A Short Discussion of Engine Reversing Linkages and Their Operation

Simple Engine Diagram

Figure 1 Simple Engine Diagram

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Shortly after my article on governor design appeared in the IMA, I received a letter from Helen Hooks, writing for her husband, William Hooks, of Toronto, Ontario. She states that William would like to know 'what the reversing lever does to actually make the engine reverse' and that he has seen reversing levers marked with 'Best Driving Position' and wasn't sure what that means. In answering these queries for William (who I understand is nearing 90) and Helen, I thought that some of the IMA readers would also be interested. So, for those interested, here is a short discussion of engine valve linkage and. hopefully, an answer to William's questions.

In order to discuss the effect that moving the reversing lever has on the engine, we should first see what the actions of the engine are. In figure 1 we have a simple design of a steam engine (without the valve mechanism or linkage). From this figure we can see that, if steam is admitted to port 'A,' the piston will move to the left causing the engine flywheel to rotate in the 'A' direction (counterclockwise). As the piston moves to the left, spent steam (from a previous cycle) is exhausted from port 'B.' Conversely, if steam is admitted to port 'B,' the engine will reverse its rotation to the direction of 'B' and spent steam will be exhausted from port 'A.' The basic function of the steam engine valve is to channel steam from the supply to the piston cylinder and from the cylinder to the exhaust. The purpose of the valve linkage is to move the valve back and forth, alternately opening and closing the ports 'A' and 'B.' This is done, maintaining the proper relationship with the crank for the direction of rotation.

In figure 2, a valve, inside a steam chest, has been added to the cylinder. In the position shown, steam from the chest would enter the left side of the cylinder, forcing the piston to the right, thus rotating the flywheel in the 'B' (clockwise) direction. The valve has also opened the port to the right of the piston to the exhaust passage, allowing spent steam to leave the cylinder. With the crank in the same position, and the valve positioned to the left, such that the right hand side of the piston were open to steam chest pressure, the engine would rotate in the opposite (counterclockwise) direction. Note that it is the valve's position with respect to the crank that determines the direction of rotation.

The reversing mechanism in the valve linkage, then, controls the relative position of the valve with respect to the crank. Figure 3 is the diagram of an engine with complete valve gear, including the reversing lever. This linkage, although shown only in diagram and not to scale, is similar to the Walschaert Valve gear in use on many railroad locomotives. It is used here for its ease of demonstrating the valve and linkage motion. Figures 3 through 6 represent the motion of the piston, valve and linkage as an engine makes one complete rotation in the clockwise direction.

Note how the valve slides back and forth as the wheel rotates, alternately opening and closing the cylinder ports to steam of the exhaust. The reversing link is moved by another link attached to a valve crank or eccentric which follows 90 degrees after the main crank. The reversing lever in the right hand position holds the sliding link in the upper position in the reversing link, causing the port openings to be sequenced for rotation in the clockwise direction.

Figure 3 shows the valve in its rightmost position with the steam being inlet to the left side of the piston and the right being exhausted. As the crank continues to rotate, the valve begins to move to the left, reaching the position shown in figure 4 as the piston reaches its rightmost position. At this point, both ports are briefly closed to either steam or exhaust. As the valve continues to travel to the left, the port to the right of the piston is opened to steam pressure and the left side to the exhaust, forcing the piston on to the left and continuing the rotation. As the valve reaches its leftmost travel, shown in figure 5, both ports are fully opened and the piston is in the middle of its stroke to the left. The valve then begins to move to the right, closing off the ports until, in figure 6, it again blocks both ports as the piston reaches the end of its left stroke. As the rotation continues through this point, the valve continues to the right and returns to the starting position, figure 3, with the ports fully opened and the piston in the middle of its stroke to the right. The dead spots in the cycle, that is those points in the rotation where no steam is left into the cylinder, are shown by figures 4 and 6.

Figures 7 through 10 show the same engine, this time with the valve linkage in reverse, that is with the reversing lever to the left. In this position, the sliding link is held in the lower position on the reversing link. Comparing figures 3 and 7, both having the crank in the same position, one can see that the valve changes its position from the rightmost to the leftmost as the sliding link is moved down the reversing link by the reversing lever. Figure 11, an intermediate position, shows the sliding link in the center of the reversing link, with the valve held centered.

