposed of as follows:

A certain portion of it is converted into work through the medium of the piston.

Another portion is conveyed away by the walls of the cylinder, this portion including the heat of convection and that of radiation.

Yet another portion is converted into internal work. Referring to the latter, suppose that steam is permitted to expand and its atoms will be in motion, which motion has been derived at the expense of or from the conversion of a certain quantity of heat.

The amount of the heat so converted obviously depends upon the amount of the motion. Suppose, for example, that steam is generated in a closed vessel as in a steam boiler, and that a certain pressure having been attained, the steam is permitted to pass off as fast as it is formed from the boiler, then the amount of atomic motion will remain constant, because the pressure remains constant; but suppose instead of the steam passing off, it be confined within the boiler, then the pressure will increase and there will be a greater resistance to the motion of the atoms, hence their motion will be less, and less of their heat will therefore be converted into atomic motion, and, as a consequence, more of it will exist in the form of sensible heat; hence while the pressure of steam continues to increase, its heat is increased, not only by reason of the heat it receives from the furnace, but also by reason of that abandoned by the steam, because it is prevented by the pressure, from expending it in atomic motion.

Spotted a typo or a problem in this chapter? Tell us

XL. THE INDICATOR

The indicator is an instrument which marks or draws a figure, or diagram as it is called, which shows the pressure there is in the cylinder at every point in the piston stroke, while it also shows the resistance offered by the same body of steam to the piston on its return stroke. From the form of this figure or diagram, the engineer is enabled to discover whether those parts of the engine whose operation regulates the admission of the steam to and its exhaust from the cylinder are correctly adjusted.

From the diagram the engineer may find the average or mean effective pressure of steam on the piston throughout the stroke, for use in calculating the power of the engine.

He may also locate the point of cut off, of release, the amount of back pressure, the degree of perfection of the vacuum in a condensing engine, and the amount of compression.

From the area of the diagram the engineer may also estimate the quantity of steam that is used, and supposing it to be dry steam, he may calculate the amount of water used to make the steam, and assuming one pound of coal to evaporate so much water, he may calculate the amount of coal used to produce the steam.

The indicators commonly used upon steam cylinders contain two principal mechanical movements; first, a drum revolving the piece of paper upon which the diagram is to be marked, and second, a piston and parallel motion for moving the pencil to mark the diagram upon the revolving paper.

The drum is given a motion that, to insure a correct diagram, is exactly timed with the piston motion.

The pencil is given a vertical movement; this movement must bear a constant and uniform relation to the pressure of the steam in the engine cylinder.

An indicator may be attached to each end of the cylinder or in the middle, with a pipe passing to each end of the cylinder, as in Fig. 3358, but an indicator of the usual construction and such as here referred to, can take a diagram, or card as it is sometimes called, from but one end of the cylinder at a time. The stop valves A and B are used, so that the communication between the indicator and one end of the cylinder may be shut off while a diagram is being taken from the other end, while both ends may be shut off when the indicator is not being used.

In the figure a piece of paper (or card, as it is commonly called) is shown in place upon the drum with a diagram upon it.

The Thompson Indicator is shown in Fig. 3359, and in section in Fig. 3360.

The Tabor Indicator is shown in Fig. 3361, and in section in Fig. 3362.

Both are made with the piston and parallel motion as light as possible, in order to enable the taking of diagrams at as high a speed of engine revolution as possible.

Each consists of a cylinder and piston, the bottom surface of the latter being in communication with the bore of the engine cylinder, so as to receive whatever steam pressure there may be in the cylinder.

This indicator piston receives, on its upper surface, the pressure of a spiral spring, which acts to resist the steam pressure.

The indicator piston rod actuates an arm or line on the end of which is a pencil, which, by means of a parallel motion, is caused to move in a straight line.

The paper or card being in place upon the drum, and steam let into the indicator, the pencil lever is moved until the pencil touches the paper as lightly as possible, and as a result of the combined movements of the pencil and drum, the diagram is marked, its form being illustrated in Fig. 3363, which represents a diagram placed above a cylinder, and the engine piston in three positions; first at the beginning of the stroke; second, at the point of cut off (which is supposed to be at one-third of the stroke); and third, at the point of release where the valve first opens the port for the exhaust. For convenience, the diagram is shown as long as the cylinder, but the actual diagram usually measures about 2-1/2 inches high and 4-1/4 inches long.

Supposing the cylinder to be filled with air, and the engine piston in position 1, and the indicator piston would be at the corner A of the diagram; but if steam were admitted, the pencil would rise vertically, marking the line from A to B, which is therefore called the admission line, or by some, the induction line.

If on reaching B the pressure was enough to move the engine piston, that piston and the indicator drum would move simultaneously, and as long as live steam was admitted the line from B to C would be drawn, hence this is called the steam line, its length denoting the live steam period.

The cut off occurs when the engine piston is in position 2, and the indicator pencil at C.

From this point the pencil will fall, in proportion as the steam pressure falls from expansion until the exhaust begins, the piston then being in position 3, and the pencil at D.

The line from C to D is therefore called the expansion line or expansion curve, and the point D the point of release or point of exhaust.

We have now to explain that in reality the whole of the remainder of the line of the diagram is, in reality, the exhaust line, yet there is a difference between the part of the line from point D to the end E of the diagram, and that part from E to A, inasmuch as that during the period of exhaust from D to E, the pressure is helping to propel the piston, while after E is reached, whatever steam pressure there may remain in the cylinder acts to retard the piston.

The line from D to E is therefore the exhaust line, and that from E to A is the back pressure line or counter pressure line.

In this example it has been supposed that while the piston was moving from position 3 to the end of its stroke, and the pencil from D to E, the indicator piston would have a steam pressure on it equal to atmospheric pressure, hence the line from E to A, in this case, represents the atmospheric line, and also the back pressure line.

The atmospheric line is a line drawn when there is no steam admitted to the indicator, and represents a pressure above a perfect vacuum equal to the pressure of the atmosphere. Its use is to show the amount of back pressure, and in a condensing engine to show the degree of vacuum obtained.

It also forms a line wherefrom the line of perfect vacuum, or that of full boiler pressure, may be marked.

The steam pressure at any point in the stroke is denoted by the height of the diagram above the atmospheric line, but the steam pressure thus taken is obviously above atmospheric, and is thus the same as the pressure of a steam gauge, which is also above the atmospheric pressure, and therefore represents the pressure that produces useful effect in a non-condensing engine.

This is what may be called a theoretical diagram, because, first, it supposes the steam not to be admitted to the cylinder until the piston was at the end of its stroke, and to attain its full pressure in the cylinder before the piston lead begins to move, whereas, in order to attain a full steam pressure at the beginning of the stroke, the valve must have lead.

Second, it supposes the cut off to be effected simultaneously, whereas the valve must have time to move and close the port, and during this time the steam pressure will fall, and the curve C of the diagram will therefore be rounded more or less according to the rapidity with which the valve closed.

Third, it supposes the steam to have exhausted down to atmospheric pressure by the time the piston had reached the end of the stroke, whereas the piston will have moved some part of the back or return stroke before the steam will have had time to exhaust down to atmospheric pressure; and,

Fourth, it supposes the steam to remain at atmospheric pressure until the piston arrives at the end of its return stroke, whereas the valve will begin to close the port and cause the steam remaining in the cylinder to compress before the piston has completed its return stroke.

In practice the diagram will, under favorable conditions, accord nearer to the shape shown in the lower part of Fig. 3363, in which the closure of the port for the cut off is shown by the curve at F. At the point denoted by g the valve began to close, and at the point denoted by h the cut off was completely effected, and the expansion curve began.

The curve beginning at D is caused by the gradual opening of the exhaust port.

The height of the line of back pressure above the atmospheric line shows the amount of back pressure.

