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Safety valve spring length

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Garry Coles	25/06/2020 13:37:47
121 forum posts 100 photos	Hi. I am now making the safety valves for my 2 in D & NY traction engine. The springs on the drawing state 22 SWG .125" I.D. & .181" O.D. but does not give the free length. The working boiler pressure is 85 psi. The plunger overall length is 3/4 in and with the adjusting centre screwed down to the approx position, there is a gap of about 1/2 in for the spring. I think I could trial & error when the boiler is in steam, but I need a starting point. Can anyone help me please. Garry
Baz	25/06/2020 13:50:53
1033 forum posts 2 photos	Seems to me if you have bout half inch gap for spring it needs to be slightly longer so perhaps start off at five eighths long, it’s trial and error.
Paul Kemp	25/06/2020 19:24:45
798 forum posts 27 photos	See similar post for Mamod SV, same principles apply. Spring rate is the important thing, 1/2" compressed for a 3/16" spring sounds reasonable you need to make sure it won't become coil bound before fully open though. Free length can be calculated by working out the area of the valves exposed to pressure with the valve closed and multiplying by lift pressure, this will give you the load the spring has to exerpt in the closed position. The Spring rate will be the increase in force per extension or compression of the spring being used (Hooke's Law) so if you know your compressed length you can work back to get your free length. Hope that makes some sense? Paul.

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