Illustrations for Ch. 14
Introductory. —When mathematicians were
investigating the expansive action of steam, and the saving that might be thus
effected, a professor in Providence,
R. I., looking over some of the calculations, became interested, and took them to a young man who had shown inventive ability while working at the harness makers trade. The latter was George H. Corliss, who then turned his attention to the steam engine, and invented a valve gear for using steam expansively. The great success of this gear made Corliss famous. NOTE
The first engine designed by Corliss with circular valves, the kind in general use now, was constructed in the year 1850. The expansion of steam had been tried with poppet valves and a fixed cut off, but had not met with much success. With the Corliss gear, the cut off can be varied and controlled by a governor; thus the degree of expansion is automatically changed to suit the load conditions.
Conditions Necessary for High Efficiency.—Economy in
the use of steam requires that it shall be: NOTE NOTE
1. Quickly admitted to the cylinder at as near boiler pressure as possible.
2. Cut off with great rapidity quite early in the stroke.
In general, the most economical cut off is from one-seventh to one-fifth stroke condensing, and from one-fifth to one-fourth stroke non-condensing. The cut off should be rapid to reduce wire drawing.
3. Pre-released as late as possible without increasing the back pressure;
The full benefit of the available expansion is thus obtained.
4. Exhausted with a minimum of back pressure;
5. Compressed only sufficiently to absorb the momentum of the reciprocating parts, and fill the clearance space, at a pressure most suited to the conditions of operation;
6. Used in an engine having very little clearance.
The small clearance of the Corliss engine is one of its economic features, being only 2 or 3 per cent of the cylinder displacement. In some short stroke high speed engines, the clearance is as much as 10 per cent.
Defects of the Slide Valve. —The economic steam distribution described in the preceding paragraphs is not obtained with the slide valve on account of its limitations, previously explained.NOTE
The chief defect of the slide valve is its inability to handle steam expansively, except to a small degree. The valve itself cannot cut off shorter than about six-tenths stroke without excessive travel. When operated with a movable eccentric, early cut off is obtained at the expense of reduced port opening. Moreover, early cut off is attended with premature release and compression; at still earlier cut off these defects become more marked. One may be corrected by changing the inside lap, but this involves a correspondingly larger error for the other.
Corliss Valves.—The limitations of the slide valve are practically overcome with the Corliss gear. As shown in fig. 851, there are four valves, two at each end of the cylinder. Steam is admitted by the upper valves, figs. 865 and 866, and exhausted by the lower ones, figs. 867 and 868, this arrangement reduces the clearance to a minimum, and provides proper drainage for the water of condensation.
Each valve has one acting edge for the steam; and is shaped as a sector of a circle; it works within a cylindrical seat over ports in line therewith. The valves are made single, double and sometimes triple ported; the usual construction is shown in figs. 865 to 868 which illustrate a steam and exhaust valve.
Each end is cylindrical, fitting the circular end of the valve chamber which acts as a guide. Part of the cylindrical section is cut away to a smaller diameter as at A, leaving a space B, which allows the valve to raise from its seat and relieve the cylinder in case an excess of water be carried in with the steam.
The valve stem has a T head which fits in a slot cut in the valve making a flexible connection. Figs. 869 and 870 show more clearly this construction. A collar forms a part of the valve stem; its object is to protect the bonnet from any cutting action due to the edges of the T head.
Figs. 874 and 875 show two views of the crank end of a Corliss cylinder with valves assembled. The valve stem collar as seen, fits in a circular recess in the bonnet.
The exhaust valve is cut away between A and B, giving the steam access to that portion opposite the port, thus pressing the valve firmly against its seat which tends to keep it tight. The valves may be removed from the cylinder by taking off the bonnets C, C’.
In figs. 877 and 878 is shown the end of the valve chambers with bonnets removed; each valve has a threaded hole H, in which a “pull rod” may be screwed to facilitate the removal of the valve. It should be noted that mark P, is scribed on the seat to locate the steam edges of the port, and another V, on the valve to indicate the position of its steam edge. These marks are used in setting the valves, since the ports and edges are not visible.
The Valve Gear.—The mechanism by which Corliss valves are operated is quite different from the plain slide valve gear. For ordinary service, where the maximum cut off is under one-half stroke, all the valves receive their motion from a single eccentric. Heavy duty engines, requiring a long range cut off, have two eccentrics, one for steam valves, and the other for the exhaust valves. Fig. 880 shows the general arrangement of the Corliss gear with a single eccentric. The reciprocating motion obtained from the eccentric is transmitted as usual by the eccentric rod to a rocker, or carrier arm; this arm is connected by a carrier rod to a wrist plate, or wheel pivoted to an arm which projects from the side of the cylinder.