At the point m, where the back pressure line rises into a curve, the valve had closed, shutting in the cylinder a portion of the exhaust steam, which is afterwards compressed by the piston.

This curve is therefore called the compression line or compression curve. The point at which it begins cannot be clearly seen when the exhaust port is closed slowly.

The compression curve ends at p, where it merges into the admission line, but the exact point where the compression ends and the admission begins cannot always be located, this being the case when the port is opened slowly or the compression extends through a large portion of the stroke.

The admission line is, however, in most cases nearly vertical when the valve has lead, because the valve opens the port quickly while the engine piston is moving at its slowest.

A diagram as drawn by the indicator does not account for all the steam that is used in the cylinder, however, as will be seen from Fig. 3363, because, as the paper drum of the indicator receives its motion from the engine cross head, its length represents the length of the piston stroke, whereas, there is a part of the cylinder bore between the piston (when it is at the end of the stroke) and the cylinder cover that is filled with steam as is also the steam passage.

This steam performs no useful work during the live steam period, but obviously expands during the expansion period, and therefore affects the expansion curve, and must be taken account of in calculating the consumption of steam, of water, or of coal from the diagram, or in marking in the true expansion curve.

In calculating the horse power, however, it may be neglected, as it does not enter into that subject.

But in any calculation involving the amount of steam used, the clearance must be marked in by a line at a right angle to the admission line and distant from the nearest point of the admission line to an amount that bears the same proportion to the whole length of the diagram as the clearance does to the whole contents of the cylinder.

The clearance line is shown at L, L´, in Fig. 3363, its distance from the admission line representing the amount of clearance which includes the contents of the steam port and passage, as well as that of the cylinder bore that is between the cylinder cover and the piston, when the latter is at the end of the stroke.

A method of measuring the amount of clearance has already been given with reference to stationary steam engines.

A diagram for a condensing engine is shown in Fig. 3364, which corresponds to Fig. 3363, except that the line of perfect vacuum or no pressure is marked in.

It represents a perfect vacuum, and must be marked on all diagrams from which the consumption of steam is to be calculated, because the quantity of steam used obviously includes that which is used in counter balancing the pressure of the atmosphere.

Learners often get confused on this point, hence it may be more fully explained as follows:

Suppose the engine piston to be blocked, in the middle of the cylinder, and has on one side of it a pressure of 20 lbs. of steam by steam gauge, and on the other the pressure of the atmosphere, and we might pump out the steam, thus leaving the cylinder empty on that side of the piston.

The atmosphere would then exert a pressure of about 14-1/2 lbs. per square inch on one side of the piston, and if we slowly admitted steam again, it would have to get up a pressure of 14-1/2 lbs. per square inch before the atmospheric pressure would be counterbalanced and the piston be in equilibrium.

But the steam gauge would at this time stand at zero, and not show that there was any steam in the cylinder, because the zero of the steam gauge is atmospheric pressure.

When, therefore, the steam gauge showed a pressure of 20 lbs. of steam in the cylinder, there would actually be a pressure of 34-1/2 lbs. of steam per square inch.

The clearance line and the vacuum line must both, therefore, be marked on the diagram when the quantity of steam used is to be computed from the diagram, and also when the proper or theoretical expansion curve is to be marked on the diagram.

This is clear, because in finding the expansion curve for a given volume of steam the whole of its volume must be taken into account, and this whole volume is represented by the area inclosed within the clearance line, the steam line, the expansion curve, the exhaust line, and the line of perfect vacuum, or line of no pressure.

The atmospheric line should be drawn after the diagram has been taken, and while the indicator is hot, as the expansion of the indicator affects the position of this line. It is drawn with the steam shut entirely off from the indicator, whose piston therefore has atmospheric pressure on both sides of it.

Whether the engine is condensing or non-condensing, the same amount of steam (all other things being equal) is used, the only difference being that in a condensing engine a greater portion of the steam is available for driving the piston.

If the condenser produced a perfect vacuum, the whole of the steam would be utilized in propelling the piston.

The "line of no pressure," or of perfect vacuum, is marked as far below the atmospheric line as will represent the pressure of the atmosphere, which is, at the sea level, about 14.7 lbs. per square inch when the barometer stands at 29.99 inches.

THE BAROMETER.

A barometer is an instrument for denoting the pressure or weight of the atmosphere, which it does by means of a column of mercury inclosed in a tube, in which there is a vacuum, which may be produced as follows:

A tube having a parallel bore and closed at one end is filled with mercury and while the finger is placed over the open end of the tube, it is turned upside down and inverted in a cup of mercury that is open to receive the pressure of the atmosphere.

The finger is then removed from the end of the tube and the mercury will fall, leaving a vacuum at its upper end.

The pressure of the atmosphere on the surface of the mercury in the cup forces the mercury up the tube, because the surface of the mercury in the tube has no atmospheric pressure on it, the action being the same as that already described with reference to the principles of action of a pump.

The weight of the atmosphere is equal to the weight of that part of the column of mercury that is above the surface of the mercury in the cup, hence lines may be drawn at different heights representing the weight of the atmosphere, or of any other gas, when the column of mercury stands at the heights denoted by the respective lines.

But as mercury expands by heat, a definite degree of temperature must be taken in marking a column, to represent the weight, this temperature being 32° Fahrenheit.

Similarly, as the weight of the atmosphere varies, according to the height at which it is taken from the surface of the earth, a definite height must be taken.

The sea level is that usually taken, the mean or average atmosphere (at that level) being 14.7 lbs. per square inch.

For higher altitudes, the mean atmospheric pressure in lbs. per square inch may be found by multiplying the altitude or height above sea level by .00053, and subtracting the product from 14.7.

Each pound on the square inch is represented by a height of 2.036 inches of mercury, hence the height of a column of mercury at a temperature of 32° that will balance the mean weight of the atmosphere is 29.92 inches, and to avoid fractions, it is usual (for purposes not requiring to be very exact) to say that the atmospheric pressure at sea level is represented by 30 inches of mercury.

The atmospheric pressure is also, to avoid using fractions, taken roughly at 15 lbs. per square inch at sea level.

Each 2 inches of mercury will, under these conditions, represent 1 lb. of pressure.

Vacuum gauges are based upon the same principles and subject to the same variations as to altitude as mercury gauges or the barometer.

To find the absolute pressure, or pressure above zero, or a perfect vacuum, we may add the pressure of the boiler steam gauge to that shown by the mercury gauge or barometer.

In Fig. 3364 the line of no pressure is marked at 15 lbs. per square inch below the atmospheric line of the diagram, the atmospheric pressure being for convenience taken as 15 lbs. above a perfect vacuum.

The line of no pressure serves as a guide in showing the effectiveness of the condenser, as well as for computing the volume of steam used, but is not necessary in computing the horse power of a non-condensing engine, because the gauge pressure has its zero marked to correspond with the atmospheric pressure.

In computing the consumption of steam or water from the diagram, therefore, both the clearance line and the line of no pressure must be marked on the diagram, and lines of the diagram extended so as to include them, thus accounting for all the steam that leaves the steam chest from the piston stroke.

Indicator springs are varied in strength to suit the pressure of steam they are to be used for.

The scale of the spring is the number of lbs. pressure per square inch represented by a vertical motion of the pencil; thus, a 40 lb. spring is one in which a pressure of steam of 40 lbs. per square inch would cause the piston to rise an inch above the atmospheric line of the diagram.

The strength or tension of the spring is so adjusted as to cause the diagram to be about 2-1/2 inches high, let the steam pressure be what it may. The following are the scales of springs of the Thompson and Tabor indicator.

THOMPSON INDICATOR.

Scale of Used for pressure above atmosphere spring. if not more than

15 lbs. 21 lbs. per square inch. 20 " 38 " " " " 30 " 94 " " " " 40 " 90 " " " " 60 " 143 " " " "

TABOR INDICATOR.

