Sometime ago I read or heard that letting a steam locomotive coast along using its own momentum, throttle off, was not a good idea. Why was this?
I was reminded of this while reading John P. Hankey’s, excellent article about roundhouses in the March, 2010 issue of TRAINS magazine. A picture on page 31 shows a worker adjusting the “drifting valve” on a locomotive mounted above the cylinder assembly.
Did this valve have something to do with allowing the locomotive to coast or drift?
Nick
Yes, the drifting or snifter valves prevents the pistons from forming a vacuum in the cylinders when steam isn’t being applied.
As I understand it the issue is that you are admitting no steam so when the cylinder works it creates a vacuum and will, when the exhaust stroke happens, suck in cinders and oil and such from the smoke box.
I recall this is also called a Wagner valve.
Note that in spite of the obvious issues for the longevity of the locomotive, some hoggers used this as a sort of dynamic brake when coming down grade.
Nick
Lubrication in the form of a tallow was carried by the steam to lubricate the valve and pistons/packings. The snifter valve was also used for when the locomotive was light stationary. It prevented the steam to get past a leaking valve and cause the loco to move by itself. Steam leaking into a cylinder would over power the independent brake. With better lubrication in latter years the wear from drifting/coasting was minimal. Often times pushers when cut off would just coast back down the hill and wait for the next train.
Pete
Thank you very much to cprted, benburch and Pete. I appreciate that you took the time to reply to my question.
Nick