A Short Discussion of Engine Reversing Linkages and Their Operation

| November/December 1994

Ph. D., P.E. 7490 Woodridge Lane Bremerton, Washington 98310

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.


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