The carrier rod has a U shaped bend B, which when the gear is connected, engages with pin P, on the wrist plate. As shown in the figure, the carrier rod is disconnected or unhooked. The object of this is to allow the gear to be operated by hand in starting; after working the condensed steam out of the cylinder, and giving the engine a few turns with the wrist plate operated by hand, the U bend of the carrier rod is allowed to drop on the pin and the engine come to speed; the gear then receives its motion from the eccentric.
When the carrier rod is hooked to the wrist plate, the motion of the eccentric causes the latter to oscillate about its center.
Attached to the wrist plate are four rods which transmit motion to the valves. The upper ones are called the steam rods because they operate the steam valves, and similarly, the lower ones are known as the exhaust rods.
The success of the Corliss gear is due in part to the peculiar motion given the valve by the action of these rods and the wrist plate. This is illustrated by the diagram, fig. 881, which shows the valves and wrist plate connection at one end of the cylinder.
In operation, the angularity of the steam and exhaust rods varies in such a manner that the valves move rapidly at the time a quick movement is desirable, as at admission and release, and slowly after the ports have been fully opened, and closed. The latter feature makes it possible to use smaller valves than would be otherwise practicable because, on account of the retarded movement, a given angular motion yields a larger port opening than could be had with a straight motion of the rod. In the diagram the valves are shown in several positions. It should be noted that as the steam valve opens through the angle A 0 B, the angularity of the steam rod with respect to the two arcs of motion A B and A’ B’, is at a minimum, hence the motion is rapid, giving a quick admission.
When the wrist plate is moving from C’, to A’, during the closed period of the valve, the angularity is greatest, resulting in a retarded valve movement.
A positive motion is given the exhaust valves, from the wrist plate, but a different method of operation is employed for the steam valves. The closing motion of the valves at cut off is not positive, but is obtained by a releasing device which at the point of cut off disconnects the valve from the main gear and allows it, with the aid of the dash pot, to drop or close with great rapidity *
This is one of the chief features of the Corliss gear, and its action is both an advantage and disadvantage.
Its good features are the rapid cut off obtained which reduces wire drawing, and thus increases the economy; also the great range of cut off secured without any reduction in port opening.
It has the objection, however, that the rotative speed of the engine is limited on account of the non-positive action of the steam valves. High piston speed with this gear, therefore, involves a long stroke, and the slow rotative speed, a large fly wheel. On account of these limitations, a new type or so called “Corliss” has appeared, in which there is direct connections between the wrist plate and the steam valves, the cut off being regulated by varying the travel of the wrist plate motion through the travel of the wrist plate motion through a governor and link.
Engines of this class are properly called four-valve non-releasing engines; as they are fully described in a separate chapter.
The action of the releasing gear being somewhat complicated, will now be explained in detail.
The Releasing Gear.—This part of the Corliss valve gear, as designed by the different builders, varies in detail, but not in principle. There are, however, two general classes of releasing gears:
1. Those in which the valves rotate toward the center of the cylinder in admitting steam, as in fig. 897.
2. Those in which the valves rotate toward the ends during admission, as in fig. 898.
Among the gears included under the first division are the familiar crab claw, and half moon types; those of the second division are generally of a form known as the oval arm gear.
Crab Claw Gear.—This type of releasing gear was used on the original Geo. H. Corliss engine, and is still to be found in operation.
As shown in figs. 901 and 902, the valve stem passes through a stuffing box, and a bracket which forms a bearing. Keyed to the end of the valve stem is a crank having two arms A, and A,’* the arm A, is for admission, and A’, for cut off.
The admission arm terminates in a bearing, through which passes the steam rod D; this bearing contains a stud die F. There is a disc which turns loosely on the valve stem between the crank and the bracket bearing; a projecting arm I, connects this with the governor rod. Attached to the circumference of the disc is a knock off cam or button H, and a safety cam or button H’. These cams are shown more clearly in fig. 900.
A “crab claw” is pivoted to the steam rod at B, and pressed against the disc by a spring. Attached to the lower member of the crab claw is a hook die G.
In operation the hook die engages the stud die when the steam rod is at the outer end of its travel, and on return moves the admission arm, thus opening the valve. At a point determined by the governor, the crab claw is depressed by contact with the knock off cam which releases the admission arm, allowing the valve to suddenly close by the action of the dash pot pulling down on the cut off arm. The operation of the dash pot will be described later.
An important part of the releasing gear is the safety cam H’; its object is to prevent the engine running away in case the governor belt break. This cam, as shown, is attached to the governor disc at such a point that when the latter is in its extreme position (in the direction of late cut off), the cam will depress or knock off” the crab claw, and thus prevent engagement of the hook and stud dies, resulting in the valve remaining closed.
