Piston Seals | The Hobby-Machinist

Am attempting to prototype a machine that has brass pistons running in an aluminum .955" bore It would probably simplify my life to go to a 1" bore for an off the shelf solution but that is not feasible.

Need to seal the piston. 166fpm and probably 150psi. Low friction is the most important consideration. Picked up a bar of raw teflon on ebay but am finding it very difficult to machine and hold tolerance. What about a cup like a brake cylinder? Perhaps a simple leather wiper? This thing needs to run for an hour tops to prove the concept, after that I'll either figure out it won't work or start to build a better one. O-rings in grooves and see how well it does. Should be capable of running fir an hour or more with this set-up. Temp and pressure should be no problem at all for such a short proto run anyway. Good luck and post up some pics of your project when you get it ready to run

Bob Lemo---some more info and application would help us to give you more options---if your piston is solid or thick enough to cut deeper grooves--then you could use a seal that uses an o ring to spread the seal (this type is used in Hydraulic cylinders), but simple o rings will work if your piston is a thinner wall and you are not dealing with super high pressure---it also would be good to know how much clearance between piston and cylinder wall and how long your piston is------Dave my 2 cents..
Viton o-rings can put up with a little more heat than std black o-rings.
another possibility is the use of quad-rings.
a quad ring is like an o-ring but it's cross section is ,you guessed it, square!!
they can be had at hydraulic seal suppliers and MC Master Carr
opposing hydraulic lip seals would also work as a 2 seal method.
teflon could be utilized as a compression ring similar to an internal combustion engine piston ring.
make the ring OD slightly oversize and file the ends to set a tight ring gap.
most full size car/truck engines use 3 or 4 compression rings and 1 oil wiper ring.
you wont need a wiper but 2 or 3 rings should suffice in sealing compression.
whichever road you choose.good luck
mike ) If low friction is important, then I’m not sure about any kind of O ring fitting the bill. For me, Teflon is easy to machine, MUST have a razor sharp tool bit, very important. Also have a good clearance angle(s) on the bit too. Sometimes a bur on the Teflon corners can be a nuisance. But, if you do your machining steps in proper order then it’s not. Holding size should not be a problem unless it’s thin wall or something like that. Then again, plan your machining steps for the metrology part of it…Good Luck, Dave. I think that there are better materials than Teflon. Some of the common beliefs about Teflon are not very accurate. The primary commercial use is chemical resistance. It is not strong, tough or heat resistant. If you heat it or burn it it releases deadly Florine gas. HDPE and Delrin (acetol) and other plastics specifically designed for use as a bearing will work better in many cases. If you use an aluminum alloy with a lot of copper and have a highly polished surface, you may be able to get away with not having any seals at all. I have seen model steam engines with no piston seals and spool valves are metal on metal as well.

Having the cylinder made of brass would make this even better since the parts would expand at a more similar rate with temperature changes. Also brass is a better bearing material.