10 lbs. 14 " " " " 12 " 20 " " " " 16 " 30 " " " " 20 " 40 " " " " 24 " 48 " " " " 30 " 60 " " " " 32 " 64 " " " " 40 " 80 " " " " 48 " 96 " " " " 50 " 100 " " " " 60 " 120 " " " " 64 " 128 " " " " 80 " 160 " " " "

A spring that is strong enough for a given pressure may be used for any less pressure.

The height of the diagram will, however, be less, and accuracy is best secured by having the diagram up to the limit of about 2-1/2 inches, using a spring that is light enough to secure this result.

Diagrams of high speed engines, however, will have their lines

more regular in proportion as a stronger spring is used.

This occurs because the spring, being under more tension, is less liable to vibration.

An indicator requires careful cleaning and oiling with the best of oil, as the slightest undue friction seriously impairs the working of the instrument.

Instructions upon the care of the instrument, and how to take it apart, etc., are usually given by the makers of the indicator.

There are various methods of giving to the paper drum of the indicator a motion coincident with that of the engine piston, but few of them give correct results.

Reducing levers, such as shown in Fig. 3365, are constructed as follows:

Fig. 3365 represents a reducing lever with the indicators attached. A C is a strip of pine board three or four inches wide and about one and one-half times as long as the stroke of the engine. It is hung by a screw or small bolt to a wooden frame attached overhead. A link C one-third as long as the stroke is attached at one end to the lever, and at the other end to a stud screwed into the cross head, or to an iron clamped to the cross head by one of the nuts that adjust the gibs, or to any part of the cross head that may be conveniently used. The lever should stand in a vertical position when the piston is at the middle of the stroke. The connecting link C, when at that point, should be as far below a horizontal position as it is above it at either end of the stroke. The cords which drive the paper drums may be attached to a screw inserted in the lever near the point of suspension; but a better plan is to provide a segment, A, B, the centre of which coincides with the point of suspension, and allow the cord to pass around the circular edge. The distance from edge to centre should bear the same proportion to the length of the reducing lever as the desired length of diagram bears to the length of the stroke. On an engine having a stroke of 48 inches, the lever should be 72 inches, and the link C 16 inches in length, in which case, to obtain a diagram 4 inches long, the radius of the segment would be 6 inches. It is immaterial what the actual length of the diagram is, except as it suits the operator's fancy, but 4 inches is a length that is usually satisfactory. It may be reduced to advantage to 3 inches at very high speeds. The cords should leave the segment in a line parallel with the axis of the engine cylinder.

The pulleys over which they pass should incline from a vertical plane and point to the indicators wherever they may be located. If the indicators and the reducing lever can be placed so as to be in line with each other, the pulleys may be dispensed with and the cords carried directly from the segment to the instruments, a longer arc being provided for this purpose. The arm which holds the carrier pulleys on each indicator should be adjusted so as to point in the direction in which the cord is received.

In all arrangements of this kind the reduced motion is not mathematically exact, because the leverage is not constant at all points of the stroke.

Pantagraph motions have been devised for overcoming these defects. Two forms have been successfully used, which, if well made, well cared for, and properly handled, reproduce the motion on the reduced scale with perfect accuracy. They are shown in working position in Figs. 3366 and 3367.

Fig. 3366 represents the manner of attaching the pantagraph motion, or lazy tongs, as it is sometimes called, when the indicators are applied to the side of the cylinder. It works in a horizontal plane, the pivot end being supported by a post B erected in front of the guides, and the working end receiving motion from an iron attached to the cross head.

By adjusting the post to the proper height and at a proper distance in front of the cross head, the cords may be carried from the cord pin C to the indicators, without the intervention of carrier pulleys.

If the indicators are attached to the side of the cylinder, the simplest form of pantagraph shown in Fig. 3367 may be used. The working end A receives motion from the cross head, and the front piece B is attached to the floor. The cord pin D is fixed in line between the pivot and the working end, and the pulleys E, attached to the block C, guide the cords to the indicators.

The indicator rigging that gives the best results at high speeds is a plain reducing lever like that first described, provided at the lower end with a slot that receives a stud, screwed into the cross head. The length of the lever should be one and one-half times the engine stroke, as given on the preceding page.

Whatever plan is followed, it is desirable to avoid the use of long stretches of cord. If the motion must be carried a long distance, strips of wood may often be arranged in their place and operated with direct connections. Braided linen cord, a little in excess of one-sixteenth of an inch in diameter, is a suitable material for indicator work.

To take a diagram, a blank card is stretched smoothly upon the paper drum, the ends being held by the spring clips. The driving cord is attached and so adjusted that the motion of the drum is central.

For convenience two diagrams, one from each end of the cylinder, may be made on the same card, as shown in Fig. 3368.

TESTING THE EXPANSION CURVE.

The usual manner of testing the expansion curve of a diagram is to compare it with a curve representing Mariotte's law for the expansion of a perfect gas.

A theoretic expansion curve that will accord with Mariotte's law may be constructed on the diagram by the following method:

The diagram, as drawn by the indicator, will have the atmospheric line upon it, and from this as a basis we may mark in the line of no pressure or line of perfect vacuum.

To do this we draw, beneath the atmospheric line, a line as far beneath it as will represent the atmospheric line, on the same scale as the spring used, in the indicator, to draw the diagram.

Suppose, for example, that a 30 lb. spring was used, and assuming the atmospheric pressure to be 15 lbs. per inch, then the line of no pressure would be drawn half an inch below the atmospheric line, because 15 lbs. pull on the spring would cause it to distend half an inch.

The clearance line must then be drawn in, according to directions that have already been given.

The next thing to do is to divide the length of the diagram into any convenient number of equal parts, by vertical lines parallel to, and beginning at, the clearance line, as shown in Fig. 3369. These lines are numbered as shown, ten of them being used because that is a convenient number, but any other number would do.

We next decide at which part of the diagram its expansion curve and the test curve shall touch, and in this example we have chosen that it shall be at line 10.

We have now to find what pressure the length of line 10 represents on the scale of the indicator spring, which in this case we will suppose to be 25 lbs., the line measuring 25/30 of an inch, and a 30 lb. spring having been used to draw the diagram. Next multiply the pressure (25 lbs.) by the number of the line (10), and divide the product (250) by the number of each of the other lines in succession, and the quotient will be the pressures to be represented by the lines.

For example, for line 9 we have that 250 divided by 9 gives 27.7, hence line 9 must be long or high enough to represent a pressure of 27.7 lbs. above a perfect vacuum, or in this case 27.7/30 of an inch.

For line 8 we have that 250 divided by 8 gives 31.25 lbs., hence line 8 must be high enough to represent a pressure of 31.25 lbs. above a perfect vacuum.

The atmospheric line is, in this case, of no other service than to form a guide wherefrom to mark in the line of no pressure, or of perfect vacuum.

Now take the case of line 5, and 250 divided by 5 gives 50, hence the height of line 5 must represent a pressure above vacuum of 50 lbs.

Having carried this out for all the lines from line 10 to line 1, we draw in the true expansion curve, which will touch the tops of all the lines.

Another method of drawing this curve is shown in Fig. 3370. Having drawn the clearance line B C, and vacuum line D C, as before and chosen where the curves shall touch (as at a), then draw from a a perpendicular a A.

Draw line A B, parallel to the vacuum line, and at any convenient height above or near the top of the diagram.

From A draw A C, and from a draw a b parallel to D C, then from its intersection with A C, erect the perpendicular b c, locating on A B, the theoretical point (c) of cut-off.

From a number of points on A B (which may be located without regard to equally spacing them), such as E, F, G and H, draw lines to C, and also drop perpendicular lines, as E e, F f, G g, H h.