If the governor belt should break while the engine is running, the governor balls would fall to their lowest position, and thus turn the knock off lever to the extreme position which will bring the safety cam into engagement with the crab claw. The hook then will not raise the steam arm, and the valve will remain closed, thus shutting off the steam supply.
To avoid disaster in case the governor belt break, the engineer should not fail to put the governor safety stop in “running position,” after starting the engine, otherwise the safety cam will not operate.
Attached to the steam arm is a rod connecting with the dash pot, which, by its action, as soon as the dies release, quickly closes the valve. The dies are released by the left hand end of the hook striking against the knock off cam, thus causing the supply of steam to be cut off.
The knock off cam is a part of the knock off lever, and the latter is connected with the governor by the governor cam rod. The point of cut off, therefore, is determined by the position of the knock off cam which is controlled by the action of the governor.
Half Moon Gear.—This type of releasing gear is illustrated in figs. 905 and 906. A single crank, or steam arm is keyed to the valve stem, motion being transmitted to the valve by this an-n, both for admission and cut off. The valve stem passes through stuffing box and its outer extremity is supported by a bearing in the bracket. At the cut off end of the half moon, is a stud upon which is pivoted the hook arm.
A hard steel block, or die, projects at the extremity of this arm, forming a hook. Another block called the stud die, is attached to the steam arm. A spring presses the hook arm toward the stud die.
The governor disc, which has an arm, or knock off lever, works loosely on the valve stem, and has attached to it the knock off and safety cams. The hook arm has a branch called the tail arm projecting toward the governor disc, there being a button fastened to the end for engagement with the cams. A rod connecting with the dash pot plunger is pivoted to the steam arm about half way between the hub and stud die.
When the steam rod moves in the direction for admission, the half moon will rotate and raise the hook stud.
As the hook arm rises, the die engages the stud die thus lifting the steam arm.
During the movement, the hook stud moves in an arc about the valve stem, and at a point determined by the governor, the button on the tail arm strikes the knock off cam which knocks the hook die away from the stud die, allowing the valve to drop shut by the action of the dash pot.
A safety cam is provided which engages with the button when the governor is in its lowest position, thus holding the dies apart in case the governor belt break.
Oval Arm Gear.—This is practically the same as the half moon gear, with the exception that the admission arm as shown in figs. 927 and 928 projects upward in order to get the opposite motion of the valve for outside admission. The similarity of the two gears is seen from the illustrations.
Dash Pots.—The quick closing of the steam valve giving a sharp cut off, is one of the excellent features of the Corliss gear. In the earlier engines, the steam valves were snapped shut by springs, or weighted plungers, but later, a more satisfactory operation has been obtained by means of dash pots, whose operation depends on:
To obtain this pneumatic action, the dash pot is fitted with an air tight plunger. A dash pot is simply a cylinder closed al one end, and accurately bored to receive the plunger. A rod connects the latter to the steam arm of the valve. The steam arm, then, as it is raised by the hook, lifts the plunger which produces a partial vacuum in the clash pot. When the hook releases the steam arm, the pressure of the atmosphere on top the plunger causes it to quickly drop and close the valve. The compression of air remaining below the plunger forms a cushion which prevents shock.
The pressure in this type of dash pot, then, must be below that of the atmosphere during the first portion of the downward stroke in order to shut the valve, and greater during the latt0r portion, to bring the parts to rest. From this it follows that the pull on the dash pot rod is greatest at the beginning of the downward stroke of the plunger, and on account of the low initial vacuum, it is very little if any at the point where the steam valve closes. The speed of the valve at this point, therefore, is not as great as it might be if the effect of a good vacuum were available during the entire period of closure of the steam valve.
In order to obtain this action, the single plunger dash pot, as just described, has been modified by the addition of another plunger or piston. Here the compression is carried on in a separate cylinder, which permits of a higher vacuum for closing the valve.
Separate compression dash pots are used on most Corliss engines and a typical construction is shown in fig. 931.
As shown, the dash pot consists of a vacuum cylinder V, and a compression cylinder c. Fitted to these are a plunger P, and piston F’, forming one casting. At the end of the vacuum chamber is a sensitive check valve K; there is a passage M, leading from the compression chamber to the atmosphere, and its opening is regulated by a screw valve S.
The operation is as follows: As the steam arm rises, the dash pot rod lifts the plunger P, causing the check valve S. to close, producing a high vacuum in the vacuum cylinder V. The travel of the plunger being longer than the compression cylinder C, the piston P, is carried past the end of this cylinder, thus giving access to the atmosphere.
When the steam arm is released, the high vacuum in the cylinder causes the plunger to drop with great speed until the piston enters the compression cylinder. The air contained therein is then compressed which quietly brings the moving parts to rest. The amount of cushion thus obtained may be regulated by the screw valve S, allowing more or less air to escape during the compression.
