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

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