Valve Springs

For my single cylinder 4-stroke, I'm copying the valve arrangement for ETW's Seal with minor tweaks. The valve area is the same - about 9/32" diameter valves and a 1/4" diameter passage. I've now made valve guides, valves, tappets and tappet guides, I need to sort out springs.

I have all but the last one or two installments of the construction series for Seal - but I can't see anywhere where he gives a specification for the valve springs.

Can anyone suggest what sort of spring load will be needed on such small valves - I have no idea if it should be a couple of grams or a couple of ounces!

Thanks

Neil

Neil,

You should be able to work out the valve acceleration from the cam profile and max engine speed. From this, together with the weight of the valve components you can work out the force required to counteract the kinetic energy of the valve gear and keep it in contact the cam.

So much for the theory. In practise you can compare it with other similar designs. What is the capacity of your single? As a start, the 10cc Matador OHV uses a 4 turns of 22 swg 1/4" OD and 3/8" long. My own design 10cc side valve, which a has a more aggresive cam than the Matador uses 5 turns of 22swg with an ID of 1/4" and 0.6" long. Both of these seem to work fine in my experience up to 10,000 rpm.

I can also let you have copies of the missing instalments if you wish.

cheers,

Rod

Edited By Roderick Jenkins on 13/01/2013 20:19:06

Thanks Rod,

My last installment is Vol. 96 No. 2405 26 June 1947, which covers the carburettor. I'll pm you my email address if you are able to scan them for me.

My engine will be about 3.5CC. the springs I have are 1/4" OD about 22 swg like the matador ones, but are about 3/4" long and more like 10 turns.

Set to what would be the likely position (with no camshaft yet I can't be precise) the inlet requires 23lbs and the exhaust 2 1/lbs! IThis seems way more than is needed. My guess is 8oz would be OK, but that is just a guess, no more.

Have you a rough idea how much force it takes to fully open the valves on one of your engines? If I work to about 1/3rd to 1/2 of that, i don't think I'd be far wrong.

Many thanks

Neil

A crude measurement with a spring balance shows about 3 1/2 lbs to open the Matador valves.

Looking at my Mastiff book, Len Mason used 22swg 1/2" long, 6.5 turns and 7/32" ID. This for a four cylinder 24cc . I've got the drawing for ETW's Kinglet: no mention of valves springs on that either- in fact the drawing leaves quite a lot to the imagination.

cheers,

Rod

Gosh! Seems my random springs aren't far off. I can trim a spring experimentally to get about 1lb - I have four.

Thanks for going to the trouble to help.

Neil

Edited By Stub Mandrel on 14/01/2013 20:24:43

Had an idea it might help, so I looked at Edgar T. Westbury's book "Model Petrol Engines", but there is no mention of the specs for valve springs on any of the engines, Ooh well its a nice wee book, does'nt tell too much about building, but it gives some ideas. Ian S C

Looks like even the great E.T.W. had a blind spot

Neil

I've been whiling away the progression of a never ending cold by reading my newly acquired ME backnumbers. In April 1945 ETW tells us that the springs for his 15cc Kiwi are: 18SWG, 5/16" ID, 6 coils in 3/4".  He gives the impression that these were just a best guess but they seem to work.  I'm a bit surprised because he usually got Dennis Chaddock to do hisdifficult sums.

Rod

Edited By Roderick Jenkins on 16/01/2013 22:10:23

ETW, redesigned and republished 20 years later (24 NOVEMBER 1960 issue, p. 635) his Kiwi engine, with weaker valve springs !
quote
"I am often severely rapped by readers for neglecting to give details of valve springs, but it is difficult to specify essential dimensions, because ultimate strength and durability depend on several factors, including not only the quality and temper of the spring steel, but also the pre-stressing of the material in manufacture. For the best results, some experiment is called for; the springs which have been found satrsfactory are 3/4 in. free length, 5/16 in. outside diameter, by 20 s.w.g., with six complete turns, not including the flattened end turns." ME, 123, p. 635, (issue 3098, 24 Nov. 1960).

I recently did a 10 cc engine (video in my album) and was surprised by the raise in performance with the progressive increase in stiffness of the springs, about 2,2 kg to open the valve now.
Cheers

https://picasaweb.google.com/113249908069844921550

Edited By gedeon spilett on 17/01/2013 08:22:48

We won't ever know who the great model engineers are if they don't put their head above the parapet. I think there are Great Experts and they are the ones who have offered us the benefit of their knowledge by writing for ME. With his style, designs and plain good sense I think ETW was the greatest, surpassing LBSC because ETW told us much of the work and reasoning behind his offerings (and by admitting that there was no definitive to the size of valve springs!). ETW was clearly also respected by his more academic peers; Chaddock and Nevil Shute who clearly, in his patrician way, partly based his hero in "Trustee from the toolroom" on ETW. In my opinion only Tubal Cain has come close to having the same authority on his subject.

cheers,

Rod