Corliss Engine Speed.—One inherent defect of the Corliss engine is that the releasing gear is not adapted to high rotative speed. In many power installations, this is a drawback, not only because of the increased first cost, but on account of the space occupied, and in general because higher speeds are attended with greater saving in the use of steam.*
For many years 70 to 80 revolutions per minute for the small and medium sizes was considered the safe limit of rotative speed, and later from 90 to 100. Since then Corliss engines are commonly run at 100 to 120, and those especially built for high speeds, as high as 200 revolutions.
Various refinements have been made in the design of the releasing gear to secure satisfactory working at high speed.
The inertia of the releasing parts has been reduced as much as possible by making them of minimum weight, thus rendering the dash pot action more effective.
Larger dash pots have been employed to secure quicker action in closing the valves. The motion of the gear has been reduced by shorter leverage and double ports.
These several refinements permit considerably higher rotative speeds, than is permissible with the ordinary construction.
An example of a high speed Corliss valve gear designed as above outlined is shown in figs. 934 and 935.
Besides the lightness of parts, short leverage, and reduced motion, the steam arm is located inside the bracket between the stuffing box and bearing The stem is thus supported on both sides of the steam lever instead of having the latter overhung. This gear, according to the builders is designed for speeds up to 175 revolutions per minute.
In figs. 936 and 937 is shown the Franklyn gravity releasing gear for which is claimed satisfactory working up to 200 revolutions per minute.
Its chief feature is the placing of the hook arm in such a position that gravity assists the spring in moving the arm to engage the stud die of the steam arm.
Ques. In what respect is the Corliss gear limited when operated by a single eccentric?
Ans. The releasing mechanism will not act to cut off later than about one half stroke.
Ques. Why is this?
Ans. If the eccentric be set at 90° ahead of the piston, it will reach its extreme position when the crank arrives at half stroke. If at this point, the hook, which was rising and opening the valve, has not struck the knock off cam, it cannot strike at all, since the motion of the wrist plate is reversed at this point; hence, the hook and steam arm will begin to descend thus gradually closing the valve. This will take place near the end of the stroke but it will not be the sharp cut off as produced by the sudden drop of the dash pot plunger when the dies are released.
Ques. Why is the single eccentric gear unsatisfactory for a cut off later than one half stroke?
Ans. In order to cut off later than half stroke, the eccentric must be turned back until it is less than 90° ahead of the crank. This decrease in the angular advance causes release and compression to occur too late. If exhaust lap be added to correct one, it will increase the error of the other in like proportion.
For instance, if the exhaust valves be given negative inside lap to obtain earlier release, compression will be delayed an equal amount.
The Use of Double Eccentrics.—In the preceding questions, it is seen that the Corliss gear with one eccentric is limited to cut offs not exceeding half stroke.
If a single eccentric be set with the least possible angular advance, in order to get the maximum range of cut off, the engine when heavily loaded will not be able to exhaust the steam early enough to sufficiently reduce the pressure at the beginning of the return stroke.
If the eccentric be advanced to secure early release, the range of cut off available with the releasing gear is so reduced that the knock off cam may not strike the hook for one or two revolutions thus increasing the trouble which it was desired to remedy.
The limitations of the single eccentric gear are overcome by the addition of a second eccentric; one to operate the steam valves, and the other for the exhaust valves. Admission and cut off are thus made independent of release and compression. The steam eccentric may therefore be set with negative angular advance to obtain a late cut off, and the exhaust eccentric with positive angular advance to secure early release and compression.
It should not be inferred from the preceding paragraph that a single eccentric gear, with the addition of another eccentric, will give as good results for long range cut off as if it were originally designed for double eccentrics. The reason for this is because in order to get a long range cut off, the steam eccentric must be given considerable negative angular advance, and the more the eccentric is set back the slower is the movement of the valve in opening the port.
If a single eccentric gear be provided with a second eccentric, and the steam eccentric be given a negative angular advance not exceeding the original positive angular advance, a moderate increase in the range of cut off is obtained with satisfactory steam distribution. However, if a later cut off be required, additional negative angular advance must be given the steam eccentric, and the valve will open slower than it did when operated by the single eccentric gear.
It is quite common practice to put a second eccentric on single eccentric engines, where only a moderate lengthening of the cut off is sufficient to meet the increase in the running load which has accrued since the engine was installed.
It should he noted that if the load I e increased so as to require a cut off beyond the range of the releasing gear, the knock off cam will not strike the hook, and cut off will not take place until the valve is closed by the return motion of the eccentric. Further, the negative angular advance given the eccentric may be sufficient to delay the closing of the valve until after release in which case live steam would blow through the cylinder into the exhaust pipe. To guard against this, valve gears when designed for two eccentrics have a maximum cut off of almost full stroke within the range of the releasing gear. This insures the release of the steam arm for any load within the capacity of the engine.
