Zwicker's Revised Practical Instructor
Q. What is an eccentric?
A. An eccentric is a subterfuge for a crank; it is anything out of center.
Q. How would you find the throw or stroke of an
A. By measuring the heavy and the light side; the difference between the two is the stroke or throw.
Q. What throw should a common slide valve
engine eccentric have?
A. Generally double the width of the entry or steam ports.
Q. If you changed the size of the eccentric
would it alter the throw of the valve?
A. No, it would not, but if you changed the position of the eccentric on the shaft it would.
Q. What is a cam?
A. A cam has no definite meaning; it has 1, 2, 3 or 4 motions; they are used on puppet valve engines, such as are in use on high pressure river steamboats.
Q. How would you measure your valve and
A. By placing the crank-pin at its dead-center, the center of the eccentric straight or plumb above the center of the shaft, the rocker-arm perpendicular, and the valve covering both ports equally; then take a tram and measure from the center of the eccentric to the center of the pin where the eccentric rod hooks on (generally the lower pin) for the eccentric rod, and from the outside center of the pin where the valve rod is attached to the furthermost end of the valve, allowing for two nuts at each end of the valve, called adjusting and jamb nuts.
Q. How would you plumb an eccentric?
A. By dropping two plumb lines, one at each side of the shaft, and half the space between the two lines will be where the center of the eccentric should stand, with heavy side up.
Q. What kind of a tool would you use to find
the exact center?
A. A pair of hermaphrodite calipers, one leg of which has a sharp point and the other leg has a short foot, so as to feel the line.
Q. What does an eccentric rod consist of?
A. An eccentric rod consists of a strap, yoke, rod and two nuts; when taking the measure, couple the yoke and strap together, then put a half-inch thick piece of wood between the two straps and find the center of the circle from four sides, with a pair of hermaphrodite calipers, then put the rod in the yoke and adjust it to the proper length by the two nuts; if that will not do, the rod must be shortened or lengthened, by cutting out or adding a piece, whichever the case may be. Then take the measure with a tram from the center of the straps to the center of the rod where the rod hooks on lower rocker-arm pin.
Q. How long is the thread on a valve-rod?
A. Long enough to allow two nuts at each end of the valve, and space for adjustment.
Q. Now, if your rocker-arm stood at a quarter,
and your eccentric out of plumb, how would you take the measure for
A. Simply bring them plumb and take the measure; that is the only right way.
Q. After you have measured the rods, what would
A. They should be put on and the valve set.
Q. What do you move or do first, to set a valve
after connections are made?
A. Move the eccentric in the direction the engine is to run, until the valve begins to take steam or lead, then tighten the eccentric temporarily with set screws, then move the crank-pin over to the other dead center, and see how much lead it has; if equal the valve is set.
Q. What is meant by the lead of valve?
A. The opening the valve has when the piston is at the beginning of its stroke.
Q. What lead should large engine have?
A. About 1/16 of an inch. High speed engines must have a quick opening or good lead.
Q. Now if you find the valve laps out ? of an
inch on one end, and the proper lead on the other, what would you
A. Divide the difference, by moving the valve one-half it is out, by adjusting the valve-gear.
Q. How much?
A. The valve has 1/16 of an inch lead at one end and laps ? of an inch at the other end; the valve is out 7/16 of an inch; then the valve must be adjusted by the nuts one-half it is out, making 7/32 of an inch. then throw the crank on the other dead center, move the eccentric whichever way will bring you back to 1/16 of an inch lead, then tighten temporarily with set screws, throw crank over the other dead center, and the valve will be set. After valve is set, tighten the eccentric for good.
Q. But if it is not set, what would you
A. Go through the same performance until it is set. Some valve-rods have a yoke that slips over the valve, while the adjusting and jam-nuts are between the stuffing box and the rocker-arm pin. When a valve-rod has no nuts, the adjusting must be done at the eccentric rod. To lengthen or shorten the stroke of valve-rod, raise or lower the eccentric-rod pin in the slot, at the bottom of the rocker-arm, whichever way suits the circumstances.
