Practical application of the indicator with reference to the adjustment of valve gear on all styles of engines

Dedication

This work is dedicated, with the highest regard, to my friend the working engineer, with the hope that it may in some manner benefit and assist him in his endeavors to rise to a higher standard of excellence in his calling.

The Author.

January ist, 1894.

Preface

While engaged in my work as consulting engineer, it has been my fortune to come in contact with a large number of working engineers; some more or less experienced in the use of the Indicator, others knowing practically nothing concerning it. It was gratifying, however, to note that whenever called upon to use my Indicators, I invariably found an interested spectator in the engineer, who, in most cases, expressed a desire to learn their use. In some cases, where desired I have given personal instructions, but in many cases the engineer could not afford such an expensive method of learning. To these, I recommended various works on the subject, but found the very general complaint that most of the works on the Indicator now published are not sufficiently definite for the beginner. These circumstances, together with the fact that I have personally felt the want of more practical information on the subject than is contained in existing books, have, to a great extent, induced me to prepare this volume; and it has been my aim to cover the ground as thoroughly as possible, and produce a work which will meet the requirements of the beginner as well as the experienced engineer. As so many of our engineers have not had the benefit of even a common school education, I have endeavored to use only such wording as would be readily understood by all, and avoid all intricate formulas which tend to confuse

6 # Preface.

and mislead the reader. The importance of the Indicator is now so generally recognized that the time is rapidly approaching when no engineer will be considered competent unless he is proficient in the use of the Indicator; and if this effort on my part shall have made the subject plainer to some, or in any degree assisted those who desire to better their condition, I shall feel amply repaid for the time and labor expended in the preparation of this work.

Lewis M. Ellison.

Chicago, Jan. i, 1894.

Introduction

The steam engine indicator is an attachment to the steam engine, which has been too long neglected. In fact, only a few years have elapsed since it was practically unknown to the average working engineer. However, questions were continually coming up which called for a frequent resort to its use, until now, in the hands of a skillful engineer, it is a very important factor, in the running of the steam plant. The first indicator, I believe, was invented by James Watt and was a very crude affair, adapted for use only on engines of low pressure and running at slow speeds. It would be wholly unfit for use on the high pressure and high speed engines of the present day. Meantime, the changes in the styles of engines necessitated changes and improvements, and the makers of indicators have endeavored to keep pace with the times; and there is no doubt that the American indicators of •to-day, for simplicity, accuracy and durability, far surpasses any like devices in the world. The benefits derived from the use of the indicator becoming more and more apparent, have greatly increased the demand, and recently there have been put upon the market several new makes and styles which are duly appreciated and which show the enterprise and mechanical skill of the maker. Nevertheless, we regret that they have not aimed to produce a more reliable and accurate instrument rather than one where relia-

8 Introduction .

bility is sacrificed to price. While some of the older makes of indicators are reasonably correct, the majority of the recent makes are practically worthless when accuracy is desired and few of them will give two readings alike. It is to be hoped that the day will soon pass when such instruments will be recognized by intelligent engineers. I wish to state here that I have no connection or interest, financial or otherwise, with any indicator or attachment, steam engine or steam appliance, and wherever comparisons are made they are used simply as facts for the benefit of the readers and without any preference or prejudice on my part. With few exceptions the diagrams used in this work were taken by myself in actual practice and are engraved as nearly as possible as taken, full size and contain all the peculiarities of the original diagram. The demonstrations are put in the simplest language, so that all can fully understand them without reference to the higher mathematics, and some of them have never before appeared in print and will no doubt prove interesting.

PRACTICAL

APPLICATION OF THE INDICATOR.