Since the necessary negative angular advance which must be given the eccentric to secure a very late cut off, reduces the speed at which the valve opens the port, poor admission is avoided in designing a double eccentric gear, either by increasing the port opening, or by the use of double ports.
An example of a double eccentric valve gear is shown in fig. 943. There are two wrist plates, which work independently of each other. The steam valves are connected to the upper plate S, which is operated by the steam eccentric, and the exhaust valves are connected to the lower plate E, operated by the exhaust eccentric. Motion is transmitted from the eccentrics to the wrist plates with the usual gear consisting of carrier rods, carrier, rockers, and eccentric rods.
Corliss Engine Frames.—In the design of the frames, care is taken to distribute the metal so as to best resist the stresses transmitted from the piston to the crank pin. Modern frames are designed with special regard to the service for which they are intended. With respect to this point, Corliss frames may be classified as:
For ordinary service the girder frame is generally used. This type was one of the distinguishing features of the original Corliss engine and is well adapted for light duty.
Figs. 944 and 945 show a front and rear view of a girder frame; a heavy backbone projects from the rear, as illustrated in fig. 945, directly in line with the strain. The form of the girder frame is such that the required strength and stiffness is secured with the least amount of metal.
Where the conditions of operation are more severe, a frame of heavier construction is desirable, such as shown in figs. 946 and 947.
Here the metal extends down to the foundation; the frame is hollow and of rectangular cross section, forming a kind of box girder of the so called “straight line” design in which the walls extend in practically straight lines from the main bearing to the cylinder, this bringing the metal, as near as possible, in the direction of the strain so as to reduce bending, or twisting stresses.
With the advent of electric railways, and the plan of transmitting power by electricity, a new type of frame came into use, which has become generally known as the “heavy duty,” or Tangye. The true Tangye frame was designed by an English engineer of that name,* and is to be found on Corliss engines only in its modified form, such as is shown in fig. 948.
Starting with a massive bearing, the metal is placed in straight lines between it and the guide casting to which it is bolted, although in the smaller sizes the frame proper and guide are sometimes cast in one piece.
The frame terminates in lines of symmetry and strength to form a circular end which receives the guide. The graceful down sweep curve at this point is characteristic of the Tangye design.
The end and side walls are carried down, and sometimes an enclosing bottom is provided, which distributes the weight and stresses to the foundation, and also forms a receptacle to catch oil.
To meet the severest conditions of service where high speed and high pressure are required, a frame of the most substantial construction is necessary. This type is known by some as the rolling mill frame, because of its universal adoption for that service.
An example of an extra heavy duty frame is shown in fig. 950; it is similar to the Tangye frame illustrated in the preceding figure, but of more massive construction. The frame and guide are cast in one piece, and it should be noted that the metal of the latter is carried down to the foundation giving additional support.
Another design of extra heavy duty frame is shown in figs. 951 and 952. This is also a one piece frame, and its very substantial and massive construction is indicated by the cross sections, figs. 953 to 956.
Setting Corliss Valves; Single Eccentric.—Adjusting the Corliss valve gear should present no difficulty when once its construction and principles of operation are understood. In fact, part of the work has been done by the engine builders in scribing the necessary reference marks on the gear during its construction. Marks corresponding to the steam edges of the valves and ports are scribed on the end of each valve and seat respectively, as shown in figs. 877 and 878.
There are on the back of the wrist plate hub three marks, A B, C, as shown in fig. 957, and one, D, on the wrist plate stud. When A, registers with D, the wrist plate is in its neutral position; similarly when B, or C, registers with D, the valve is in one of its extreme positions. In some cases the marks are arranged as shown in fig. 960.
The several operations to be performed in setting Corliss valves are as follows:
1. Squaring the Wrist Plate and Rocker.—The first step in setting the valves is to unhook the carrier rod, and put the wrist plate in its neutral position, that is, the position shown in figs. 957 and 960, where line A, is opposite D.
The wrist plate should be clamped in this position by placing a piece of paper between it and the washer on the supporting pin. If the marks A, and D, have been correctly located, a plumb line from the center of the hook pin as shown in fig. 959 should register with the center of the wrist plate.
The rocker should now be placed in a vertical position with a plumb bob, and the length of the carrier rod adjusted, if necessary, so that it will engage with the hook as shown in the figure.
The rocker is conveniently brought into a vertical position by turning the engine over until it is vertical as determined by the plumb bob, taking special care before doing this to shorten the dash pot rods enough to prevent the steam arm die being forced against the hook arm shoulder.
Similarly, the exhaust valves are put in line and line position or given a very small amount of lap by adjusting the exhaust rods L, L.
