The valve-setter's guide; a treatise on the construction and adjustment of the principal valve gearings used on American locomotive

Contents

3hapter page

Preface 7

I The Steam Engine 9

Preface

It would be difficult indeed to write anything particularly new on a subject that has been so much commented on as the valve gearings of steam engines ; but it will be readily admitted that things that are already familiar

to us may be seen in a new light. It will also be observed that in this age of rapid changes even the steam engine is found to be capable of improvement. The recent application of the Walschaerts valve gear to American locomotives, and also the more recent invention of the Baker-Pilliod valve gear has given a new interest to the subject. It is to be hoped the result will be that a greater degree of reliability in the harnessing of steam in locomotive service will be accomplished. It is with the hope of adding something to the general mass of information, especially with a view of being helpful to young railway men, that these articles have been written.

Other valve gearings claim attention. Many others besides those described in this little book are in successful operation. Some still in embryo are full of promise. In selecting those that are the most popular forms of locomotive valve gearings in use in America, it is not with the intention of ignoring or belittling other ingenious devices, but with the assurance that a fair knowledge of the organic structure and methods of adjustment of valve gearings commonly in use will readily help the thoughtful mechanic to a more easily acquired knowledge of the mechanism of any valve gearing, no matter in what shape the parts may appear.

Not only so, but as the valve gearing is the most intricate part of the reversible steam engine, so the study of valve gearing is calculated to develop the faculty of giving thoughtful attention to the details of all forms of involved mechanism.

A word may be added in regard to the brevity with which the subject has been treated. In an experience of many years among apprentice machinists and young railway men, I never found it advantageous to pour a mass of tedious information into the ears of listeners however willing they might be to hear. In fact, the more anxious they were to learn the less they needed to be told. A hint in the right direction was generally enough. Many exceptions arise in practice which can hardly be foreseen in imagination. The intelligent mechanic or engineer learns much that does not appear in print. Practical experience unfolds the mysteries of all arts. The object of a book of this kind should be to help the beginner to help himself. In the hope of aiding in kindling this noble aspiration these brief articles were written for the pages of Railway and Locomotive Engineering and met with much popular favor, and are now collected in more convenient form in the belief that they may be of further service to railway men. J. K.

New York, Jan. 1, 1910.

THE STEAM ENGINE

I. THE STEAM ENGINE

Apart from the many varieties of form in which steam engines are made, there are two general divisons into which they may be classified, those of the condensing and non-condensing engines. The locomotive and nearly all the factory engines are of the non-condensing type. In this class the steam after it has done its work in moving the piston, is exhausted into the open air. An engine of this kind is readily discerned by the puffing sound of the exhausted steam, each re-percussion indicating the completion of the piston stroke, and sudden release of the pent-up steam, which it may be observed has still much more than sufficient force left in it to overcome the pressure of the atmosphere. In a condensing engine the steam after passing through the cylinder is admitted into a box or receptacle called a condenser on account of its being cooled by contact with water or with pipes through which cold water is passing. The condensation of the exhausted steam has the effect of producing a partial vacuum in the condenser, which permits the steam being used until it has reached a lower pressure than in a non-condensing engine. Marine engines are mostly of this type, as a constant supply of cold water can readily be obtained.

It will be remembered that the atmospheric engine previous to James Watt's invention of the steam engine, was operated by the admission of steam into a vertical cylinder, while the piston was at the upper end of the stroke and when a jet of cold water was admitted into the cylinder, the steam was condensed, thereby producing a vacuum and the weight of the atmosphere acting on the uncovered upper face of the piston was sufficient to press down the piston. Watt conceived the idea of condensing the steam in a separate vessel, thereby effecting a great saving and also made the important advance in applying steam alternately to both sides of the piston, thereby creating or inventing an entirely different engine from that of Newcomen.

The rapid condensation of steam into water renders suitable openings into all steam cylinders a necessity. These openings, or cylinder cocks, as they are called, are usually operated by a hand lever which is placed in the open position when starting the engine. This should be carefully attended to, as water unavoidably gathers in the cylinders while engines are not running, that is if a pressure of steam is in the boiler.