From the intersection of E C with b c, draw a horizontal line to e. From the intersection of F C with b c, draw a horizontal line, and so on; and where these horizontals cut the verticals (as at e, f, g, h) are points in the curve, which begins at c, and passes through e, f, g, h, to a.

But this curve does not correctly represent the expansion of steam. It would do so if the steam remained or was maintained at a uniform temperature; hence it is called the isothermal curve, or curve of same temperature. But in fact steam and all other elastic fluids fall in temperature during their expansion, and rise during compression, and this change of temperature slightly affects the pressure.

A curve in which the combined effects of volume and resulting temperatures is represented is called the adiabatic curve, or curve of no transmission; since, if no heat is transmitted to or from the fluid during change of volume, its sensible temperature will change according to a fixed ratio, which will be the same for the same fluid in all cases.

A sufficiently close approximation to the adiabatic curve to enable the non-professional engineer to form an idea of the difference between the two may be produced by the following process:

Taking a similar diagram to that used for the foregoing illustrations, as in Fig. 3371. Fix on a point A near the terminal, where the total pressure is 25 pounds. As before, this point is chosen in order that the two curves may coincide there.

Any other point might have been chosen for the point of coincidence; but a point in that vicinity is generally chosen, so that the result will show the amount of power that should be obtained from the existing terminal. This point is 3.3 inches from the clearance line, and the volume of 25 pounds 996, that is, steam of that pressure has 996 times the bulk of water.

Now if we divide the distance of A from the clearance line by 996, and multiply the quotient by each of the volumes of the other pressures indicated by similar lines, the products will be the respective lengths of the lines measured from the clearance line; the desired curve passing through their other ends. Thus, the quotient of the first or 25 lb. pressure line divided by 996 is .003313; this, multiplied by 726, the volume of 35 lbs. pressure, gives 2.4, the length of the 35 lb. pressure line; and so on for all the rest.

The application of either of the above curves will show that some diagrams are much more accurate than others, even though taken from engines of the same design and quality of workmanship.

As a general rule, those from large engines will be more correct than from small ones, and those from high more correct than from low speeds, and in either case efficiently covering the steam pipes and jacketing the cylinder, to prevent condensation, will improve the diagram.

The character of the imperfection in the expansion curve, shown by the application of a test curve, is generally too high a terminal pressure for the point of cut off, the first part of the curve being generally the most correct, and nearly all the inaccuracy appearing in the last half.

The usual explanation of this is, that the steam admitted during the live steam period condenses because of having to heat the cylinder, and that this water of condensation re-evaporates during the latter part of the stroke when this water of condensation is at a higher temperature than the expanded steam, and thus increases the pressure.

A leaky admission valve may generally, however, be looked for (or else wet steam), if the expansion curve rises much during its lower half.

TO CALCULATE THE HORSE POWER FROM A DIAGRAM.

In calculating the horse power of an engine, the only assistance given by the indicator is, that it provides a means of obtaining the average pressure of the steam throughout the piston stroke.

There are two methods of doing this, one by means of a planimeter or averaging instrument, and the other by means of lines called ordinates.

The ordinates or lines are drawn at a right angle to the atmospheric line, as shown in Fig. 3372, and each line is taken to represent the average height or length of one-half of the space between itself and the next lines.

Suppose, for example, that we require to get the area of that part of the diagram that lies between the dotted lines in the figure, and it is clear that the average height of this part of the diagram is represented by the height of the full line between them.

Any number of ordinates may be used, and the greater their number the greater the accuracy obtained. It is, however, usual to draw 10.

The end ordinates A and D, in the figure, should be only half the distance from the ends of the diagram that they are from the next ordinate, as will be seen when it is considered that the ordinate is in the middle of the space it represents.

The ordinates being drawn their lengths, are added together, and the sum so obtained is divided by the number of ordinates, which gives the average height of the ordinates.

Suppose, then, that the average height of the ordinate is two inches, and that the scale of the spring of the indicator that took the diagram was 30 lbs., then the average pressure, shown by the diagram, will be 60 lbs. per square inch. Or in other words, each inch in the height of the ordinate represents 30 lbs. pressure per square inch.

The mean effective pressure having been found, the indicated horse power (or I. H. P. as it is given in brief) is found by multiplying together the area of the piston (minus half the area of the piston rod when great accuracy is required) and the travel of the piston in feet per minute, and dividing the product by 33,000, an example having been already explained.

It is to be observed, however, that when great accuracy is required a diagram should be taken from each end of the cylinder, as the mean effective pressure at one end of the cylinder may vary considerably from that at the other.

This will be the case when a single valve is used with equal lap, because, in this case, the point of cut off will vary on one stroke as compared with the other, which occurs by reason of the angularity of the connecting rod.

When cut off valves or two admission valves are used, it may occur from improper adjustment of the valves. It occurs in all engines, because on one side of the piston the piston rod excludes the steam from the piston face, unless, indeed, the piston rod passes through both covers, in which case the rod area must be subtracted from the piston area.

If the expansion curve in a diagram from a non-condensing engine should pass below the atmospheric line, then the mean effective pressure of that part of the card that is below the atmospheric line must be subtracted from the mean effective pressure of that part that is above the atmospheric line, because the part below represents back pressure or pressure resisting the piston motion.

The planimeter affords a much quicker and more accurate method of obtaining the average steam pressure from a diagram.

Coffin's averaging instrument or planimeter is shown in Fig. 3373. The diagram is traced by the point O, and the register wheel gives the area of the diagram.

A quick method of approximating the mean effective pressure (or M. E. P. as it is called) of a diagram is to draw a line a b, in Fig. 3374, touching the expansion curve at a, and so inclined that the space e is, as near as the eye can judge, equal to the space d. Then the line f drawn in the middle of the diagram, and measured on the scale of the spring that was used to take the diagram, represents the mean effective pressure, or M. E. P. of the diagram.

CALCULATING THE AMOUNT OF STEAM OR WATER USED.

The amount of water evaporated in the boiler is not accounted for by an indicator diagram or card, and the full reasons for this are not known.

It is obvious, however, that the loss, from the steam being unduly wet or containing water held in suspension, is not shown by the diagram, and this amount of loss will vary with the conditions.

Thus the loss from this cause will be less in proportion as the point of cut off occurs earlier in the stroke, because, as the water is at the same temperature as the steam, it will, as the temperature of the steam reduces from the expansion, evaporate more during the expansion period, doing so to a greater extent in proportion as the cut off is early, on account of there being a wider variation between the temperature of the steam at the point of cut off and at the end of the stroke. On the other hand, however, in proportion as the cut off is earlier, the proportionate loss from condensation during the live steam period is greater, because a greater length of the cylinder bore is cooled during the expansion period, and it has more time to cool in.

Whatever steam is saved by the compression, from the exhaust, must be credited to the engine in calculating the water consumption from the indicator card or diagram, since it fills, or partly fills, the clearance space.

In engines which vary the point of cut off, by varying the travel of the induction or admission valve, the amount of compression is variable with the point of cut off, and increases in proportion as the live steam period diminishes; hence to find the actual water or steam consumption per horse power per hour, diagrams would require to be taken continuously from both ends of the cylinder during the hour; assuming, however, that the point of cut off remains the same, that the amount of compression is constant, that the steam is saturated, and neither wet nor superheated, steam and the water consumption may be computed from the diagram as follows:

WATER CONSUMPTION CALCULATIONS.--An engine driven by water instead of steam, at a pressure of 1 lb. per square inch, would require 859.375 lbs. per horse power per hour; the water being of such temperature and density that 1 cubic foot would weigh 62-1/2 lbs. If the mean pressure were more than 1 lb., the consumption would be proportionately less; and, if steam were used, the consumption would be as much less as the volume of steam used was greater than an equal weight of water. Hence, if we divide the number 859.375 by the mean effective pressure and by the volume of the terminal pressure, the result will be the theoretical rate of water consumption in pounds per I. H. P. per hour.