It should be noted that the object in giving steam lap to Corliss valves is quite different from the result sought with the slide valve:
Lap is given a slide valve to obtain a desired cut off, while with the Corliss gear, its object is to secure the most favorable angular advance of the eccentric.
As has been mentioned, the eccentric should have some angular advance in order to secure pre-release, and the proper amount of compression.
The usual amount of steam lap for small engines is from 1/16 to 3/4 inch, and from 1/4 to 1/2 inch for the larger sizes.* The builders of the Reynolds-Corliss engines recommend that the valves be set according to the following table lap and lead.
Table for setting valves
Diameter of Cylinder
|8 to 12||
|14 to 20||
|22 to 30||
|32 to 36||
3. Adjusting the Dash Pot Rods.—This is a very important adjustment. If the rods be too short, the steam valves will not open, if too long, the rods will be bent, or the bonnets broken, or both. To make this adjustment, the wrist plate is turned by hand to its extreme position, that is, until B, or C, of fig. 957 registers with D.
When the plunger is down as far as it will go, as in fig. 962, the dash pot rod D, should be adjusted so that the hook on shoulder H, will safely clear the steel die S, on the steam arm, leaving a margin M, between the hook die B, and steam arm die S.
4. Setting the Eccentric.—The paper which was inserted between the wrist plate and its washer may now be removed so that the wrist plate can turn. After doing this, the engine is placed on the dead center and the eccentric located “by eye.” The lead is measured and the engine turned over to the other center. If the length of the eccentric rod be correct, both leads will be the same; if not, the lead must be equalized by adjusting the eccentric rod to the correct length. The equalized lead will probably be too great or too little since the eccentric was set by eye; it remains then to change the position of the eccentric until the valves show the desired lead as given in the table.
The foregoing, of course, applies to engines having the eccentric fastened with set screws. In some cases, especially on large engines, when the eccentric is keyed to the shaft the position of the eccentric is fixed, and the steam lap as given in the table is subject to correction.
After the wrist plate, valves and rocker have been squared, and the lead equalized, if the correct lead be not obtained, the engine is placed on each dead center, and the length of the steam rods (K, K, fig. 961) adjusted until the valves show the proper lead as given in the table.
If for any reason it be desired to change the position of the eccentric as for instance, to increase the range of cut off, it may be done by fitting an offset key.
The setting of a Corliss eccentric is governed by the same principles that apply to the slide valve eccentric; the operations differ only in minor details. For instance, it is not necessary to place the Corliss eccentric “by eye” a little ahead of its correct position when equalizing the lead as both positive and negative lead may be easily measured from the reference marks. Instead of wedges for measuring the lead, this is conveniently done by means of dividers.
5. Adjusting the Governor Connections.—There are two governor cam rods H, H, by which the controlling action of the governor is transmitted to the knock off levers, as shown in fig. 961. By lengthening or shortening these rods, the point of cut off may be adjusted. In doing this, the governor sleeve is raised by means of the safety stop.
This prevents the governor reaching its lowest position, and brings it in the lowest position in which the hook should engage the steam arm.
With the governor in this position the carrier rod is unhooked and the wrist plate turned by hand to one of its extreme positions. The corresponding steam valve will now be wide open, and in this position the governor cam rod H, (fig. 961) is to be adjusted so as to bring the knock off lever G, in the proper position to release the steam arm M, thus allowing the valve to close.
The wrist plate is now turned to the other extreme position, and a similar adjustment made at that end of the cylinder.
To check the correctness of the cut off adjustments, the governor should be raised to an intermediate position and blocked. The carrier rod is then connected to the hook pin, and the engine turned over slowly in the direction in which it is to run, noting the positions of the crosshead at which each cut off takes place. If equal cut off be obtained for each stroke, no further adjustments are necessary, if not, the length of the cam rods should be adjusted until the points of cut off are at equal distances from the beginning of the stroke. The valve bonnets may now be replaced, and the block removed from the governor which completes the setting of the valves.
Setting Corliss Valves, Double Eccentrics.—The work of setting the valves of a Corliss engine having two eccentrics, is not so complicated as supposed by some. It is rendered easy by keeping in mind the principles involved.
The reason for two eccentrics, as before explained, is to obtain a long range cut off without distorting the action of the exhaust valves.
If all four valves were operated by one eccentric, cut off could not take place later than one-half stroke without causing release and compression to occur too late. This is due to the negative angular advance which must be given the eccentric to secure late cut off.
With two eccentrics, then, negative advance may be given the steam eccentric to get a late cut off, and positive advance to the exhaust eccentric to secure early release and compression.
The arrangement of the steam rods of a single eccentric gear is such as to give a slow initial* movement to the valve while the port is still covered. with positive lap; this causes the quick motion period of the valve movement to occur while the valve is opening the port.
