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Piston valve movement

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Peter Simpson 130/01/2017 17:18:17
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206 forum posts
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I'm attempting to set the Walschaerts valve gear up on my 5" gauge "Speedy". At mid gear should there be any movement on the valve piston shaft ? There appears to be to much movement and do not know what is correct. At the moment there is nearly as much valve movement in mid gear as there is in full reverse ???

John Purdy30/01/2017 18:38:58
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431 forum posts
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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 webster30/01/2017 18:56:42
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Further to JP's reply, you won't necessarily get full port opening in full gear, but the ports should remain open to steam until about 75% or more of the stroke.

John Purdy30/01/2017 20:05:22
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431 forum posts
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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

Peter Simpson 130/01/2017 20:55:08
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206 forum posts
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Hi Guy's

Thanks for the replies. Later this afternoon I removed both valve pistons and made a C/L mark on both pistons, I refitted the valve pistons. When looking down the steam inlet I could see the valves were slightly out of centre. I have now got the loco running on air. Will spend tomorrow fine tuning the motion . Onwards and Upwards.

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