The most important openings into the cylinder, however, are the ports by which the steam is admitted and that bv which it escapes after its work is accomplished. This leads us to the slide valve, the most common form of valve used in steam engines, and although simple in construction it is a most ingenious contrivance for admitting and cutting off the supply of steam at the desired instant of time. The cylinder is so constructed as to form a flat portion on a part of its outer surface on which there are three rectangular openings, the middle being the exhaust port, is wider than the other two, and leads directly from the cylinder face to an opening on which the exhaust pipe may be bolted. The two other ports are the steam ports, one leading to one end of the cylinder and the other to the other end. The slide valve is so constructed as to cover these three openings and is shaped like a hollow rectangular dish with an inner cavity, the edges of the dish forming the face of the valve. This face is carefully fitted so that when the valve is placed in position it forms, in conjunction with the surface of the cylinder, a steam-tight joint. It may be added that in the early days of the steam engine much care was taken in fitting the valve face and valve seat together so that every part of the metals were bearing equally on each other. It was latterly found that the friction incident to the movement of the valve very quickly caused the valve to adjust itself to the valve seat so that a fine bearing at the start is not essential, the bearing being perfected in the course of a few hours' service.

The slide valve, when in the central position, covers the two steam ports and leaves the middle port open in the inner cavity of the valve. If the valve was so constructed as to cover the two steam ports exactly it can be readily perceived that a movement of the valve in either direction would open one of the steam ports leading to the cylinder and open the other steam port to the inner cavity of the valve, thereby opening the communication to the exhaust pipe. A valve so constructed would admit steam during the entire length of the piston stroke, so that when the piston stroke was completed the steam in the cylinder would be at or near

FIG. i. SLIDE VALVE WITHOUT LAP.

boiler pressure and when released by the opening of the exhaust port, as already alluded to, the loss of steam at each stroke would be equal to the full capacity of the cylinder and steam spaces. In its primitive form the slide valve was constructed in this way and was used on some of the earlier engines of James Watt. Fig. I shows such a valve in the central position. This form of valve had several serious objections. The smallest degree of lost motion had the effect of admitting the steam in a sufficient quantity to obstruct the piston at the end of the stroke. The tendency to create severe pounding was

THE STEAM ENGINE very great. The most important drawback was in the fact, already alluded to, that the steam was released while at full pressure, and no advantage could be taken of the expansive power of steam. During Watt's lifetime the pressure of steam as applied to steam engines was extremely low, so that the loss was not so very great as it became when, with the improvement in boiler construction, higher pressures of steam became available. • In the early days of the locomotive fifty pounds pressure of steam per square inch did not admit of much advantage being taken of the expansion of steam in the cylinder. The change of the form of the slide valve was very simple, but very important. It consisted merely of lengthening the valve face, so that when the valve stood in the center of the seat the edges of the valve extended a certain distance over the steam ports, as shown in Fig. 2.

This extension of the valve faces is called outside lap, or simply lap. It has the effect of closing the steam port at a certain distance before the piston reaches the end of the stroke, and this point, which with the variations incident to the action of the radial link is a variable point, usually called the point of cut-off. It can be readily understood that when the supply of steam is cut off from the boiler before the piston stroke is com-

FIGu 2. VALVE WITH OUTSIDE LAP.

pleted, the piston is moved on its further course by the expansive power of steam, and when the stroke is completed the steam in the cylinder having increased in volume is correspondingly reduced in pressure and when released the loss is not great, as the pressure of the steam is not greatly in excess of the pressure of the atmosphere.

II

II. THE SLIDE VALVE

Having described the construction of the slide valve and its relation to the steam ports, and pointed out the advantage derived from the face of the valve being made more than sufficient to cover the steam ports, it remains to be stated that the opening and shutting of the steam ports by the movement of the valves is of the utmost importance in the running of the steam engine. It will be readily understood that when the piston is at one or other of the extreme ends of the cylinder and it is intended that it should be moved in the other direction, the steam port should be rapidly opened so that the steam may exert its full pressure on the face of the . piston. Many eminent engineers held the opinion that the opening of the port should occur in advance of the arrival of the piston at the end of the stroke. This is what is known as the lead of the valve. It has been the subject of much discussion and is still an open question. At first sight it would appear to be a hindrance to the piston to meet with a pressure of steam working adversely to the direction in which the piston was moving, but when it is borne in mind that even steam at high pressure, moving at a high velocity, requires some small instant of time to move through the intervening space between the steam chest and cylinder, and also that the piston itself may be moving at a high speed, it is easy to understand that it may be an advantage to begin to open the valve slightly in advance of the reversing of the movement of the piston.