The steam rods of a double eccentric gear are arranged to give a quick initial valve movement, because negative lap is used here, which causes the valve to open the port at an earlier point in its travel.
It follows then, that a valve gear designed to he operated by a single eccentric cannot very well be made to cut off much later than half stroke, even if a separate eccentric he added, because the slow initial movement of the single eccentric gear cannot be corrected without a re-arrangement of the steam rods.
In setting the valves of a double eccentric gear, the operations of squaring the valves, equalizing the travel, etc., is practically the same as with the single eccentric engine. The various steps are as follows:
1. Squaring the Wrist Plates and Rockers.—These are squared in the same way as with the single eccentric gear. That is, the wrist plates are placed in the neutral position as indicated by the reference marks and clamped; the setting is then verified with the plumb bob. Similarly, after unhooking the carrier rods, the rockers are placed in a vertical position and the carrier rods adjusted to the proper lengths, using plumb bobs as in fig. 959.
In squaring the rockers, the eccentrics should be unloosened on the shaft and the rockers moved by turning the eccentrics rather than the engine.
2. Squaring the Valves.—These are squared by adjusting the steam and exhaust rods the same as with the single eccentric gear. However, with two eccentrics, the steam valves are set with negative lap, which, in amount, is usually a little less than half the port opening as shown in fig. 973.
The object of this negative lap is to so locate the position of the eccentric that it will give a quick movement to the valve in opening the port.
The exhaust valves are set in line and line position as illustrated in the figure.
3. Adjusting the Eccentric Rods.—The travel of the valves should be equalized, by adjusting the lengths of the eccentric rods. After unclamping the wrist plates, each eccentric is turned on the shaft so as to bring the wrist plates in the extreme positions, noting if the reference marks register at these points. If not, the eccentric rods are to be adjusted until the marks come opposite each other.
In case the eccentrics are keyed, or not easily turned on the shaft, the engine may be turned over instead, in equalizing the travel. If this be done, great care should be exercised to see that the dash pot rods are not too tong, otherwise the valve gear may be injured as previously explained.
4. Adjusting the Dash Pot Rods.—The length of these rods is adjusted in the same way as with the single eccentric gear, hence no additional instructions are necessary. It is well to repeat, however, that this is an important adjustment and should be carefully made. If the rods be too short, the steam valves will not open; if too long, the rods will be bent, or the bonnets broken, or both.
5. Setting the Exhaust Eccentric.—This is usually set first as it is next to the shaft; the other eccentric then may be turned out of the way if necessary in tightening the set screws.
To locate the position of the exhaust eccentric, the engine is turned in the direction it is to run until the piston is brought to the point where compression should begin.
The distance of this point from the end of the stroke, may be taken at 5 per cent of the stroke. This is easily measured by scribing reference marks on the cross head and guide.
The eccentric is now turned in the direction the engine is to run, until and line as shown in fig. 974, and then fastened in position on the shaft.
The diagram to the right of the figure shows the relative positions of eccentric.*
6. Setting the Steam Eccentric.—The engine is now placed on the dead center, and the steam eccentric turned in the direction in which the engine is to run until the valve has the proper lead, as shown in fig. 975; for a double ported valve this will be less than required for the single ported type. If the eccentric rod adjustment previously made be correct; the lead for the other end of the cylinder will be the same, otherwise a further adjustment of the eccentric rod is necessary.
It should he noted that the steam eccentric will be either in its normal position of negative angular advance, or at a half turn (180° degrees) from this position, depending upon the type of the valve gear. Thus, with the half moon gear and inside admission as shown in fig. 977, the motion of the valves must be reversed, hence the steam eccentric must he given a half turn to the right as shown in the figure. If these valves had outside admission, the steam eccentric would be in its normal position as shown in fig. 976.
7. Making the Governor Adjustments.—The method of adjusting the governor with the double eccentric gear is the same as with single eccentrics, as described on page 515.
8. Final Adjustments with Indicator.—It is desirable after setting the valves that an indicator he applied to the engine when it is in operation, to verify the valve setting, and make more accurate adjustments than would otherwise be possible. In using the indicator the following directions for the final adjustments should be noted:
If with the average load on the engine, the indicator cards from the two ends of the cylinder be unequal, showing that one end is doing more work than the other, the governor rods should be adjusted to cut off a little earlier at the end with the larger load, and a little later at the other end.
If the toe of the diagram turn up, showing that pre-release is too late, the exhaust rods, with single eccentric gear, may be shortened a little, not forgetting that this is at the expense of reducing the compression. Lengthening the rods will increase compression, but make release later.
With the double eccentric gear, a later release should be corrected by giving the exhaust eccentric more positive angular advance.
If the card appear a little late all around, the eccentric (or eccentrics) should be set a trifle forward.