Chapter I. The Indicator

The indicator is an instrument for registering on paper the varying steam pressure on the piston of an engine during both the forward and return stroke. It consists, essentially, of a small steam cylinder, having on its axis a small drum upon which the paper is rolled. This drum revolves backward and foiward by a motion derived from the crosshead of the engine. The cylinder is provided with a piston whose motion is resisted by a spiral spring. Steam is admitted beneath this piston, and as it is at a greater or less pressure, so is the spring more or less compressed and the piston rises and falls accordingly. The motion of the piston is conveyed by a series of levers to a pencil which is pressed against the paper on the drum. Before using the indicator clean the bearing surfaces of the cylinder and piston and lubricate them with the best cylinder oil that is clean and does not stick or gum. Use refined porpoise jaw oil on all the other bearings. On tests of long duration, the piston should be taken out and together with the cylinder should be cleaned and oiled frequently as necessary. The central bearing upon which the drum rotates, is subject to much wear and should be frequently cleaned and oiled. After using the instrument, thoroughly clean and oil it, being especially particular in regard to the springs as a very slight rust impairs their accuracy. It is of importance that an indicator be kept in good order and to determine that it is properly adjusted and constructed, the following is a good method. With the indicator in a vertical position and without the spring (with piston attached) ; raise the pencil motion to its highest position and when released it should drop with freedom from friction to its lowest position. The piston, cylinder and oil must be warm, otherwise the piston is apt to stick . As a further test, again raise the pencil motion to its highest position and cover the hole through which steam is admitted to the indicator with the thumb when the pencil should sink at a slow and uniform rate through the whole range of its motion. Should there be a suspicion of a catch from any position within this range, detach the piston from the pencil motion and try the pencil motion and piston separately. If both are in good order the fault lies where the piston is attached to the pencil motion. A piston which is too free may occasion serious defects from leakage, especially in condensing engines. The pencil motion should be parallel to the drum surface and the drum be perfectly round and properly centered. By adjusting the pencil stop so that only a ray of light can be seen between the pencil point and the drum surface, the piston is moved upward (with a

Practical Application of the Indicator.

lead pencil) and the pencil point should be carefully watched to see that it keeps the same distance from the drum surface. With the pencil, both in the highest and lowest position, rotate the drum through its whole range by drawing out the cord and again watch the pencil point. Then apply the paper and allow the pencil to touch the paper and repeat the operation by forming two horizontal lines and one vertical line.

Practical Application of the Indicator.

If the reduction is correct, the lines drawn will be straight throughout th,eir whole length and at right angles to each other which may be proven with the angles as shown in Fig. I. Those made of celluloid are preferred. To determine that all joints are without play, insert a spring of high tension and carefully feel at the end of the pencil bar. In case two indicators are used they should be either both right or both left hand.

PAPER.

The paper used with the indicator should be of good quality, having a smooth surface and capable of receiving an impression with the lightest possible pressure of the pencil against the paper. Paper having a glossy surface should be avoided. The paper should reach to the top of the drum and about one inch longer than the circumference of the drum. Printed blanks containing spaces for recording various observations for future references are preferable. Those used by the writer are prepared for the particular test on which they are to be used. Those shown in Fig. 2, are shown one-half size and arranged for a

Lewis M. Ellison, Consulting Engineer, 25 W. Lake St., Chicago.

DIAGRAM FROM ENGINE AT

OIAM OF CYLINDER

OIAM OF PISTON BOD

DIAM OF STEAM PIPE

OIAM OF EXHAUST PIPE.

. PER-CENT.CF CLEARANCE,

. STROKE IN INCHES

. REV.PER MIN.

. SCALE OF SPRINQ

BOILER PRESSURE.- INITIAL PRESSUaE._

BACK.PRESSURE.

MEAN EFFECTIVE PRESSURE..

. VAC.PER GAUGE, INCHES

. BAROMETEft.REAUS

.TEMP.HOT WELL

INDICATE DMORSE POWEK_ u, a D o

Sill

Fro 2.

drum two inches in diameter, will usually be sufficient for the engineer. Remember, that without all necessary data the diagram is worthless except as an indication of the valve setting. In getting the correct data in making indicator tests remember the golden rule is, never take anyone's word for anything. But note these observations yourself which must be undertaken with a degree of precision, as many a man's evidence in court has been disallowed for want of sufficient and reliable data.

The paper should be placed so that the clips hold it on the drum as smooth as a glove. To accomplish this, close to the clips, with the thumb and forefinger pinch the ends of the paper together. Never attempt this operation by pulling on the ends of the paper, as it springs the clips outward which in turn sets the paper away from the drum.

PENCIL.

Use hard lead and of good quality such as usually accompanies the indicator. It should be sharpened with fine sandpaper to a smooth point, as fine lines are preferable for their distinction. The lead should project through the socket as little as possible, so as to avoid undue twisting of the pencil bar. The pressure of the pencil on the paper should be just sufficient to make a legible mark, a greater pressure only creates undue friction. The friction of the pencil tends to lessen oscillations, but at the same time it falsifies the diagram. A more satisfactory result can be obtained by using a metallic point in place of the pencil and it, like the lead, should be as light as possible, for, its weight acting at the end of the lever where the velocity of motion is at its maximum will tend to increase oscillations. It should be sharpened to a fine point, rounded off on the end so as not to tear the paper while at the same time make a fine but distinct line. The metallic cards are chemically prepared for the metallic points.

