Slide valves worked well with low pressure, low temperature steam applications. They had the advantage of "wearing in", rather than "wearing out"; hence they required less maintenance.<p>However, for high pressure, high temperature (i.e. superheated) steam, piston valves were better. Slide valves and their lubricants did not hold up under high-pressure steam application.<p>Hence, on Mallet articulated locomotives, piston valves were used for the high-pressure (small) cylinders, while slide valves were used for the low pressure (large) cylinders. The steam leaving the superheater and the throttle entered the high pressure cylinders first, then exhausted into the low pressure cylinders, then out the stack. Using different types of valves for each pair of cylinders was common practice not just for N&W but for Mallets in general.<p>Simple expansion articulated locomotives (i.e. non-Mallets) had four equal-sized high pressure cylinders. So they were equipped with piston valves on all four cylinders. The UP "Challenger" and "Big Boy" are examples of simple expansion articulated locomotives.<p>(There are others on this board that can explain it better than I can, but that's the essance of it.)



Surviving World Steam Locomotives
james1@pernet.net

The N&W used piston valves on both the high pressure and low pressure cylinders of most of their Y class 2-8-8-2 Mallet compounds. The intercepting valves on these locomotives supplied superheated steam to all 4 cylinders for starting in simple mode. Once they started to get up some speed they were changed to compound mode to reduce the steam consumption.<br> Roger Mitchell<br> Master Mechanic<br><br>



n0mcr@netzero.com