Explaining 'Cut Off'

This forum is for all off-topic (ie. non-railway) discussion.

Moderators: 52D, Tom F, Rlangham, Atlantic 3279, Blink Bonny, Saint Johnstoun, richard

Hatfield Shed
LNER A4 4-6-2 'Streak'
Posts: 1664
Joined: Fri Dec 23, 2011 3:34 pm

Re: Explaining 'Cut Off'

Post by Hatfield Shed »

PiotrJames wrote: Mon Jan 25, 2021 10:41 am ... I assume "using the reverser" to control cut off means that it would be used either forward of "zero" (when in forward gear) OR after zero (when in reverse gear) i.e your are "using" the reverser but you may not be in reverse...
Correct. The earliest steam locos had separate valve drives for forward and reverse, one of which the driver engaged as required for the direction of movement.Then 'someone' on Stephenson's team realised that the two drives could remain coupled to the valve rod via a moving link which enabled the valve motion - and thus the engine action - to be reversed without any disconnection and reconnection of the valve drives.
PiotrJames wrote: Mon Jan 25, 2021 10:41 am ...Also, using the motor car gear analogy, sounds like you have to manipulate the lever or screw straight through the higher gear positions to get to the first gear position to get the locomotive going, either forward or reverse, and then pull back to the higher positions from there?
Got it in terms of the gear movements. Stand on a terminal platform to watch an outside Walschaerts gear loco arrive and the driver wind the gear into position to reverse the loco away.

But it's unlike the motor car gear for an internal combustion engine in a very significant way. The steam engine is a mechanically fixed gear transmission; what this gear varies is the percentage of the piston stroke during which steam is admitted: most of the stroke for reliable starting so that the engine doesn't go 'blind', less of the stroke for more efficient expansive working as speed is worked up.
Post Reply