This is more readily apparent in the case of engines running at high speeds than otherwise, because the velocity with which the steam moves through a certain space at a certain pressure may be safely called a constant velocity, while the rate of speed at which the valve or the piston travels varies according to the kind of work accomplished. This fact has led to the claim that the increase in the amount of lead in the valve which is caused by the shortening of the travel of the valve in the case of engines equipped with the Stephenson valve gear is an advantage to locomotives running at high speeds when the reverse lever is "hooked up." The action of the steam in striking with sudden force at the beginning of the piston stroke may be likened to what is known as "advancing the spark" in gasoline engines, which has the effect of increasing the velocity of the engine partly from the more complete compression of the inflammable gases, and partly from the fact that the gasoline engine piston receives the full shock of the explosion

THE SLIDE VALVE before it has moved any considerable distance from the end of the cylinder.

The parallel between the action of steam pressure and gasoline explosion, however, is not an exact one. The variation is very great, steam being a constant pressure and gasoline explosion an intermittent shock. Increasing the amount of lead in the valve of a steam engine does not by any means increase the speed of the engine. It has the contrary effect, and it is a growing fact that the idea which obtained in many quarters that a locomotive could not run freely unless

SLIDE VALVE SHOWING LEAD.

the valves were set with considerable lead is rapidly losing its adherents. This fallacy may readily be dispelled by marking the degree of smoothness with which a well-balanced locomotive will run when an opportunity occurs to shut off steam altogether. The alleged shock which a certain amount of lead of the valve is said to overcome is not apparent, and there is undoubtedly a growing opinion that the older methods of having a valve opening of one-eighth of an inch is gradually losing ground, and this amount after being cut down to one-sixteenth is now reaching the thirty-second mark, and if the present trend of thought continues the so-called lead of the valve will have become a minus quantity.

It may be added that the idea that the increase in the amount of lead of the valve caused by the shortening of the valve travel, in the case of the Stephenson valve gearing, was an advantage, has lost credence largely from the fact that since the rapid introduction of the Walschaerts valve gearing, with which many of the new and larger locomotives are equipped, it has been clearly demonstrated that the constant lead maintained by the latter valve gearing at varying amounts of valve travel and speed, is a decided advantage in its favor. Indeed, many experienced engineers do not hesitate to assert that the excessive amount of lead formerly allowed in locomotive practice, and the large increase in that amount occasioned by the shortening of the valve travel, was the chief cause of much of the troubles incident to the involved mechanism of the Stephenson valve gearing. Be this as it may, the opinion is rapidly taking firm root that the amount of valve opening at the end of the piston stroke should be kept at the lowest possible amount, and that better results in locomotive and engine running generally could be obtained by having the valve closed at the end of the piston stroke than by having an opening wide enough to admit sufficient steam that cannot do other than counteract the motion of the piston as it approaches the dead center.

THE VALVE-SETTER'S GUIDE.

III. THE LINK MOTION

A peculiarly important quality of the steam engine is the fact that it lends itself with ease to perfect control. That motion of such amazing force can be suddenly stopped and reversed within a brief ments are simple enough, and if taken into consideration one by one can be readily understood by the average student in a short time.

The invention of the link motion has

U Valvt 8tm

BockerSha/t a* Central Lint of Motion % period of time is one of the most wonderful results of applied science.

The most common form of reversing gear is what is known as the link motion. The combination of levers and rods and eccentrics and radial link have an involved air of mystery to the young engineer, but a little attention will show that the movebeen the subject of much controversy, but it is now universally conceded that Mr. William Williams, a young Welchman, a draughtsman employed in the Stephenson Locomotive Works at Newcastle, England, submitted the first sketch showing the idea of a movable link suspended from the arm of a movable shaft and equipped

THE LINK MOTION with means of being attached to two eccentrics, the movable link being attached to the valve rod, effecting the reversing of the engine and also a variation in the valve travel and consequent variation in the expansive use of steam. It was a masterly invention and the unconscious young inventor, to whom the thought had come as an inspiration, had evidently no comprehension of the momentous importance of the device. It may be said to explained his device to some self-seeking shopmates who were not slow to take advantage of his ingenuousness. Claims of improvements were made by others, but the fact is that Mr. Williams' finished invention was incapable of any improvement. The device was not patented. The Stephensons began applying it to their locomotives in 1842. It came into immediate favor and has been the most popular form of valve gearing used on

&)■ i l *co. fry.

REPRESENTATION OF THE ANGUS SINCLAIR CO. VALVE MODEL WITH D-SLIDE

VALVE.

have completed Watt's wonderful engine. At one stroke the harnessing of the titanic force of steam was completed. Engines could henceforth be run backward or forward. Variable expansion, variable speed, with all the delicate varieties in force and velocity, were at once accomplished.

It is a matter of regret that so little is known of the gifted mechanician. With the frankness of youth he seems to have locomotives ever since. In the absence of any legal claim to the invention the device became generally known as the Stephenson valve gearing or shifting link, the latter name being used in contradistinction to link motions where the link oscillates on a fixed center as in the Walschaerts valve gearing.

