Poppet Valves - Trains Magazine
On the T1, both the oscillating cam and rotary cam drives were mechanical. Oscillating cams took their motion from a gearbox containing what amounted to a pair of miniature Walschaerts valve gears. The valve drive cam moved through a small arc but did not fully rotate 360 degrees. Cutoff was controlled within the gearbox mechanism.
As the name suggests, on rotary cam drive, the cam operated more like those in IC engines, i.e., it rotated 360 degrees and the valve timing was controlled by the contours machined into the cam. Others will have to fill in how the cam position was varied to effect cutoff positions. I know it was done, but don't know exactly how. I'll keep looking for a good explanation.
Some early poppet valve gear were steam actuated, but I never heard that they were successful, at least in the US environment. PRR had an L1 so equipped IIRC.
No. 5500 was converted to rotary cam valves 7/3/48. Some of the enginemen considered it the best of the lot. No. 5547 followed about a year later entering service 7/30/49 as a T1a. This was actualy very close to the original duplex concept as designed by Baldwin in the late 1930's before modifications for PRR in 1940-41. The crews have said they couldn't tell any difference in performance between 5547 and the rest of the T1s, at least at normal operating speeds.
The oscillating cam drive, as it was installed on the T1 was a mechanic's nightmare. Although the drive boxes themselves were considered fairly reliable, their location was about as inaccessible as one can imagine and routine mainenance suffered accordingly. The front box was under the "hood" of shrouding on the pilot deck, behind the aftercooler and between the air pumps. On a conventional locomotive with Type A gear, the drive box usually sat exposed on the pilot beam and was much easier to get to. For the rear engine set, the box was located behind the cylinder saddle and positioned vertically between the frames. Nice place to work. Oh, and just for further complication, it was a mirror image of the front drive box. Both the Walchaerts equipped 5547 and the outside drive rotary cam 5500 were vast improvements regarding accessibility and probably maintenance cost.