For the terminal pressure we may take the pressure at any convenient point in the expansion curve near the terminal, as at A, Fig. 3375, in which case the result found must be diminished in the proportion that the portion of stroke remaining to be made, A a, bears to the whole length of the stroke a b; and it may also be diminished by the proportion of stroke remaining to be made after the pressure at A has been reached in the compression curve at B. In other words, A B is the portion of the stroke A B, during which steam at the pressure at A is being consumed. Hence the result obtained by the above rule is multiplied by A B, and the product divided by a b.

To illustrate, suppose the mean effective pressure of the diagram to be 37.6 lbs., and the pressure at A, 25 lbs., of which the volume is 996.

Then 859.375/(37.6 × 996) = 22.94 pounds water per I. H. P. per hour, the rate that would be due to using an entire cylinder full of steam at 25 pounds pressure every stroke. But as the period of consumption is represented by B A (b a being the stroke), the following correction is required:

(22.94 × 3.03)/3.45" = 20.15; 3.03 inches being the portion B A, and 3.45 inches being the whole length b, a. This correction allows for the effects of clearance as well as compression, since, if more clearance had existed, the pressure at A would not have been reached till later in the stroke, and the consumption line B A would have been longer.

But such a rate can never be realized in practice. Under the best attainable conditions, such as about the load indicated on the diagram, or more on a large engine with steam tight valves and piston, and well protected cylinder and pipes, the unindicated loss will seldom be less than 10 per cent., and it will be increased by departure from any of the above conditions to almost any extent. It will increase at an accelerating ratio as the load is diminished, so that such calculations applied to light load diagrams would be deceptive and misleading; in fact, they have but little practical value, except when made for comparison with tests of actual consumption for the purpose of determining the amount of loss under certain given conditions.

DEFECTIVE DIAGRAMS.

In seeking the causes that may produce a defective diagram, the following points should be remembered:

The indicator must be kept in perfect order, thoroughly clean and well lubricated, so that its parts will move freely. It should always be cleaned throughout after using.

The motion of the indicator drum should be an exact copy, on a reduced scale, of that of the piston at every point in the stroke.

The steam pipes from the cylinder to the indicator, if any are used, must be large enough to give a free and full pressure of steam, and care must be taken that the water of condensation does not obstruct them or enter the indicator.

The cord should be as strong as possible, or if long, fine wire should be substituted.

The pencil should be held to the card with just sufficient force to make a fine line with a sharp pencil.

The diagram should be as long as the atmospheric line, any difference in this respect showing unequal tension of the cord, probably from unequal pressure of the pencil to the paper or card.

A fall in the steam line could arise from too small a steam pipe, and this could be tested by a diagram taken from the steam chest. It could also occur from too small a steam port or an obstructed steam passage as well as from a leaky piston.

An expansion curve that is higher than it should be may arise from a leaky valve, letting in steam after the cut off had occurred, or if at the later point of expansion curve, it may be caused by the steam being wet or containing water, which evaporates as the temperature falls from the expansion.

An expansion curve that is lower than it should be may be caused by a leaky piston, by a valve that leaks on the exhaust side but not on the steam side, or if the exhaust valve is separate from the steam valve, it may leak while the steam valve is tight.

It may also be caused by the cylinder being unduly cooled, as from water accumulating in a steam jacket.

There are many defects in the adjustment of the valve gear, or of improper proportion in the parts, that may be clearly shown by a diagram, while there are defects which might exist and that would not be shown on the diagram.

It is possible, for example, that a steam valve and the engine piston may both leak to the same amount, and as a result the expansion curve may appear correct and not show the leak.

Insufficient valve lead would be shown by the piston moving a certain portion of its stroke before the steam line attained its greatest height in Fig. 3376, in which from A upwards, the admission line, instead of rising vertically, is at an angle to the right, showing that the piston had moved a certain portion of its stroke before full pressure of steam was admitted.

That too small a steam port or steam pipe did not cause this defect may be known from the following reasoning:

The port opened when the pencil was at A, which shows that the valve had lead. At this time the piston was near the dead centre and moving slower than it was when the pressure reached its highest point on the diagram, and since the steam line is fairly parallel with the atmospheric line, it shows that the port was large enough to maintain the pressure when the piston was travelling fast, and therefore ample when the piston was moving slow.

The remedy in this case is to set the eccentric back.

With less compression the point A would be lower.

Excessive lead is shown in Fig. 3377 by the loop at A, where the compression curve extends up to the steam line, and the lead carries the admission line above it, because of the piston moving against the incoming steam.

To mark in the theoretical compression curve, the vacuum line and the clearance line must be drawn in as in the figure, and ordinates must be drawn.

According to the diagram, in Fig. 3377, the compression is clearly defined to have begun at C, and at that time the space filled by steam is represented by the distance from C to the clearance line. The pressure above vacuum (or total pressure) of the steam in the cylinder when the compression began is represented by the length or height of the dotted line 1.

Now suppose the piston to have moved from the point C, where compression began, to line 2 (which is midway between line 1 and the clearance line), and as the compressed steam occupies one-half the space it did when the piston was at C, therefore the steam pressure will be doubled, and line 2 may be drawn making it twice as high as line 1.

Line 2 is now the starting point for getting the next ordinate, and line 3, must be marked midway between line 2 and the clearance line, and twice as high as line 2, because at line 3 the steam will occupy half the space it did at line 2. Line 4 is obviously midway between line 3 and the clearance line.

Through the tops of these lines we may draw the theoretical compression curve, which is shown dotted in.

To find the amount of steam actually saved by the compression, we have to consider the compression curve only, beginning at the point of the diagram where it is considered that the compression actually began, and ending where the compression line joins the admission line, and the horizontal distance between these two points represents the length of the cylinder bore filled by the compression.

To find the average amount to which the steam is compressed, we must draw within this length of the diagram, and within the boundaries of the compression curve, and the line of no pressure ordinates corresponds to those given for finding the average shown pressure of a diagram, as explained with reference to that subject, taking care to have the end ordinates spaced half as wide as the intermediate ones, as explained with reference to Fig. 3372.

Spotted a typo or a problem in this chapter? Tell us

XLI. AUTOMATIC CUT OFF ENGINES

An automatic cut off engine is one in which the valve gear is so acted upon by the governor as to keep the speed of the engine uniform under variations of the load the engine drives, and notwithstanding variations in the boiler pressure. This it accomplishes by varying the point in the piston stroke at which the live steam is cut off. This is economical because it enables the engine to use the steam more expansively than is possible with engines having throttling governors, which govern the engine speed by wire drawing the steam.

There are two principal forms of automatic cut off engines, first, those in which the steam valve spindle or rod is released from the parts that move it to open for admission, while dash pots, weights, or springs close the valve to effect the cut off; and second, those in which the travel of the valve is varied so as to alter the point of cut off.

The first usually employ fly ball governors which actuate cams or stops to trip the valves for the steam cylinders. The second usually employ wheel governors or speed regulators, as they are sometimes termed.

The distinctive features in the action of the first, of which the Corliss engine is the most important, is that as two admission and two exhaust valves are used, therefore the amount of the valve lead, the point of exhaust and amount of the compression remain the same at whatever point in the piston stroke the cut off may occur; whereas in the second, the lead increases, the cut off occurs earlier, and the compression increases in proportion as the cut off occurs earlier in the piston stroke. In this class of engine the steam valve travels as quickly when opening the steam port for a short and early period of cut off as it does for a late one, hence the amount of steam port opening is as full, with reference to the piston motion, for an early as it is for a late point of cut off. In other words, there is the same amount of steam port opening for the first, second, third, and fourth inch of piston motion, let the point of cut off occur at whatever point in the piston motion it may. In engines which vary the point of cut off by reducing the travel of the slide valve, this is accomplished by using double ported valves or griddle valves.

