Piston ring

[JimbobParticipant @jimbob]

Hi all

I have recently acquired a part finished single cylinder double acting steam engine model. I find that the piston ring is very tight in the bore of the cylinder. The chamfered ends of the ring have a suitable gap when the ring is in the cylinder, and the piston head itself has a good working clearance. The ring seems to be of brass or bronze.

Any suggestions on how to relieve the tension would be much appreciated.

[JimbobParticipant @jimbob]

[Jeff DaymanParticipant @jeffdayman43397]

Hi Jim,

Might be worth a try to take out the hard ring and pack the ring groove with either PTFE strands or graphited yarn packing. Oil it up good on assy and I’ll bet it works fine. Either of these soft packings will not injure any bore regardless of material.

Otherwise you’re into some serious messing around to fixture and re-machine the hard ring. Is the bore cast iron ?

One other item to check – is the piston moving freely at the operating temp?

JD

[JasonBModerator @jasonb]

You will have quite a bit of friction until the engine has been run in on the bench for an hour or so particularly if teh bore is a bit rough with tooling marks.

Whats the gap on the ring when its not compressed and whats the bore you should be able to work out what this gap should be, if its too much then that may explain why its tight, if its correct then see above.

J

[Ian S CParticipant @iansc]

Jim, Is the ring groove deep enough/ the ring too deep for the groove? Ian S C

[MichaelRParticipant @michaelr]

Jim, As Ian S C says the ring groove needs to be deeper than the ring thickness,about 3 thou deeper than the thickness of the ring should be about right.

Stick.

[JimbobParticipant @jimbob]

Hello, and thanks for the advice.

The ring is free in the groove, the groove suitably deep all round and the bore has no visible machining marks. Don’t know what it would be like when hot, as I haven’t got that far yet! However, when I got it, it was sufficiently assembled to try it with compressed air, needing approx. 100psi to make it run.

I think I will try the other methods of sealing, as suggested, first, and see what happens.

Thanks again

Jim

[Phil AshmanParticipant @philashman88468]

Jim, What conclusion did you come to in the end? I have exactly this problem with my loco cylinders. They’re cast iron cylinders and pistons and commercially made rings. The chassis does run on air, but the rings are so tight I’m afraid to run it any more for fear of causing damage. I don’t know whether to press on and try and run it in, or remove the rings and use soft packing instead.

Phil

[DustyParticipant @dusty]

Hi all

Remove the rings from the piston and insert them into the cylinder bore. If there is no gap or you cannot insert them then they will need re-gapping, if the gap is O.K. then they will probably just need running in, or you have problems with the depth of the ring groove.

[mgjParticipant @mgj]

Yesss – or you may have rings which are too thick. You only need about .050 D-d.to give the right wall pressure which is not a lot.

 

If you go into the Model Engineers Handbook section 11.5 its is all covered. The problem is that to get the thickness you have to resolve a bit of an equation. One of the few times that TC wasn’t all that helpful because it doesn’t have wall pressure and thickness in the right places.. It isn’t in fact more than O level arithmetic, but if one is a bit out of practise one might have to think about it..

 

Alternatively a very nice gent called Allan Beasley set it all out in 2 articles ME 29/4/2009. All you have to do is read the info off his charts, so well done him. Couldn’t be easier. His second article tells you how to make your own.

 

Alternatively, you can just send off to Stuart Models, because the rings they supply are to the Tubal Cain/Prof Chaddock low wall pressure low friction thickness – or the 3 sets that I have bought have been.So I haven’t honed anything – just a good tool finish, set a couple of thou gap and both engines (3 cylinders) whirred away like gooduns. Other ppeople may supply the same or similar rigns, but I haven’t done the maths for them so I don’t know. Those are iron rings – the formulae for iron or brnze rngs are hte same but hte vlues to be plugged in are different – see ME Handbook.

 

Check the gaps of course, and check the difference between the free gap and the installed gap. You are, for a steam engine looking for a ratio of d/10 installed to free gaps.

 

Well worth checking to get it right. Because of the maths very small differences in section of a ring make a great deal of difference to the wall pressure and hence base friction, and thats before the gas gets round the back of the ring and starts to make it seal. So its worth trying to understand the situation and getting it right.

 

 

 

Edited By mgj on 25/03/2011 18:39:50

[mgjParticipant @mgj]

Are you sure it is the ring. It all rotates freely without the ring – or is there some other misalignment or embarrasing unconcentricity? (as well?)

 

Please correct the post above: D-d to be about .100 or 50 thou on radius. Apologies

[JimbobParticipant @jimbob]

Posted by Phil Ashman on 25/03/2011 15:24:58:

Jim, What conclusion did you come to in the end? I have exactly this problem with my loco cylinders. They’re cast iron cylinders and pistons and commercially made rings. The chassis does run on air, but the rings are so tight I’m afraid to run it any more for fear of causing damage. I don’t know whether to press on and try and run it in, or remove the rings and use soft packing instead.

Phil

Hello

Everything else was free turning. Following advice from members of the forum, I replaced the metal ring with plumbers gland packing ptfe “string”. While the result may not be quite the best for power or long life, I can now move the piston up and down the cylinder just by blowing by mouth into the inlet. As I don’t envisage using the engine to power anything, I think this should be ok.

Thanks again to all who responded, your comments are much appreciated.

[mgjParticipant @mgj]

Fine – so what was wrong with the ring? Something has to be wrong with it, because with a newly fitted ring the piston should slide with steady gentle pressure. No you won’t blow it by mouth but you shouldn’t need 100psi!

 

Thus far you have cured the sympton not the disease. Its the disease thats the interesting bit – anyone can get a steam engine to run!

[Phil AshmanParticipant @philashman88468]

Gents,

Thank you all for your inputs. I stripped a cylinder down over the weekend. There were one or two other “embarrassments”, Slide bars were slightly misaligned, and the crosshead pump had gone quite stiff. But the rings still were quite tight. The gaps seemed ok, but they’re quite a bit thicker than the D-d of .1, and the bore is finish turned only, not honed, so that probably accounts for it. But there appeared to be no damage or scoring at all to the bores, so I think I’ll gently run it in for a bit, then have another look.

Phil

[JimbobParticipant @jimbob]

Posted by mgj on 25/03/2011 21:10:05:

Fine – so what was wrong with the ring? Something has to be wrong with it, because with a newly fitted ring the piston should slide with steady gentle pressure. No you won’t blow it by mouth but you shouldn’t need 100psi!

 

Thus far you have cured the sympton not the disease. Its the disease thats the interesting bit – anyone can get a steam engine to run!

The problem with the metal ring was exessive outward pressure on the cylinder.

Ring/groove clearance and groove depth was good, as was the ring gap when fitted in

the cylinder.

Hence my initial query re. reducing the tension.

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