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.
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.