Fig. 3378 represents the arrangement of the valves in a Corliss engine, V and V^{1} being the steam valves and V^{2} and V^{3} the exhaust valves. These valves, it will be seen, extend crossways of the cylinder and are circular. In the figure the valves are shown in the position they would occupy when the piston was at the crank end of the cylinder, as in the figure.

The principles of a Corliss valve gear will be understood from the following, which is derived from a book by the author of this work, and entitled Modern Steam Engines.

In 3379 and 3380 the valve gear (which is the distinctive feature of the engine) is represented with the parts in the position they occupy when the cut off occurs at half stroke, the piston having moved from the head end of the cylinder. In Figs. 3381 and 3382 the parts are shown in position with the crank on the dead centre and the piston at the crank end of the cylinder, valve v having opened its port to the amount of the lead.

Referring to Fig. 3379, motion from the eccentric is imparted by the rod M to the wrist plate Y, to which are connected the rods C, C´, for operating the admission valves, whose shafts are seen at S, S´, and the rods F, F´, for operating the exhaust valves, whose shafts are seen at T, T´.

The mechanism for the steam or admission valves may be divided into three elements: first, that for operating the valve to open the port for admission; second, that for closing the valve to effect the cut off; and third, that which determines the point in the stroke at which the cut off shall occur.

The first consists of the rod M, wrist plate Y, and the rods C and C´, which operate the bell cranks r r, r´ r´ which are fast on the valve shafts S, S´. Upon the ends of bell cranks r r, r´ r´, are pivoted latch links u, u´, which have in them a recess for the latch blocks, of which one is seen at e (the rod R´ and its connection with the valve stem being shown broken away to expose e to view). During the admission the latch block abuts against the end y of the recess w and is tripped therefrom by the cam n´. The ends of arms g of the latch links abut against the hub of the arms d, d´ upon which are cams n, n´, and at a, a´ are springs for keeping the ends g of latch links u, u´ against the hubs and cams of d, d´.

Referring now to the valve mechanism at the head end only, suppose the piston to be at the head end of the cylinder, and latch block e will be seated in the recess provided in a to receive it, and as the bell crank moves, the latch block will be raised by the latch link, which is carried by a crank arm corresponding to that seen at x at the crank end of the cylinder, and as this crank arm is fast upon the valve spindle, the lifting of e will open the valve for admission. As soon, however, as the end g of the latch link meets the cam n´, the latch link will be moved so that the end y of its recess will leave contact with the latch block e and the dash pot will cause rod R´ to descend instantaneously and close the valve, thus effecting the cut off.

The period of admission, therefore, is determined by the amount of motion the latch link u´ is permitted to have before its end g meets the cam n´, which trips the latch link, and therefore frees e from the latch link recess.

The point at which the cut off will occur, therefore, is determined by the position of the cam n´, because if n´ is out of the way, the end g of the latch link will not meet it, the latch link will not disengage from the latch block e, and the cut off would be effected by the lap of the valve, and independently of the dash pot. As in Fig. 3379 the parts are shown in the positions they occupy at the instant the cut off is to occur, therefore the cam n´ has just tripped the latch link, and the end of e has just left contact with the end y of the recess w in the latch link u´.

The point in the stroke at which the tripping of u´ from e will occur and effect the cut off is determined by the governor, because d´ is connected to the governor through the rod G´. In proportion as the governor balls rise, d´ is moved from left to right, and the end of cam n´ meets g earlier, or, vice versa, in proportion as the governor balls fall, the arm d´ is moved to the left, g will meet the end of cam n´ later, and the point of cut off will be prolonged.

We now come to the means employed to close the valve quickly and without shock when the latch block is released from the latch link. Referring then to the crank end of the cylinder, the latch block for that valve is carried upon arm x, to which is attached the rod R from the dash pot piston (the arm corresponding to x, but at the head end being shown removed to expose the latch block to view). We may now turn again to the head end of the cylinder, rod R´ corresponding to rod R at the other end, and it is seen that R´ connects to a dash pot piston p´ having a stepped diameter, the lower half fitting into bore H´, and the upper half fitting into a bore H. The piston p´ fits the bore H´ and fills it when the rod R´ is at the bottom of the stroke, hence as p´ is raised there is a vacuum in H that acts to cause p´, and therefore R´ and x, to fall quickly and close the valve the instant the latch block is released from the latch link. To prevent the descent of rod R´ and piston p´ from ending in a blow, a cushion of air is given in H by the following construction:

At S and S´ are valves, threaded to screw and unscrew, the ends forming a valve for a seat entering H.

As the rod R´ and its piston p´ descend, the air in H finds exit through a hole at h until that hole is closed by the piston p´ covering it, after which the remaining air in H can only find exit through the opening left by the end of the valve S´, and this amount of opening is so regulated by the adjustment of S´ that a certain amount of air cushion is given, which prevents p´ from coming to rest with a blow. The head of valve S´ is milled or knurled, and a spring t´ fits, at its end, into the milled indentation, thus holding it in its adjusted position. The under surface of the upper part of p´ is covered by a leather disc, while the part that fits in H´ is kept air-tight by a leather-cupped packing.

The connection of the cam arms d and d´ with the governor is shown in Figs. 3381 and 3382, in which the parts are shown in the position they would occupy when the crank is on the dead centre and the piston at the crank end of the cylinder. The rod G´ connects the cam arm d´ with the upper end of lever A, which is connected to the governor and vibrates on its centre as the governor acts upon it.

Now suppose the speed to begin to diminish, and the governor balls to fall, and the direction in which A will move will be for its lower end to move to the right, thus moving d to the right and carrying its cam away from the end of the latch link, which will therefore continue to open the port for a longer period of admission. Or, referring to Fig. 3381, it is plain that, if the governor balls were to lower from a reduced governor speed, G´ would move to the left and cam n´ would be moved away from contact with the end g of the catch link, which, not being tripped, the admission would continue. On the other hand, suppose the governor balls to rise from an increase of governor speed, and d´ (Fig. 3379) would be moved to the right, and the cam n´ meeting g earlier, correspondingly hastening the cut off.

The governor is driven by a belt from a pulley on the crank shaft to the pulley W, Fig. 3381, whose shaft conveys motion to the governor spindle through the medium of a pair of bevel pinions in which v represents (referring again to Fig. 3378) the steam or admission valve for the crank end port, and v^{1} that for the head end port, while v^{2} is the exhaust valve for the crank end, and v^{2} that for the head end of the cylinder. All four valves are shown in the positions they would occupy when the crank was on the dead centre and the piston at the crank end of the cylinder, hence the valve positions shown correspond to the positions the parts of the valve motion occupy in Fig. 3381.

The faces of the valves are obviously arcs of circles of which the axes of the shafts s, s´ are the respective centres. Valve v has opened its port to the amount of the lead, which in this class of engine varies usually from 1/32 to about 1/16 inch. As separate exhaust valves are employed, the point of release, and (as the same valve edge that effects the release also effects the compression) therefore that of the compression, may be regulated at will by adjusting the lengths of the rods F, F´, Fig. 3379, which have at one end a right and at the other a left hand screw, so that by turning back the check nuts and then revolving the rods their lengths will be altered.

