Re: Inside-/Outside-Admission Valves

Hi, Dan -

Outside ADMISSION valves are like those on #315 - a "D-" or "Hat-" shaped valve
which slides on top of the ports inside a square valve chest. The steam enters on
top of the valve, and is admitted outside (to the front of the cylinder when the valve
slides to the back of the chamber, and to the back of the cylinder when the valve
slides to the front of the chamber). All of the existing ex-D&RGW 2-8-2's have piston
valves, which have a spool-shaped piston inside a cylindrical chamber. The steam
enters the center of the spool, and is admitted to the front of the main cylinder when
the valve piston is to the front, and to the back of the main cylinder when the valve
piston is to the back of the valve chamber.

Since slide valves (like on #315) were an old design, and the Walschaerts valve gear a more modern design, it was relatively rare to find the two used together (as on C&C #1 and C&C/SP #18). An even odder example is the D&SL 200-series 2-6-6-0 mallets - true compounds - which had outside valve GEAR - Walschaerts - on both the front (low pressure) and rear (high pressure) engines. These used INSIDE ADMISSION piston valves on the high-pressure (rear) cylinders and OUTSIDE ADMISSION slide valves on the low-pressure (front) cylinders!

- Russ