O-Rings on Pistons

[Wrist Pin]

If you read my introduction you will know that I have been out of the machining trade for a number of years. But like riding a bicycle, one can get right back into the game and carry on.

Many years ago, more than I care to admit, I started a two cylinder something or other. Based on a slug of hex aluminum for a block and two air cooled jugs, it was going to be a airplane fueled two stroke, a model air compressor, an air powered twin, a four stroke valved engine and just about anything else I could conjure up over the years.

Now I want to finish it off. The cylinders are sleeved with bronze bushings and mic around .800". I started making pistons but the bores are tapered. I could hone them out but there are holes drilled for exhaust ports that I no longer want to use. Now I have decided to sleeve the sleeves to close off the exhaust ports, bore and hone the cylinders to .750 and use pistons with O-rings. It will now become a compressed air engine to join another on on my shelf.

In my O-ring kit, I have .750 O.D. O-rings. What bore diameter would give me a snug running fit? Does the bore have to be a "mirror finish"? I figure .005 under size for the aluminum pistons. Anyone have any nuggets of experience with O-ringed pistons they would like to share?
Thanks in advance.

 

[picclock]

I've never made one but in the course of some research found that they have a high coefficient of expansion.

One of the solutions to this was to cut the ring groove depth oversize and allow the ring to float. The sealing is achieved by pressure on the upper and lower faces of the ring, and any ring expansion is in a radial direction. The recommended ring gap is fairly loose, according to the article from memory.

I'll see if I can dig up some more info or a link on the article.

Best Regards

picclock

 

[stevehuckss396]

It will now become a compressed air engine to join another on on my shelf.

If I were in your shoes I would sleeve it and make a piston between 1 and 2 thousands smaller than the bore and run it without a ring. In a compressed air engine the ring is not nesessary and will cause un-needed friction. I have 6 or 8 air engines with bores between 1/4 and 1 inch. None of them have rings.

The engines have

Aluminum bore with delrin piston
Brass bore with aluminum piston
Aluminum bore with brass piston.

I lean to the delrin or aluminum piston because there is less weight moving up and down.

Good luck!!

 

[Brian Rupnow]

Stevehucks is right---But---If you look at my thread on the Kerzel build, there is a spec sheet on groove diameters for viton o-rings used in a 3/4" bore in my thread. The Kerzel is an internal combustion engine, but the o-ring seal technology will be the same. I have built 10 "air/steam" engines, and never found it necessary to have rings on any of them.---Brian

 

[shred]

I put o-rings on my Paddleducks pistons. It runs at much lower air pressure now. The key is to only squeeze the ring 10% or so, we're not looking for a fluid-tight fit.

 

[Wrist Pin]

Hi picclock
A link would be appreciated, thank you.

Steve
My vertical twin has steel pistons running in bronze bushings without rings and it runs quite nicely. I could do the same with this one as well. I just figured I would try something new. See what I could learn. Perhaps I could try the delrin.

Brian
Do you have a link to the Kerzel?

Shred
How smooth did you make the cylinder walls to run an O-ring?
Thanks guys!

 

[Brian Rupnow]

http://www.homemodelenginemachinist.com/index.php?topic=10831.0---Brian

 

[Wrist Pin]

Thanks Brian
I have "speed read" up to page 15 and bookmarked page 13 where the o-ring info lives.
I see I'm not the only one who has "trials & tribulations" while on the machining trail.
You do great work! Thanks for the link!
Jim

 

[shred]

Shred
How smooth did you make the cylinder walls to run an O-ring?

The usual reamer-finish, maybe with a little fine sandpaper. For an air engine as long as it's reasonably smooth, it'll work ok.

 

[n4zou]

Just cut 3 oil grooves in the piston. You need oil lubrication anyway so duel purpose it. Simply use a 60 degree single point threading tool and make the grooves about .005" deep. Oil will collect in the grooves and pressure differential between the grooves will cause a near perfect seal as long as you have between .002 to .003 clearance between the piston and cylinder walls.

 

[picclock]

Hi Wrist pin

Link as promised

http://www.rlhudson.com/O-Ring Book/designing-dynamic.html

They have a lot of useful oring stuff there

Best Regards

picclock

 

[Wrist Pin]

Thanks Guys!

 

[Wrist Pin]

Hi Guys
OK, I'm a bit further along. I turned aluminum plugs and pressed them into the bronze bushing cylinders. Drilled them and used a sharp .750 2-flute end mill to create new bores. Darn, if they didn't come out nice! Taper is about .0003".

For pistons I used some white plastic stuff my Dad had laying around, (I took most of his raw stock when he passed, God only knows what half this stuff is!). It turns nice but is not thermally stable. I turned one a tad under size and it was tight in the bore. I got called away from the shop for a few minutes and when I came back, it fell in the bore. I think it's nylon.

Last night I put one piston in he freezer for a while as I got some tooth paste. Took the piston out of the freezer, coated it with tooth paste and ran it up and down the cylinder. As the piston warmed up it got tighter in the bore. Stop, rinse, repeat. This is terribly slow. Anyone have an better ideas???