SPRINGS.

In order to obtain a correct diagram, the height of the pencil (caused by the compression of the spring) must exactly represent in pounds pressure per square inch the pressure on the piston of the engine at any point of the stroke and the velocity of the drum must at all times bear a constant ratio to that of the piston.

In selecting a spring to suit different conditions, as to steam pressure, speed, etc., the aim should be to obtain a diagram about two inches high on slow speed engines and about one and one-fourth inches or less on high speed. This is accomplished by using a spring that is numbered one-half the highest pressure. Thus, if a 40 spring is used, forty pounds pressure per square inch will raise the pencil one inch, and eighty pounds two inches and so on for other pressures. The spring is the measuring factor of the indicator and the most vital part of the instrument. Indicator springs give to-day, as a rule, more power on an engine than they would give if the springs were correct. For tests of importance the scale of deflection should be verified before and after each test. The rule for giving the maximum pressure to which each spring is subjected varies with different makes of indicators; therefore it is not given here.

Chapter II. Application Of The Indicator."

The application of the indicator to an engine is a scientific experiment and should be undertaken with a degree of care and precaution against error, as the value of the diagram depends on its correctness. The value of the indicator is now so generally recognized that every concern which pretends to manufacture an engine makes provision for its application. When no provision for its application has been made, the cylinder must be drilled and tapped for not less than one-half inch pipe thread, in the side where the pantograph reducing motion is used, but usually on top for high speed and in such position in the clearance space that the holes are not the least covered over by the piston at the end of its travels. Should the clearance space be too small to allow for this, access may be had by chipping a channel in the cylinder head from the tapped hole out into the clearance space. In drilling, the piston should be taken out and the cylinder cleaned from chips and cuttings. The indicator should be connected as directly as possible to the cylinder. For the reason that it requires a difference in the pressure in order that the steam can flow from the engine cylinder into the indicator, thus causing a loss of pressure in the steam line and an increase of pressure in the expansion curve and with long connections, such as the use of side pipes connected at the middle with a three-way cock as shown in Fig. 3, or still worse a T with an angle valve at each end which is one

Fig. 3.

of the most objectionable forms, the results will show up later in the indicator than when it took place inside of the steam engine cylinder.

The cock should be screwed into the cylinder itself. When the cylinder is tapped upon the side this will bring the instruments horizontal. Where nipples and elbows are used to bring the indicator into a vertical position, the ends of the nipples must be reamed out, as the burr made in cutting it if not reamed wire-draws the steam. Use no leads or paints in connecting, it is not necessary and they are liable to get into the instrument. When, however, a single indicator is to be used upon both ends of the cylinder, and where engines are constantly changing their load, the three-way cock is sometimes used to equalize the work between the two ends of the cylinder. The most proper arrangement is to have an instrument upon each end of the cylinder from which simultaneous diagrams may be taken.

Practical Application of the Indicator.

REDUCING MOTION.

Whenever the stroke of the engine is greater than the length of the diagram desired, some means must be provided to reproduce upon a reduced scale a perfect duplicate of the piston movement, many devices in use fail to do so, and the diagrams taken with them are incorrect. The pantograph, as shown in Fig. 4, is a correct reducing motion, and various lengths of

Fig. 4.

diagrams may be had by changing the position of the cross-bar D, moving it toward the end pivot A for longer diagrams, and toward the end pivot B for shorter ones. The cord pin C must be in line with the end pivots A and B. The cord pin is grooved for the application of the cord, but its diameter is usually so great that it tends to throw the cord. To prevent this it should be regrooved to a diameter of about one-eighth of an inch. The joints can readily be tightened when they are too loose, by upsetting the rivet heads. When not in use, the whole apparatus should be kept in a bath of oil which at the same time lubricates the joints. The arrangement for sup-

Fig. 5.

porting and attaching the pantograph differs as the ingenuity of the operator may suggest.

The simple device as shown in Fig. 5 is constructed of half-inch nipples and fittings. A is screwed into the side of the crosshead, the oil cup being removed and placed in the tee. The length of A should be about four inches so as to prevent the corners, E. E. of the pantograph from coming in con-

Practical Application of the Indicator.

tact with the crosshead. The height of B should be about four inches so as to bring the pantograph to the

Fig. 6.

proper level. B can be turned at any angle, as shown in Fig. 6, to suit various sizes of engines. A wooden

&~ 4" ■$-

X

Fig. 7.

plug is driven into C and bored out to receive the end pivot A of the pantograph.

