O-rings on air engines??

[Brian Rupnow]

In my never ending quest for small engine knowledge, I have seen a few fellows post about using o-rings on the pistons of compressed air engines. Not Viton, which I have used succesfully on i.c. engines, but just plain garden variety buna-m black rubber or urethane o-rings. I have installed and honed the cylinder liner in the current overcrank engine I am building, and I find that the cylinder has a finished bore that will allow .624" to enter it, it will not allow .628" to enter it. Unfortunately, my brass piston mikes at about .616" dia. I now have the choice of A-making a newer larger piston, which is not really a big deal, or B--putting an o-ring groove in the existing piston sized for a "dynamic" o-ring application. I kind of like the idea of trying something new. I have a dozen air engines I have built which run very well with no rings at all, but this undersized piston situation gives me the opportunity to try a new solution. It also seems to me that a rubber o-ring will allow for a small misalignment where the piston rod passes thru the cylinder end cap without binding. I seen Chuck Fellows post something about "discovering" o-rings just the other day, but I don't know if that was a static or dynamic application. So---what are the opinions from you engine builders out there? The only down-side that occurs to me is that the "wear life" of a rubber o-ring will never come close to that of a lapped brass piston. Thats okay---If it wears out quickly, THEN I can build a new piston.

 

[rkepler]

I've used plain round section and quad-rings, both to good effect. The quad-rings seem to give better sealing with less friction, but that might just be wishful thinking. The quad-rings may be installed without the slack needed for an o-ring to roll back and forth in the groove.

 

[Longboy]

Do air engines use a lube for the top end like I/C does with the WD40 blend but rather may be.......air tool oil in the air stream? I'm really pleased with the O-ring pistons in my low performance pair of I/C engines. I would say with a top end lube and no combustion heat you would be hard pressed to wear one out! Dave.

 

[dvbydt]

Brian, in my experience you only need 5% squeeze on the "O" ring, not the recommended 10%, otherwise the friction is too great. Yes you do need some light oil, but as this stays in the groove, not a lot.

Ian

 

[tel]

I know a lot of people have success with them, but I'm afraid I don't like the use of O rings in dynamic situations - I'll stick to my twisted plumber's tape wound in to the groove!

 

[Blogwitch]

It is perfectly OK to use O-rings on an air driven piston, I have used either Viton or Buna, depending on what I had in my boxes, but DO NOT USE SILICONE RINGS WITH WD40 (or anything similar) lubrication, you will end up with a sticky mess.

Your main problem is the slack piston. If you use just one O-ring at the top, you will be liable to get piston slap, where the piston will wobble side to side at the bottom end as it is going up and down, and so wear unevenly, both the piston and bore can end up oval shaped. To cure this, if your piston skirt towards the bottom of the piston is thick enough to cut another O-ring groove, then another ring there will cure the problem.

They are usually a little tight to begin with, but once they have worn their flat running face on the outside after say one or two hours running, they will then usually last for years.

I use just a drop of standard engine oil to lube not just the outside, but the inside as well. Just one drop into the engine intake will give you hours of trouble free running.

 

[Brian Rupnow]

Thank you, gentlemen for your answers. Since its about 6 miles from my house over to the o-ring suppliers, I am going to have a go at a larger plain piston this morning. If I am not pleased with it, then an o-ring is in my immediate future.---Brian

 

[Dale]

I was wondering if knurling the bottom of the piston would work,say from the ring land on down? It has been done on full size applications. it could save on making a new piston

 

[Brian Rupnow]

I was wondering if knurling the bottom of the piston would work,say from the ring land on down? It has been done on full size applications. it could save on making a new piston

Thought about that one Dale, but the entire piston is only 1/4" long. It was quicker and easier to make a new piston. ----piston is only 5/8" diameter.

 

[Dale]

Wow! I was visualizing a complicated piston that took hours to build. Machining a new one was a good idea. BTW how in the world do you fit an o ring groove and a wrist pin hole in a piston that's only 1/4 inches long? Must both be small diameters

 

[tel]

No wrist pin (gudgeon) Dale - most steamers are more or less a plain disk with oil or ring grooves and the rod rigidly attached! The function of the wrist pin is carried out by the (external) cross-head.