Q. Now, after you have set your valve, keyed
everything up properly, and there was a thud or dead sound in the
engine or cylinder, what would you do, or where would you look for
A. In the exhaust being choked. The steam chest cover must be taken off, then uncouple the valve, turn the valve up sideways and move it until the steam edge has the proper lead with the steam-port, then place a square on the valve-seat of the cylinder, and against the valve-face, to see how the exhaust lead on the opposite steam-port corresponds; if it is choked, then scribe it by allowing a little over double the steam lead.
Q. How is the exhaust made larger?
A. By chipping out the exhaust cavity in the valve, and rubbing file over it to smooth it.
Q. Do you think a little over double the
steam-lead would be sufficient for the exhaust?
A. Yes, if not, take out a little more.
Q. Where should the exhaust be?
A. It should be the furthest from the steam-port that is receiving.
Q. What would you do in case your eccentric
slipped around on the shaft?
A. Set the valve the same as before.
Q. Is the principle of valve setting the same
on all engines?
A. Yes; some engines have two steam and two exhaust valves, but that makes no difference, the principle is the same.
It often happens that engineers, under whose control Corliss engines are placed, are not practically acquainted with the operation of the Corliss gear, and are at a loss what to do should the gear need adjustment. By carefully observing the following questions and answers, the desired information will be found.
Q. Into how many classes are the different
types of Corliss valve gear divided?
A. Into two general classes.
Q. Which are they?
A. To the first class belong the crab-claw gear, originally used by George H. Corliss, and later by Harris and other prominent Corliss engine builders.
To the second class belong the half-moon valve gear, as used on the Reynolds Corliss engine built several years since, and followed in some recent designs of Corliss engines.
Q. Which are the more favorable and widely
known type now in general use?
A. The half moon type.
Q. Why so?
A. Because the old style crab-claw steam valve opens toward the center of the cylinder, which obstructs the supply passage and forces the steam to pass over and around the valves. This fault is overcome in the half moon type, as the steam valve opens away from the center of the cylinder, thus leaving a clear and direct passage for the steam into the cylinder.
Q. Do the two different styles make any
difference into the opening of the exhaust valves?
A. No. The difference in the two classes is simply in the direction of movement of steam valves; the exhaust valves open the same in either class, viz.: away from the center of the cylinder.
Q. What name has the Corliss valve gear?
A. It is called a detachable valve gear.
Q. Why is it called detachable?
A. Because the steam valves open positively at the proper time by the direct action of the working parts of the engine, and continue to open until the connection with the working parts of the engine are broken by detaching or tripping the hook, by action of the cut-off cams.
Q. How are the steam valves
A. When the steam valves are detached they are closed by the action of springs, weights, or more generally vacuum dash pots, thus cutting | off the supply of steam.
Q. How is the detachment or tripping
A. The time in the stroke at which the tripping takes place is known by the position of the cut-off cams, which are moved and controlled by the governor.
Q. Does the cut-off cam trip the hook always at
the same point?
A. No. The cut-off is determined by the requirements of the load on the engine.
Q. By what name is this cut-off known?
A. The automatic cut-off.
Q. How is the theory of the Corliss valve
motion easily understood?
A. The theory is easily understood by considering the four valves as the four parts (or edges) of a common slide valve.
Q. Why are the four valves of the Corliss
engine considered as the four parts (or edges) of the common slide
A. The working edges of the two steam valves answering as the two steam edges of the slide valve, and the working edges of the two exhaust valves as the exhaust edges of the slide valve.
Q. The Corliss having four valves, and the
common slide valve only one, does it not make any difference in
A. As far as the setting the principle is the same; the only difference is in the adjustment.
Q. Why does the adjustment make a
A. The four working edges of the common slide valve are in one solid valve, so that any change or adjustment of one of the edges interferes with the other three. If one edge is to be changed in reference to the others, it must be done by altering the valve itself. The Corliss valves on the other hand, are adjustable, each by itself, and any one of the valves may be changed without disturbing the other three.
Q. Can the adjustment be made while
A. When the engineer is familiar with his engine and knows what changes are necessary, the adjustment may be, and is frequently, made without stopping the engine.
Q. How many edges has a slide valve?
A. Four two steam and two exhaust.
Q. Have the Corliss valves the same number of
edges as the common slide valves?
A. No. Each Corliss valve represents an edge of the common slide valve, viz.: two steam edges, two steam valves, two exhaust edges, two exhaust valves.
Q. How are the valves connected to the
eccentric and worked on Corliss engines?
