O ring Pistons cylinder Clearance?

Looking for advise and experience on O ring/piston/cylinder clearances on cylinder bore, piston size and O ring groove dimensions on steam design vertical engines using compressed air. Cylinder is Cast iron, piston is Delrin and O rings are black rubber. As this is my first attempt using this combination any assistance and advise would be appreciated.

I've never used delrin or rubber rings, usually aluminium 6082 and Vitron O rings.

For air running allow 0.001" per 1" diameter bore, I don't generally measure this but just go by feel. If made well this will allow you to run the engine slowly with no load on a couple of psi of air WITHOUT the ring, just put the ring in if you want to load the engine.

At the very least hone the bore but preferably lap it.

Edited By JasonB on 25/09/2018 18:36:23

Hi Robert

I suggest you download a seal manufacturer's design guide where you will find advice on the design of housings for seals, including grooves for O rings. I have a guide from James Walker , would not be without it.

Andrew

if you go down the small clearance no ring route machine some vee shaped grooves in the piston. I'm not sure whether it acts as a labyrinth seal or just traps a film of oil but it seems to work by all accounts

The problem with using the size guides from O ring manufacturers is that they tend to be for industrial applications at higher pressures. This means the ring is compresses more and quite tight in the bore which will not make for a smooth slow running engine. Best to use the tables in Model Engineers Handbook or in Reeves paper catalogue which give figures that suit our needs.

+1 for the "no ring" sealing method described by Duncan Webster.

After reading the explanation of how this labyrinth seal system works in Henry Greenly's excellent book "Model engineering dating from 1915 (pages 117-119) I have used it for several model engines with great success.

Eric

I agree with Jason about the use of manufactures sizing/groove charts, when I was making dive cylinders for my model submarines I found that when the grooves were cut accurately in pistons, the O-rings had too much drag for the motor size and gear train that I was using. It would have been different if the cylinder was pumping oil which is a better lubricator than water, as they worked okay when nicely lubed and on the bench just shifting air. I think that it depends on the operating pressure of your engine and the lubrication provided .

Trevor

You probably already know this but the coefficient of thermal expansion of acetal is quite high so even small changes in temperature can cause significant dimensional change. This has caused me problems in the past using Delrin as a piston material.