Peter

In mid gear with Walschaerts valve gear the total valve movement should be 2x(lap plus lead), so the valve should be just opening the ports at each end of the stroke by the amount of the lead. As the reverser is moved forward the valve will open progressively further till the full port opening is achieved in the fully forward ( or reverse) position .

John

Duncan, Peter

What I meant by full port opening is the design port opening which may be less than the actual port width depending on the geometry of the ports and the valve. I should have been more clear, sorry about that.

  As Duncan says the ports should remain open for about 75% of the stroke in full gear, cutting off proportionally earlier as the reverser is moved to mid-gear. The 75% figure is governed by the design of the whole valve gear, and is chosen to allow some expansive working (increasing efficiency) while still providing sufficient power to start a train from stop.

John

Edited By John Purdy on 30/01/2017 20:06:37