Similarly the amount of admission lead may be adjusted by an adjustment of the lengths of rods C, C´, which also have right and left hand screws. Referring now to the admission valve v, it is seen that its operating rod C is at a right angle to bell crank r, r, hence the amount of valve motion will not be diminished to any appreciable extent by reason of the wrist plate end of rod C moving in an arc of a circle, and the point of attachment of rod C to the wrist plate is such that, during the admission, the valve practically gives as quick an opening as though rod C continued at a right angle to r. But, if we turn to valve v´, which has closed its port and covers it to the amount of the lap, we find that bell crank r´ and its operating rod C´ are in such positions with relation to the wrist plate, that the motion of the latter will have but little effect in moving the bell crank r´. This is an especial feature of the Corliss valve motion and is of importance for the following reasons:

The lap of the valve (which corresponds to the lap of a plain D slide valve) is usually, in this class of engine, such as to cut off the steam at about 7/8 stroke, but the adjustment of the cam position is usually so made that, from the action of the governor, the latest point of cut off will occur when the piston has made 5/8 of its stroke, the range of cut off being from this to an admission equal to the amount of the lead.

As the eccentric is fixed upon the shaft, the speed at which the valve opens the port for the admission is the same for all corresponding piston positions. Thus suppose the piston has moved an inch from the end of the stroke, and the valve speed will be the same, whether the cut off in that stroke is to occur at quarter stroke or half stroke, and as the valve continues to open the port until it is tripped, therefore, at the moment it is tripped, the direction of valve motion must be suddenly reversed.

As the duty of its reversal falls upon the dash pot, it is desirable to make this duty as light as possible, which is accomplished by the wrist motion, which acts to reduce the valve motion after the port is opened a certain amount for the admission.

We have, therefore, that during the earlier part of the admission, the port opening is quick because of the eccentric throw being a maximum, while during the later part of the port opening, this rapid motion is offset or modified by the wrist motion, thus lessening the duty of the dash pot and enabling it to promptly close the valve.

The range of governor action, so far as the governor itself is concerned, is obviously a constant amount, because a certain amount of rise and fall of the governor balls will move the cams a given amount. But the range of cut off may be varied as follows: At Z, Z´, are adjustment nuts, by means of which the lengths of rods G, G´ may be varied.

Lengthening rod G obviously moves arm d and its cam n further from the end of the latch link u, and therefore prolongs the admission period.

Shortening the rod G´ causes cam n´ to move around and away from the leg g of the latch link, and prolongs the admission.

The adjustment of the lengths of G and G´ may therefore be employed for two purposes; first, to prolong the point of cut off, and maintain the speed when the engine is overloaded, or to hasten the point of cut off for a given engine speed, and thus adjust the engine for a lighter load.

HIGH SPEED AUTOMATIC CUT OFF ENGINES.

What are termed high speed engines are those whose pistons run at a velocity of more than about 600 feet per minute, some making as high as 800 or 900 feet in regular work. High speed engines are usually provided with an automatic cut off, and a majority of them vary the point of cut off, by means of shifting the eccentric across the shaft, so as to reduce the eccentric throw, and therefore the valve travel. This causes the valve to cut off the steam earlier.

The eccentric, instead of being fixed upon the crank shaft, has an elongated bore, and is hung on an arm that is pivoted at its other end after the manner of a pendulum. This arm is called the eccentric hanger.

A wheel governor is usually employed to shift the eccentric across the shaft. In some cases, however, two valves are employed, one effecting the admission, the release, and the compression, and the other the cut off.

When two valves are employed, the lead, the point of cut off, the point of release, and the point of compression may be maintained equal for all points of cut off; whereas, when a single valve is employed, the lead, the point of release, and the compression will vary with the point of cut off, or, in other words, will be different for every different point of cut off.

The general principles upon which a wheel governor is constructed is, that two weights or weighted levers in moving outwards from the engine shaft, from the action of centrifugal force, move or rather shift the eccentric across the shaft, reducing its throw, and therefore by reducing the travel of the valve hasten the point of cut off and reduce the power of the engine.

In the governor of the Buckeye engine, the centrifugal force may be varied by increasing or diminishing the distance of the weights from the pivots of the arms on which they swing.

This is shown in Fig. 3382_a_, in which it is seen that the weights A are adjustable along the arms a, a. The points of attachment d, d of the springs to the weight arms are also adjustable.

When reversing is done, by shifting the eccentric across the shaft, the lead cannot be kept equal, but will, if the eccentric is swung from a pivot that is on the line of centres, when the crank is on a dead centre, be greater at the head end than at the crank end of the cylinder. The discrepancy may, however, be equalized by swinging the eccentric from a pivot that is not on the line of centres at a time the crank is on a dead centre.

But this equalization will only exist at some one point in the eccentric position, or in other words, if the eccentric is shifted across the crank shaft, simply to reverse the engine, and not to vary the point of cut-off, it will naturally be moved, in reversing the engine across the shaft, to a given and constant amount, and in this case, the pivot on which its hanger is hung may be so located with reference to the line of centres and the crank (the latter being on a dead centre when the point of suspension of the eccentric hanger is found) that the lead is equal for both the backward and forward gears.

But if the eccentric is shifted across the shaft to vary the point of cut off as well as to reverse the direction of engine revolution, the lead cannot be kept equal.

It is better, in this case, to so locate the point of eccentric hanger suspension as to let the lead be the most at the head end cylinder port, because the piston travels fastest at that end of the cylinder, and therefore requires more lead, in order to cushion the piston.

A construction for shifting the eccentric across the shaft is shown in Fig. 3383, in which D, D is a disc, having at b a pivot for the eccentric hanger. The amount the throw line of the eccentric must be shifted to reverse from full gear forwards to full gear backwards is from the line b x to line b x´, and the shifting is done by two racks F and J, having teeth at an angle of 45° to their lengths. F is fast to the eccentric, and J is carried in a sleeve that slides along the shaft, and sliding it moves the eccentric across the shaft by reason of the teeth of one rack being at a right angle to those of the other.

It is obvious that the eccentric may be moved around the shaft in place of across it, the distance its throw line requires to be moved being the same in either case.

To shift an eccentric so as to reverse the direction of engine revolution, all that is necessary is to place the crank on either dead centre and measure the amount of valve lead. Then loosen the eccentric from the crank shaft, and while the crank is stationary, move it around upon the shaft until it has opened the port full, and nearly closed it again, leaving it open to the same amount as it was before the eccentric was moved, or in other words, open to the amount of the lead.

Fig. 3384 represents a side elevation of a high speed wheel governor engine, designed and constructed by the Straight Line Engine Company of Syracuse, New York, the construction of the governor being shown in Fig. 3385, in which R is the eccentric rod, the eccentric being carried in a lever strap pivoted at A, and connected at B to two links C and D, the former of which connects to the spring E, and the latter to the weighted lever F. The centrifugal force generated by the weighted end of F endeavors to move the eccentric inwards, and thus reduce its throw, which reduces the valve travel and hastens the point of cut off.

On the other hand, the tension of the spring E acts to move the eccentric in the opposite direction, and maintain the full throw of the eccentric and maximum point of cut off. These two forces are so calculated in the design and proportion of the parts that under a maximum load the engine will run at its proper speed, while, if the load decreases, the action of F will hasten the point of cut off enough to allow for the decreased engine load, and thus keep the engine still going at the same speed.

Other novel and interesting details in the construction of this engine are as follows:

The two arms forming the frame are cast with and run in straight lines from the cylinder to the two main bearings, and rest upon these self-adjusting points of support.

There are two fly wheels, both between the main bearings, and one of which carries the governor so that the centre of the valve is brought in line with the centre of the eccentric.