A. With the wrist-plate, carrier arm, rocker arm and reach rod.
Q. Is the wrist-plate good for any other
A. Yes. It modifies the speed of travel at different parts of the stroke, in relation to each other, and gives a quick and constantly increasing speed when opening the steam valves, and a quick opening and closing of the exhaust valves.
Q. When do the steam and exhaust valves travel
A. When they are closed.
Q. Can the valves of Corliss engines be
adjusted when the reach rod is unhooked from the wrist-plate, so
the valves may be properly set, independent of the position of the
Q. Are the Corliss valves easily set?
A. If the engineer has any knowledge, as he should have, of the ordinary slide valve, and of the effect of 'lap and lead' as applied to its working, and will consider the Corliss valve gear in the light of this knowledge, he will soon master the seeming difficulties in his way and find the Corliss gear to be the simplest, most perfect and most easily adjusted of all valve motions.
Q. How would you go about setting the Corliss
A. Begin by taking off the back caps or back heads of all four valve chambers. Guide lines will be found on the ends of the valves and on the ends of the chambers, as follows: On the steam valves, coinciding with the working edges of the valves; on the steam valve chambers, coinciding with the working edges of the steam ports. On the exhaust valves and ports, guide lines are also scribed to set them by. The wrist-plate is centrally between the four valve chambers, on the valve gear side of the cylinder. A well defined line will be found on the stand which is bolted to the cylinder, and three lines on the hub of the wrist-plate, which, when they coincide with the line on the stand, show the central position of the wrist-plate and the extremes of its throw or travel. To adjust the valves, first unhook the reach rod connecting wrist-plate with rocker arm and place and hold the wrist-plate in its central position. The connecting rods between steam and exhaust valve arms and wrist-plate are made with right and left hand screw threads on their opposite ends, and provided with jamb nuts, so that by slacking the jamb nuts and turning the rod they can be lengthened or shortened as desired. By means of this adjustment, set the steam valves so that they will have inch lap for 10 inch diameter of cylinder, and inch lap for 32 inch diameter of cylinder, and for intermediate diameters in proportion.
For the exhaust, set them with 1/16inch lap for 10 inch bore, and1/8 inch lap for 32 inch bore on non-condensing engines and nearly double this amount on condensing engines, for good results. Lap on the steam and exhaust valves will be shown by the lines on the valves being nearer the center of the cylinder than the lines on the valve chambers. Having made this adjustment of the valves, the rods connecting the steam valve arms with the dash pots should be adjusted by turning the wrist-plate to its extremes of travel and adjusting the rod so that when it is down as far as it will go, the square steel block on the valve arm will just clear the shoulder on the hook. If the rod is left too long, the steam valve stem will be likely to be either bent or broken; if too short, the hook will not engage, and consequently the valve will not open. Having adjusted the valves as stated, hook the engine in and, with the eccentric loose on the shaft, turn it over and adjust the eccentric rod so that the wrist-plate will have the correct extremes of travel, as indicated by the lines on the back of hub of wrist-plate. Then place the crank on either dead center and turn the eccentric in the direction in which the engine is to run to show an opening at the steam valve of from 1/32 to 1/8 inch, depending upon the speed the engine is to run. This opening will be shown by the line on the valve being nearer the end of the cylinder than the line on the valve chamber. This opening gives the 'lead' or port opening when the engine is on the dead center. The faster the engine is to run the more lead it requires, as a general rule. Having turned the eccentric so as to secure the desired amount of lead, tighten it securely, by means of the set screw, and turn the engine over to the other center, and note if the other steam valve has the same lead. If not, adjust by lengthening or shortening the connecting rod to the wrist-plate as the case may be necessary to do.
If the engine has the half-moon, crab-claw, or other gear which opens the valves toward the center of the cylinder, the manner of the adjustment will be the same, except that the 'lap' on the steam valves will be shown when the line on the steam valve is nearer the end of the cylinder, and the 'lead' when this line is nearer the center of the cylinder than the line on the valve chamber. The adjustment of the exhaust valves and the amount of 'lap' and 'lead' will be the same in either case.