In figure 7, the valve is in the leftmost position with steam being admitted to the right side of the piston. This draws the piston to the left, rotating the flywheel in the counter clockwise direction. As the cycle continues, the valve moves to the right, blocking both ports as in figure 8, as the piston reaches its dead spot at the end of its left stroke. When the valve reaches its rightmost position, figure 9, the left side of the piston is under steam pressure and is traveling to the right. Figure 10 shows the piston at its rightmost position with the valve having shut off both ports and traveling left to reach the position shown in figure 7.

The answer to 'What does the reversing lever do?' is, then, that it changes the relative position of the valve with the crank, thereby properly sequencing the port openings and closings to cause the engine to rotate in the desired direction. The answer to the other implied question about the 'Best Driving Position' inscription can now be addressed.

As mentioned earlier, as the reversing lever is moved from one extreme to the other, the valve passes from a position of maximum opening, figure 3, through a position where both ports are closed for any crank position, figure 11, to opposite position of maximum opening, figure 7. The movement, from a maximum opening to the closed position, is a gradual closing, a narrowing of the steam port opening to the point of closure. Thus, for example, when the lever is moved half of the way to the center, or closed position, one can consider that the valve will only open half as much as when it is in its extreme position. Likewise, if the lever is moved only a quarter of the way to the center, the valve will open three quarters full. Additionally, the actual timing of the port closure is changed as the lever is moved toward the center or closed position. This timing is normally referred to as the 'cutoff' and is given a value as the percentage of the stroke during which the valve is opened. For instance, if a piston has a stroke of 20 inches, and the valve closes after the piston has traveled 15 inches, the cutoff is said to be at 75%. A cutoff of 0% would be when the lever is in the center, or closed position. A cutoff of 100% would ideally be when the lever is in the extreme position, however this position, due to design considerations, is from 80-90% for most operating engines.

Please note that this is only a rough description of the actual proportions for valve closure, and is used for illustrative purposes only. The actual port openings will depend on the type linkage in use and the specifics of the valve design. This also ignores such attributes as lead and lap, a thorough discussion of which is not necessary here.

Consider an operating engine with the reversing lever in either extreme position. As the lever is moved toward the center position on the quadrant, the valve gradually reduces the opening for the steam to enter the cylinder, thereby reducing the actual amount of steam working on the piston. As the lever approaches the center position, one can see that the engine would eventually stop for lack of steam pressure. Conversely, under most conditions, when the reversing lever is in the extreme position, more steam is admitted to the cylinder than can be used during the stroke, and is wasted to the exhaust at the end of the stroke. The most economical operation of the engine would be to position the valve such that sufficient steam was admitted to the cylinder to provide power through expansion, but not so much as to be wasteful. This position can represent a timing cutoff as short as 25% on some engines; however, it will normally be 50-60%. Some manufacturers placed markings on the reversing quadrant to indicate what position was the most economical operating position for various engine uses such as thrashing, drayage, plowing, etc. I suspect that many more indications were placed on quadrants by observant engineers, or perhaps their fireman who preferred to shovel as little coal as possible. As a point of interest, a traction engine with the governor belt removed, can be driven easily with the speed controlled only by the reversing lever. A number of railroad engines are operated this way normally.

There are quite a few different types of valve gear used on traction engines. Probably the most well known is the Stephenson gear, used on many American engines and extensively on British engines. The Woolf gear is used on Case and other engines and is probably the simplest to maintain. The Baker gear, originally designed to be used on engines built by A. D. Baker, saw much greater service on heavy railroad locomotives. A movable, variable eccentric reversing gear is used on Frick engines. The Marsh gear, which did not allow for varying the cutoff, is used on Advance engines, and the Arnold gear, similar to that used on the Frick, is used on the Rumely engines. These name just a few of the many types of reversing gear. Although they are geometrically different in many ways, they all performed the same relative operations and actions governing the positions of the valve, piston and the crank as has been shown and discussed herein.

As before, I would be happy to address any other questions you may have on this, or any other steam engineering subject.