In the excellent model before us, a duplicate of which can be procured from the

IS publishers of Railway and Locomotive Engineering, and a drawing of which is reproduced for illustration, it will be noted that the piston is in the middle of the cylinder. Both steam ports are closed, and if it is desired that the engine should be moved forward it will be necessary that the steam be admitted at the front so that the piston should be pressed backwards and so move the wheels forward on the rails. In order to admit the steam to the front end of the cylinder the valve must be moved backward. It will be readily noted that by moving the lever forward on the quadrant the radial link is dropped downward until the upper eccentric rod is in line with the lower end of the rocker. To accommodate itself to this movement the lower end of the rocker has been pushed some distance forward, the upper end of the rocker consequently moving an equal distance backward and carrying with it the slide valve. This has the effect of opening the steam port leading to the front end of the cylinder and the pressure of the steam is admitted to the front face of the piston.

If, on the other hand, it is desired to move the engine backwards, it will be found that by moving the lever to the back of the quadrant, the link is raised until the lower eccentric is in line with the lower end of the rocker. To admit of this movement the lower end of the rocker is drawn back with the consequent result that the upper end of the rocker has been moved a corresponding distance forward, thereby opening the steam port leading to the back of the cylinder and so admitting the steam to the back of the movable piston, which moving forward has the effect of moving the wheels backward upon the rails.

There are several important factors rendering these simple movements possible and wTiich, if carefully observed, will readily impress themselves upon the memory of the attentive student. The first is that with the rocker in perpendicular position, the valve rod should be of the exact length so that the valve should be in the central positon in regard to the steam ports. Presuming that the piston has moved forward in the cylinder until it has reached the extreme end of the stroke, it is necessary that the valve should be beginning to open the front port in order that the steam may be admitted to the front face of the piston for the purpose of moving the piston backward. At this particular point it is well to turn our attention to the position of the eccentrics and their relation to the driving crank. It will be understood that with the piston at the front end of the cylinder the main crank pin is at the forward center. If the valve was so constructed as to cover the steam ports exactly, the two eccentrics should be standing with their extreme points at right angles to the main crank pin, the forward eccentric with its extreme point being on the top or above the crank, and the eccentric controlling the backward motion on the bottom or below the crank.

As previously described, however, the valve in modern use is so constructed that the extreme ends of the valve when in a

THE LINK MOTION central position overlap the steam ports. In the modern locomotives this amount of lap extends to three-quarters of an inch, and if to this is added some amount of valve opening it will be readily understood that the eccentrics must be moved sufficiently from the position at right angles to the crank to a position nearer to the crank by an amount equal to the lap and lead combined. The effect of this movement toward the crank on the part of the eccentric has the effect of pushing the lower end of the rocker toward the cylinder and consequently drawing the upper end of the rocker with the valve rod backward a sufficient distance to overcome the lap of the valve.

It need hardly be stated that the sliding valve alluded to is the common D-shaped valve, but the operations of the outside admission piston valve are identical with the action of the valve described. In the case of an inside admission piston valve a change of the position of the eccentrics in their relation to the main crank is necessary, as it will be readily seen that the edge of the valve at the point of admission is at the opposite side of the port and the valve also must necessarily travel in the opposition direction to that of the outside admission valve. The relative merits of the various forms of valves are not pertinent to a general discussion of the link motion.

It will be observed that the radial link is simply a contrivance for the purpose of detaching one eccentric and placing the other in operation, besides which it has the advantage of varying the amount of travel of the valve, thereby allowing a supply of steam to be admitted for a long cr short space of the piston stroke. This is accomplished by moving the lever toward the center of the quadrant and it will be easily understood that as the eccentric rod in operation is moved some distance from the end of the rocker it has the effect of shortening the movement of the rocker arm and consequently shortening the stroke of the valve. Such are the chief features of the link motion and with these clearly in mind, other less important details may be readily mastered by the thoughtful student.

IV. ADJUSTING THE STEPHENSON VALVE GEAR.

The importance of correctly adjusting and maintaining the position of the valves cannot be overestimated. Like the valves in the hearts of animals their perfect operation is the very life of the moving force of the machine. The work of adjusting the mechanism has been absurdly overestimated, and the idea assumed by many that the operation is a difficult one is a gross error. Like other alleged mysteries that owe their importance to the lack of opportunity of witnessing their true inwardness, familiarity soon begets a spirit of common regard. There is just the same degree of accuracy necessary in adjusting the attachments of the throttle valve. Both are a simple matter of careful experiment and exact measurement. In adjusting the ordinary sliding valve it may be said at the outset that there are degrees of proficiency required from the designing of the parts of the gearing to the simple matter of squaring the eccentric rods in a brief roundhouse examination. The working machinist is very safe to assume that the designer's part is correct and he can safely proceed with the adjustment of the valve gear, which is generally among the last operations in the building or repairing of a locomotive.

