I am somewhat familiar with poppet valves and the resulting improvements in efficiency and performance to the locomotives on which they were applied. What I don't know is the difference between oscillating cam and rotary cam poppet valve gear. What is the difference between the two and the relative advantages and disadvantages of each.
The 'trainspotter view' difference is that oscillating cams are driven by a lever connected to the crosshead, somewhat similar in appearance to the combining lever of a more conventional valve motion. Rotary cams are driven by an 'eccentric that isn't' on the main driver crank pin, a crown gear centered on the axle which drives a pinion. That pinion turns a long rotary shaft more or less in line with the main rod. That shaft, in turn, drives a second pinion mated to a crown gear on the outside center of the valve chamber above the cylinder.
Exactly what goes on inside the valve chamber, and how directional control and variations in cutoff are arranged, are outside of my rather limited area of expertise.
Chuck
Franklin Railway Supply first developed the oscillating cam style poppet valves gear in about 1938. Each poppet valve was opened and closed by a cam that moved back and forth through a partial turn aka 'oscillated'. The cams were driven by four sets of Walschaerts valve gear enclosed in an oil tight metal box. To get more or less cutoff or to reverse, the link was raised or lowered on the Walschaerts gear in the box which changed the amount the cams oscillated. Two sets of the valve gear drove the left and right intake valves, the other two sets drove the left and right exhaust valves.valves.
On a two cylinder locomotive the box containing the valve gear usually was in front of the cylinders. Access for mainteance was OK. On the Pennsy T1 4-4-4-4 Duplex, the valve gear box for the rear set of drivers was under the middle of the boiler. Very very bad for maintenance access.
At about the time the US entered WWII, Franklin came out with the rotating cam poppet valve gear. Here the cam shaft rotated much like that on a gasoline engine. The cams had different profiles runing the length of the shaft and to get more or less cutoff the cam was shifted lengthwise. The advantage was that all the parts were accessable from the outside of the locomotive.
For a much more information, and history, I would suggest "One Man's Locomotives" by Vernon L. Smith ISBN 0-87046-079-X
Piper106
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