Allchin safety valves

[Ed DeeParticipant @eddee]

I'm proceeding quite slowly building a 1 1/2 Allchin and am now working on the safety valves and I’m having difficulty with the spring. The plan indicates that the spring should present a load of 7.516 lbs at a length of 11/16 (0.6875) inches; this seems correct for a 100lb/sq in boiler pressure with two 7/32 safety valves.

I have two springs purchased for Reeves of the correct 1/16 inch square wire and with the correct number of coils; they vary a little in length but lets concentrate on the shorter one – which has a free length of 0.870 inches. Borrowing the weights from the kitchen scales, 7lbs compresses the spring to around 0.768 inches – so that the force required to achieve a compressed length of 11/16 inch would be closer to 15 lbs rather than the required 7.5 lbs

Given that the space into which the compressed spring (together with its nut and cap) has to fit is only 0.75inches, the compressed length cannot really be longer than the 11/16 inch – in which case the spring pressure would be far too great (not to mention the problem of actually assembling it)..

Have I made a stupid mistake anywhere? Have others encountered a similar problem – and what was the solution? I was considering mounting the spring onto an arbour and grinding down the outside to reduce the dimensions of the wire.

Incidentally I purchased a second spring in case the first was a reject – but the second has a free length of 0.912 inches.

Any advice will be welcome!

Regards

Ed Dee

[Ed DeeParticipant @eddee]

Problem with the spring size for 1 1/2 Allchin

[Sub MandrelParticipant @submandrel]

Shorten the spring!

Neil

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hasn't the Allchin got standard Salter / Ramsbottom type valves with release lever and the centre spring in tension to hold the lever down ?

MikeW

[JasonBModerator @jasonb]

I think I would go with Neils option and shorten the spring as the adjustment on the screw does not allow for having it much longer. However I would not go rushing to shorten it now do a test once the engine is complete so it blows off when required

[Ed DeeParticipant @eddee]

Shortening the spring is not high on my list of options.

The spring as supplied is 'correctly' made with 'closed' and 'ground' ends – as well as having the correct number (7) active coils (closed ends have the pitch of the end coils reduced so the end of the non-active coli touches the last active coil)

I'm not quite such a rivet counter that I would be over upset to have 6 active coils rather than 7 but If I shorten the spring and grind the ends flat I still lose one of the closed ends – which I would really prefer not to do.

I suppose I could soften the spring and try reshaping it – either by reducing its free length or (if there would not be sufficient compressibility), shorten it and re-close and re-grind the shortened end and then re-harden the spring but its a bit outwith my comfort zone.

The softening the spring option seems more adventurous to me than reducing the spring rate by reducing the wire diameter by grinding the outside

Regards

Ed Dee

[Ed DeeParticipant @eddee]

Hi Michael

The allchin safety valve has a release lever with a central spring but it is a compression spring rather than a tension spring. The spring , together with its cap and retaining nut fits between the top and bottom of the casting that contains the safety valves. As JasonB says there is not a lot of free space – 3/4 inch in total.

Regards

Ed Dee

[John BaguleyParticipant @johnbaguley78655]

Hi Ed,

I machined up several parts for a chap for his Allchin, including the safety valve assembly. I fitted the spring supplied by him which was the one from Reeves. He subsequently successfully steamed it but made no mention of the spring being too strong when he came to set the release pressure so I assume it was fine.

As Jason suggests, I'd leave it as stock until you get around to a proper steam test but you could test the valves on air before that to give a rough idea of whether the spring is ok as is.

John

Edited By John Baguley on 23/06/2013 18:21:55

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