On crossheads similar in style to Fig. 7, A can be made of round iron, flattened and slotted near the

Practical Application of the Indicator.

end as shown in Fig. 8 when it will fit any bolt and can be clamped fast to one of the bolts uf the crosshead. Fig. 9 shows the arrangement for supporting the end pivot B of the pantograph. This device is constructed of about 4 feet of | inch gas pipe which can be made up with couplings so that it can be disconnected into parts, thereby rendering it convenient in moving from place to place. The clamps A and B are made of tough wood. A is about 1^ inch thick x 1 } 2 wide x 6 inches long and slotted at the end, and with a bolt can be made fast any place on the pipe so as to get the correct height for the pantograph. A hole is made near the other end for the end pivot B of the pantograph. The floor plate is made fast to the floor with screws. One brace is parallel with the engine and made fast to the floor and the short clamp B and another brace at right angles made fast near the top

Practical Application of the Indicator.

with a similar clamp or into a wooden plug which is driven into the top of the pipe while the other end of the brace is usually fastened to the wall or over on to the engine frame. It is immaterial whether the stationary end of the pantograph be placed in the center of motion or not so far as correctness goes and makes no difference whether the pantograph is set horizon-

Fig. 9.

tally, perpendicularly or oblique so long as the corners E. E. of the pantograph will clear.

Some writers claim that the stationary end of the pantograph must be precisely in the center of motion, which is a mistaken idea, and has caused many engineers to measure with precaution against error to locate this point correctly. Owing to the complicated construction of the pantograph, it cannot be used on high speed engines, and is frequently replaced by a

Practical Application of the Indicator.

light wooden pendulum pivoted to a fixed support usually -a | inch gas pipe screwed into the engine guide as shown in Fig. 10 and fitted at its lowest extremity with a pin which is carried by a slot made fast to the crosshead as shown in Fig. n. The length of the pendulum should be about one and a half times the stroke of the engine. The cord is looped or hooked on a pin which must be in the center line of the pendulum and on a point on the pendulum to correspond with the length of diagram, and the cord must

Practical Application of the Indicator.

be precisely at right angles to the pendulum when it is in the middle of the stroke. The point of support must be precisely in the center of motion and must not vary to the edge of a knife blade otherwise it will distort the diagram and the results will appear unequal if they really are equal.

When a link is used in connection with the pendulum as shown in Fig. 12, the link should be about one-half the stroke. The pendulum should be arranged so that the link will incline as much below the path of the crosshead when in the middle of the stroke as it inclines above the path when at the end of its travel. The fixed support of the pendulum will then be at a point as far from the support of the slotted pendulum as the distance from A to B. Frequently provisions are made near one end on the guide for making the support fast, and in order to have the same inclination, the relative length of the pendulum and link must be such that at midstroke the pendulum is perpendicular, when it is a horizontal engine, and at right angles to the plane in which the crosshead moves.

While both of the motions last mentioned are frequently used neither of them are correct. The use of a segment of a pulley, sometimes called the Brumbo pulley, introduces rather than eliminates an error. The correctness in the reduction of any reducing motion can readily be determined by placing the engine so that the crosshead is at either end of its travel with reducing motion and indicator properly attached. First, draw a horizontal line by pulling out the cord and without the spring make a vertical mark upon the card by raising the pencil and with the engine in this position make a mark on the crosshead and guide. Space off as many marks on the guide as the length of stroke is in inches, move the engine so that the mark on the crosshead coincides with each mark on the guide, and at each movement make a corresponding mark on the card. If the lines are equally spaced, as shown in Fig. 13, the principle of the reduction is correct, while its practicability when at full speed may be radically distorted. If so, it is

Practical Application of the Indicator.

due to several causes, but principally vibration of tne reducing motion or indicator. Frequently cord stretching or throw of the indicator drum will cause a distortion of the diagram which can be tested by first taking the atmospheric line while the engine is running slow, then move the card upward about 1-16 of an inch and take another line when the engine is at full speed. If there is a difference in the length of the lines, distortion is generally due to one or more of these causes.

