Re: Slide valve versus other valve gears

Slide valves use the boiler steam pressure to force the valve against the valve seat, which is a sliding joint. This worked well enough when boiler pressure was not real high but as boiler pressures increased, it required more force to actuate the valves and valve and valve seat wear increased. Piston valves contain the boiler pressure within the spool (the valve body) and the pressure does not force the valve against the valve seat so they are easier to move and do not wear as fast.

The primary knock against Stephenson gear is its location between the frame rails. This makes maintenance more difficult because of bad access and therefore, more expensive. Again, in the early days, labor was cheap but it got more expensive as life moved on.