Assuming that the four eccentrics are loose upon the driving axle of an ordinary 8-wheel locomotive, the best plan is to move the engine until one of the crank pins on the axle to which the eccentrics are placed is on the forward center, that is the point nearest to the cylinder. We will assume this is on the right side. If the eccentric which is designed to move the engine in a forward direction is intended to be next the frame, the eccentric may be promptly put in place somewhere about 85 degs. above the line of the crank pin, the exact position need not be ascertained at this time, the eccentric being temporarily held in place by one or more set screws. The "eccentric intended for the backward motion is placed a simlar distance below the crank pin or center line of the axle. The eccentrics on the other side of the engine can be similarly attached temporarily in their places. Assuming that the links have been already attached to the rockers and the link hangers adjusted to the arms of the lifting shaft, the forward eccentric rod should then be adjusted to the top of the link and the back eccentric rod to the bottom of the link. We are now ready to begin the operation of correctly adjusting the entire gearing, and it may be stated that inasmuch as the preparatory markings of centers and openings are of prime importance, it is proper that the locomotive should be levelled. It does not follow that because the engine is new or newly repaired it is exactly level. The tendency is to drop slightly in front, which is generally rectified when the water is placed in the boiler. When the engine is level the rocker should be placed exactly plumb. This can readily be done by moving the reverse lever while a fine line is hung over the rocker end with two small weights attached and the lever stopped at the exact point where the two lines are equidistant from the center of the rocker. The valve rod should then be adjusted so that the valve is exactly in the middle of the valve seat. A neglect of this important point will cause variations in the valve travel that can never be entirely overcome. At this time it should be noted whether the valve yoke is properly adjusted to suit the thickness of the valve, as in the case of an engine being frequently repaired the valves and valve faces wear rapidly and a liner may be necessary under the valve yoke to prevent undue strain upon the valve rod packing as well as a tendency to tilt the valve.

Before finally attaching the valve rod to the rocker, the best method of marking the exact positon of the valve openings is by a tram reaching from the guide yoke or other fixed part of the engine to a suitable position on the valve rod. Both ends of the tram are sharpened, one end to adjust itself to a center punch mark on the guide yoke, and the other end, which is bent so as to scratch a mark readily on the valve rod. The points marked are necessarily where the valve is beginning to open at the front and back steam ports. The marks on the valve rod should be marked lightly with a fine center punch. The steam chest lid can then be put on the steam chest and any danger of material or articles falling into the steam chest can be entirely avoided.

The next operation should be the ascertaining of the exact dead centers or positions when the crosshead and piston are standing still at each end of the piston stroke. This point should be located very carefully. If the rim of the main driving wheel is near the guide yoke the same tram may suit to locate the center, but generally a longer tram is necessary with both ends bent. The engine may then be moved to a point where the crosshead is about half an inch from the back or forward end of the stroke. A fine center punch mark may be made on the crosshead and guide yoke or guide block as the case may be, and a pair of compasses carefully adjusted to the two points. Accompanying this adjustment, the tram should be extended from the guide yoke or other point to the rim of the wheel and an arc drawn by the sharp end of the tram. The engine should then be moved over the center until the crosshead returns to the exact point where the compasses touch the center marks on the crosshead and guide yoke or block. Another arc or line should be drawn on the wheel rim and a point between those two arc lines is the dead center. It will be noted by those who have time to try the experiment that it is difficult to obtain this center mark in a positively correct positon. A repetition of the experiment with the crosshead at a greater or lesser distance from the end of the stroke will show a slight variation. This arises from the fact that while the crosshead is moving very slowly near this point the rim of the wheel is moving rapidly, and it may be added that the valve is also moving rapidly at this time. Hence the utmost care should be taken in ascertaining the dead centers. It is often noted that in looking over the valves, it may be only a few days after the engine has been running, that the exact center, as newly discovered, may show a considerable variation in the rim of the wheel. This arises from the lost motion already caused by the incidental wear of the engine bearings, so that old markings on wheel rims instead of being of any value are often misleading. Variations in the dead center marks will also show in marking while the engine is moving forward as compared with markings that are made while the engine is running backward.