Chapter III. Cord

The movement of the paper drum is derived from a closely braided hemp or metallic cord, all of which are more or less elastic. The result of the elasticity "in the hemp cord" is that at the end of the stroke, the cord having to overcome the throw of the drum, the tension of the drum spring stretches the cord more or less at the commencement of the stroke. The drum loses a part of its movement at the commencement of the stroke, but gradually regains its curtailment at the end of the stroke by giving the drum a throw, thereby affecting a motion proportionate to that of the piston movement. The result of the elasticity in the metallic cord is that the drum retains its proportional movement to the end of the stroke when the throw of the drum produces an elongation of the diagram. The hemp cord is therefore the most efficient to eliminate this distortion When any great length has to be used, it is well to partly substitute metallic wire for hemp cord. The braided cord is supplied by dealers in the instruments; but before using, it should remain suspended for at least one day with a heavy weight at its lowest end to take out any undue stretch which it may contain, after which it should be treated with a light coat of beeswax, when it will not be easily affected by moisture and will longer retain its proper condition. The cord should be looped over the cord-pin on the reducing motion which permits the pin to turn easily within it, and should be hooked and unhooked at that point. Nearly every engineer at first expects a difficulty in hooking and unhooking the cord on high speed engines, which is merely supposition.

The writer has taken diagrams on some of the highest speed engines built and never experienced the slightest difficulty. When a hook is used it must be light and hooked upon the cord-pin itself, or as near the instrument as possible. The device frequently used for adjusting the length of the loop and cord, and usually made of a small piece of stiff mefalin which are drilled four holes for the cord to pass through, thereby forming the loop, is a device about as worthless as can be made, and when it is on the middle of the line, together with the heavy hook now so common in use, it is in its most objectionable form, and will cause the cord to whip. The length of the diagram on slow speed engines should be about four inches and about two inches high, as large diagrams will show up the defects to greater advantage. The length of diagram desired can easily be determined by taking hold of the cord in one hand and allow it to pass over a rule held in the other hand with the engine running slow. When a hook and loop are used, the length of the cord is adjusted so that the hook clears the loop about one inch at the end of the stroke, and since a drum two inches in diameter unwinds the cord about six inches, this will give a clearance of one inch to the drum at each end of the stroke, when the diagram will come in the center of the card. The proper tension of the drum spring is of importance. If too little tension it will cause the drum to throw past at the end of the stroke when the cord will be constantly whipping, and if the tension is too great the drum loses a part of its stroke, a matter which can be easily tested. With the engine running slow and with a tension on the drum spring, so that it neither causes the cord to stretch nor the drum to throw, take the atmospheric line, move the card up about I- 1 6 of an inch, put the engine up to speed and readjust the drum spring to the supposed proper tension and take another atmospheric line. If the last line is the longest the tension of the drum spring is too little, which caused the drum to throw, and if shorter the tension is too great.

It is desirable to avoid the use of leading pulleys as shown in Fig. 12; however, when it becomes necessary in order to lead the cord off parallel with the guides, a leading pulley must be used and from this the cord may be carried in any direction. For high speed engines the length of the diagram should be about .2 to 2\ inches and from 1 to ij inches high and it is desirable to use a lighter and smaller drum on high speed engines.

HOW TO ATTACH THE INDICATOR AND TAKE THE DIAGRAMS.

Before attaching the indicators, steam should be blown through the cocks so as to remove any grit which may be in the connections. Fig. 14 shows the indicators properly attached to a Corliss engine, for taking steam pipe and cylinder diagrams, with the

Practical Application of the Indicator.

SO Practical Application of the Indicator.

pantograph reducing motion and its attachments complete. Before taking a diagram, steam should be admitted into the indicators during several revolutions of the engine and when dry steam blows through the relief, apply the pencil during one revolution, but where the load is constantly changing and when the power is to be computed from the diagrams, the pencil should be applied during as many revolutions as is necessary in order to obtain both the lightest and heaviest load, from which the average horse power may be ascertained. After taking the diagrams, steam should be shut off from the indicators and the atmospheric line should be taken immediately, before the spring contracts, as the indicator piston rod and spring expand when in contact with steam and contract when steam is shut off, due to the difference in temperature. The result is that the expansion of the piston rod raises the pencil and the expansion of the spring lowers it, and when steam is shut off, the contraction of the piston rod and spring affects the pencil vice versa. Now if the expansion and contraction of the rod and spring were the same, one would balance the other, but they are not the same, consequently, the atmospheric line should be taken immediately after the diagram, so that the conditions in both rod and spring may be as nearly the same as possible. After unhooking the line, let the indicator drum revolve gently back against the stop.