In order to simplify the explanation, the mechanism has been separated into three separate sections. Figs. 3386 and 3387 show such of the details of the parts between the cylinder and crank as are peculiar to this engine. The cross head is of the slipper guide style, and the illustration, Fig. 3386, shows the simple method adopted for adjusting the guide to the proper height to maintain the alignment. Another feature peculiar to the straight line not mentioned above, that of making the cross head pin fast in the connecting rod, is used in this engine also, but in a somewhat different form. As will be seen by Fig. 3387, the pin is made much larger, and this allows of its being made of "steel casting" and cast hollow with cross bars at each end for centring. These pins are held in the rod by a binding screw which catches in a groove that is milled around one-fourth of its circumference. After the pin is placed in the rod and the binding bolt is inserted, the pin is prevented from working out endwise, and the binding bolt prevents it from turning; but when the binding bolt is slackened, the pin can be rotated one-fourth of a revolution. The scheme is as follows: After running the engine for a while, the engineer is instructed to slack the binding bolt, give the pin a quarter turn and bind it fast. By repeating this, the pin can be kept more nearly round, probably, than by any other plan. By referring again to Fig. 3386, it will be seen that the plan for taking up the wear in the cross head pin bearings is simply that of setting up the common half box, and the endurance of the arrangement, with the hardened and ground steel pin running in babbitt lined boxes of double the ordinary size and length, must be satisfactory.

The drop oil cups for lubricating the cross head pin are located so as to have the drop "picked" off just as the cross head completes its stroke at the cylinder end, and while it is travelling at its slowest speed. The oil, as it leaves the wearing surfaces of the pin, is conveyed to the lower slide.

Figs. 3388 and 3389 show the parts that connect the eccentric with the valve. The method of connecting the rod to the eccentric strap is convenient. The lower joint in the eccentric strap is set up tight, metal to metal, and the upper joint left open 1/8 of an inch.

STEAM FIRE ENGINE.

In a steam fire engine the prime requisites are rapidity of getting up steam and efficiency with lightness, economy of fuel being a secondary consideration.

Fig. 3390 is a general view of a steam fire engine constructed by the Clapp & Jones Manufacturing Company.

Fig. 3390_a_ is a longitudinal section through the boiler and one steam cylinder and pump.

The construction of the boiler is shown in Figs. 3390_a_ and 3391, the former being a vertical section of the engine and boiler bearing the steam pipe and exhaust pipe shown in place, and one of the draught tubes shown in section, and the latter a vertical central section.

The outside shell is represented at a´´, a´´. This shell extends the whole length of the boiler. The fire box sheet b´´, b is less in length, extending only to the lower tube sheet.

The lower tube sheet C´´ is perforated by all the tubes; the heavy lines showing the coil tubes in fire box, the others are smoke tubes. The upper tube sheet d has holes only for the smoke tubes. The smoke or draught tubes are shown at e´´, e´´, e´´; these also answer the important purposes of drying and superheating the steam.

F´´, F´´, F´´ are the sectional coil tubes, the main feature of this boiler. They are in the form of a spiral coil, the spiral bend being enough to leave room for five others of the same size between, so that there are six of these coils in each circular row. The number of rows is determined by the size of the boiler and the amount of steam required.

Each coil is connected with the lower tube sheet by screw joints, all right hand, that require no fibrous or elastic packing, an angle elbow being used to get the short bend at the end. The tubes then make about one turn around the fire box, and are joined to the side sheet of the same, with the same union used at its upper end, which makes a joint that never gets loose from any kind of work it may be subjected to. These unions or couplings are made of different kinds of metal, and put together so that no two pieces of iron come in contact to corrode and stick together; and should it, from any cause whatever, become necessary to take these coils out, it can be done, and the same tubes replaced without destroying any part of them, or damaging any piece so that it could not be used again.

G´´, G´´ is the ornamental dome or covering for the upper end; g´´, g´´ is the smoke bonnet and pipes for concentrating the hot escaping products of combustion for the purpose of making a draught of air through the fuel. H´´ are grate bars, and I´´ fire door. J´´, J´´ is the water line. The height has been determined by experiment, yet should be varied a little to get the best drying effect of the coal. A coal that makes a flame would call for a higher range of the water line, while coal that produces heat without the flame would call for a lower range; this the engineer will soon find. The working of the boiler is as follows: The fire being started in the fire box, as soon as the water in the coils begins to heat circulation commences from natural causes (nor is it at any time necessary to use a hand pump or any other artificial means for keeping it up), the heated water passing up in the steam drum, and the colder water from the leg and drum taking its place, as is shown by the arrows in the leg, till the whole is heated to the steam making temperature. At this point steam pressure begins to show, which goes up very fast, as the water is all so near the steam temperature. Of course, it is better to carry the water at about the height shown, as a uniform pressure of steam is easier maintained, which is always desirable; yet the limit of safety is not reached till the water is nearly all out, or so long as it is not below the connection of the coils in the leg; and even at this point the only danger is in the damage to the coils from the heat when there is no water to protect them.

In Fig. 3391_a_, one engine and pump is shown in side elevation, and the other in section, the cranks being at a right angle, one to the other. A yoke from the piston rod spans the crank, so that the steam and pump pistons are in line and directly connected. From the lower end of this yoke, a rod connects to the crank shaft upon which are the two fly wheels and the eccentrics for the steam valves.

It will be seen in the longitudinal section, Fig. 3390_a_, that the steam valve face is a segment of a circle and therefore answers, so far as the distribution of the steam is concerned, to a simple D slide valve, which exhausts through the pipes m, p. The steam pipe n enters the bottom of the steam chest at n´.

The two main pumps a are made in one piece, entirely of composition; one of them is shown in section. The piston is a solid piece of brass, as well as the cylinder in which it works, but are made of different composition, one hard, the other soft, to prevent cutting. The valves are of India rubber; the discharge valve is a ring, one for each end of the pump, as shown at b, Fig. 3391_a_. One is shown open, while the other is closed. They are held in place by grooved rings of brass; these rings fit in grooves in the rubber, which, when they are put in the pump, and their set screws are in, with their points in the grooves in the brass rings spoken of above, the discharge valves are complete for work.

The suction valves are shown at K on Fig. 3391_a_, and will be easily understood. They are of a design for this special use and place, which is around the pump cylinder in a circular chamber. The water ways covered by these valves are long and narrow, one valve covering two of these openings, they being held in place by two studs that go through the centre part of the valve, a wire going through these studs, and close to the back of the valve which keeps it up to the seat, the only spring to either of these valves being the elasticity of the rubber. The opening and connection D, D is the inlet to the pump, and where the suction hose goes on, there being a pipe or chamber with branches for the two air chambers, and at each end is a discharge gate and a connection for the leading hose. The part d is the feed pump for the boiler supply, e is the air chamber on the pipe that leads to the boiler to ease off the shocks caused by the plunger striking the water, when the pump does not fill. It is drawn broken off to show the upper part of the pump barrel and stuffing box. The pipe f is the feed water pipe from the pump to boiler, shown from different points in Figs. 3390_a_ and 3391_a_. g is what we call the suction pipe to the feed pump. It connects to the main pump in the discharge part of it.

A piece of hose pipe connects to the boiler at a point just above the water line, so that hot water or steam (according to the height of the water in the boiler) may be applied to any part that may have become frozen.

Heaters are almost universally used in connection with steam fire engines to keep the water hot, and in many cases to keep a few pounds pressure to shorten the time of going to work should the fire be close at hand. This boiler has an advantage for this kind of heating; the circulation is so perfect and free that all the water in it is heated alike; so when the fire is lighted the steam starts immediately up, instead of having to wait till some cold water has been heated that had not been reached by the very limited circulation in them, there being some parts that the circulation produced by the heater does not reach, leaving, of course, this water cold.

The arrows K´´ (Fig. 3391), show the direction of the circulation when working with fire in the fire box; those marked L´´ show the direction of it when on the heater which is directly opposite.

The outside pipe connected at about the water line is the outlet from the heater, and the inlet to the boiler, which carries the heated water over the crown sheet, where, as it gets cooler, it enters the coils, descends into the leg, and from there to the pipe near the bottom of the boiler; this pipe leads to the heater, so that the water is kept moving just in proportion to the heat given it; any kind of a heater can be used with the same result.

Spotted a typo or a problem in this chapter? Tell us