Any small changes in the admission lines that may be desired without affecting the rest of the diagram are made by adjusting the steam rods. Shortening the steam rods will give earlier admission, while lengthening them will, of course, produce the opposite result. Figs. 838 to 848 (page 461) in the chapter on Valve Setting show effect of errors in valve setting as recorded by indicator diagrams.
note NOTE—The first engine fitted with the Corliss gear was of the beam type with flat slide valves. There were separate inlet and outlet ports, which were made as short as possible. The valves gave a rapid admission and cut off, the latter being obtained by releasing weights, suspended from a lever.
Note NOTE.— The Corliss valve gear possesses the following important advantages: 1. Reduced clearance volume and clearance surface, owing to the shortness of the admission and exhaust passages obtained by placing the valves close to the ends of the cylinders. In such cylinders the clearance will vary from 3 to 5% of the piston displacement. 2. Separate valves are used for steam and exhaust, the steam valves being at the top corners of horizontal cylinders and the exhaust valves at the bottom corners, by which means, during the flow of the steam from the cylinder, the exhaust surfaces are swept clear of water and a natural system of drainage is thus provided. This advantage applies more especially to horizontal cylinders. 3. It maintains a wide opening during admission of steam with a sudden return of the valve at cut off, thus preventing wire drawing of the steam during admission. 4. It permits of independent adjustment of admission and cut off, release and compression. 5. It provides an easy and effective method of governing engines of large power, by regulation of the cut off, through the action of a governor on the comparatively light working parts of the valve disengaging gear. It is frequently claimed for the employment of separate steam and exhaust valves that condensation is reduced because the entering steam coming through a separate passage, and not through that through which the steam is exhausted, does not come into contact with surfaces which have just been cooled down by the comparatively cold exhaust steam, as is the case when the port is common to both admission and exhaust; but this claim is only valid if the area of clearance surface be reduced by the arrangement of separate valves, because in any case, all the surface up to the exhaust valve must be heated up each stroke whether the steam is admitted through the same of through a separate port. One important objection to the Corliss valve gear is the limitation of the speed of rotation of engines fitted with it owing to its action being dependent upon the engagement and tripping of catches.
Note NOTE.—Corliss, like most inventors, had to force his invention on an unwilling public. He had to take all the responsibility, and in many instances take his pay in what he could save in fuel. This in the end proved fortunate for him, as in most cases he received far in excess of the price he had put on the machine. According to Crane, at the time Corliss was selling his automatic cut off engines for what he could save, the United States Government was spending money in experiments to show there was no economy in using steam expansively. Employed by Corliss was a man by name of William Wright. Wright always claimed that he was the original designer of the Corliss valve. He invented a cam motion—a cam moving around a central cam, its position being determined by the governor. This cam operated poppet steam valves and made an automatic cut off engine. The exhaust was two slide valves, each valve being placed at the cylinder ends so as to reduce clearance, aside as far as possible get the results obtained by Corliss. These engines were built for a number of years by Woodruff & Beach, at Hartford, Conn. Wright made a change in his cam and governor and went into business for himself. After a time he became convinced that the poppet was not a tight valve and built his engines with gridiron valves. When Corliss’ patents expired, a great many builders started in to build “improved” Corliss engines, and some of them made rather sad work of it.
NOTE NOTE. —In the Corliss valve gear, one important point to notice is the means which the wrist plate provides for giving the valve small movements when closed, and large, and therefore quick, movements when the port is open. This reduces the power required to drive the valve gear to a minimum. For instance, in a certain engine, during the movement of the wrist plate through the first and second half of its total arc, the steam admission valve moved through 11° and 27° respectively, and the exhaust valve 11° and 27° respectively. For ordinary engines of this type, the angular advance of the eccentric is about 15°.
* *NOTE.—The Corliss gear on account of this action is sometimes called a releasing or drop gear.
* *NOTE.—On the half moon and oval arm gears, there is only a single crank keyed to the valve stem; it is called the steam arm.
* *NOTE.—According to tests made by Prof. Jacobus, on a 17X30 engine, fixed cut off, Meyer valve, the loss in economy for about one-fourth cut off is at the rate of one-twelfth lb. of water per horse power for each decrease of a revolution per minute from 86 to 26 revolutions, and at the rate of five-eighths lb. of water below 26 revolutions. It should be noted, that the results thus obtained do not represent the absolute loss, because, an engine designed for slow speed would have correspondingly small ports and passages thus reducing the clearance, which would considerably reduce the loss above obtained
* *NOTE.—The regular Tangye frame is in one piece with the down sweep curve beginning at the cylinder; it is adapted to short stroke engines having locomotive guides, such as the Porter-Allen, Buckeye, etc.
* *NOTE—The initial movement of the valve means the beginning of its movement starting from the extreme position.