To adjust the rods connecting the cut-off or tripping cams with the governor, have the governor at rest and the wrist-plate at one extreme of its travel. Then adjust the rod connecting with the cut-off cam on opposite steam valve so that the cam will clear the steel on the tail of the hook about 1/32 inch. Turn the wrist-plate to the opposite extreme of travel and adjust the cam for the other valve in the same manner. To equalize the cut-off and test its correctness, hook the engine in and block the governor up about 1 inch, which will bring it to its average position when running. Then turn the engine slowly, in the direction in which it is to run, and note the distance the cross-head has traveled from its extreme position at dead center when the cut-off cam trips or detaches the steam valve. Continue to turn the engine beyond the other dead center and note the distance of cross-head from its extreme travel when the valve drops. If the distance is the same as when the other valve dropped the cut-off is equal. If not, adjust either one or the other of the rods until the distances are the same.
By following these directions, the engine will do good work, but to know just what it is doing the engineer should use the indicator often. No engine room is complete without a good indicator, and no engineer can be well posted as to what his engine is doing and keep it in its best possible condition for good work without having an indicator and using it often.
Q. How would you find the dead center of an
A. By placing a spirit level on the strap that goes around the brasses that connect the crank-pin to the connecting-rod, and when it is level the crank is at a dead center. If the engine is not level, then use an adjustable level.
Q. What other way could you find the dead
center of an engine?
A. By moving the engine toward the dead center until the cross-head stopped moving; then put a center punch mark in the floor, and one on the flywheel, after having marked it with a tram; then move the crank over the center until the cross-head begin to move, then put another mark; the middle between the two marks is the exact dead center; then bring the middle mark to the point of the tram; this is done with a small tram with one straight point and a short foot.
Q. If the engine had to be run in the opposite
direction to which it had been running, how could it be done?
A. It could be done by placing the crank-pin on the dead center, removing the steam-chest cover, and turning the eccentric over on the shaft in the opposite direction, until the valve has the proper lead at the opposite port, then try the engine from dead center to dead center, to equalize the lead at both ends of the valve; then the engine will run in the opposite direction.
Q. Does a crank-pin and piston travel the same distance?
A. No, a crank-pin travels 11416/10000times further than the piston each revolution, or 0.5707/10000 times further each stroke. For example, take an engine with a 12-inch stroke, the piston travels 24 inches and the crank pin 376992/10000 inches each revolution, or the piston travels 12 inches each stroke and the crank-pin 18.8496 per single stroke of piston. To do this multiply the single stroke by one-half of 3.1416, which is 1.5708, and the answer will be the distance the crank-pin travels further than the piston per single stroke. This rule answers for all engines. Another fact not generally known by many men is that a crank of an engine, at two certain points, travels a long distance while the motion of the cross-head is hardly noticed. When the center of the crank-shaft and crank-pin are in a line with the piston-rod, no steam pressure applied to either side of the piston can set the engine in motion; this is called the dead center.
Q. Is the piston-head in the center of the
cylinder when the centers of the crank-pin and crank-shaft are
plumb, or in right angles with the cylinder?
A. No, under no circumstances.
Q. What is a revolution?
A. It means a crank has turned once around, or made a circle.
Q. How many strokes has a revolution?
A. Two to each revolution.
Q. If an engine has 24 inches stroke, and makes 65 revolutions per minute, how many feet does it travel in a minute?
A. Twenty-four inches multiplied by 2 equals 48 inches, this multiplied by 65 revolutions equals 3120 inches, which divided by 12 equals 260 feet per minute.
Q. If you were asked the horse power of any
sized engine, could you tell it?
Q. Well, how would you go about it, and what is
a horse power?
A. A horse power is 33,000 pounds raised one foot high in one minute, or 150 pounds raised 220 feet high in one minute. To find the horse power of any engine, first find the area of the piston-head face, then multiply the answer by the average pounds pressure per square inch in cylinder, then multiply by the number of feet traveled in one minute, and divide by 33,000.
Cylinder 12 x 24 in. 12 diam. of cylinder. 65 revolutions, 12 Average pressure 40 lbs. 144 sq. of diameter. . 000.7854 Generally allow about the boiler pressure 113.0976 area of p.h. face. in figuring H.P. 40 average Pressure in the cylinder 4523.9040 260 No. ft. trav. by d.
We'll be continuing this reprint of Zwicker's manual in a future issue, starting with a discussion of 'The Indicator.'