Originally Posted by chrispare
Ok the ring grove is .420 the piston is .490 and.070wide
With the ring installed it is about .530 when it makes contact. The ring is a #011
Finished piston will still be the same .187 so as to not have to worry about any Interference issues.
Thanks for the measurements.
After reviewing the data I suggest that a design change is in order for the piston. Not a big change.
The sizing for the piston was based on a Cast Iron Ring or Delrin or Teflon.
Since we are using an O-ring (a good choice) the gland area needs to be changed so that O-ring will provide a good seal but not too tight.
Since the cylinder has a bore of .500" the correct O-ring size is the 012. It is designed for that bore, the .011 is for a 7/16" bore. If the 011 is used with the diameter .420" (inside diameter for the .011 is .301") it will be stretched over 25%. The manufactured suggested stretch is 0-5% with typical at 1-2%, the excessive stretch can overstress the material.
The next issue is the squeeze. For reciprocating applications 10-20% is generally acceptable. The squeeze using the current the piston / O-ring is over 43% ...very tight and unnecessary. Aiming for 15% would allow some variation the Bore and gland diameter.
Finally the gland width , typically in this application 15% is usually the norm. Due to the squeeze, heat expansion etc the O-ring will need that extra room.
So in summary:
Use a AS568-012 O-ring.
Inside groove diameter should be .375-.380" .
Grove width should be .081 to .085
The cylinder should be as close as possible to .500 and smooth (a little honing may be needed). All corner and holes should have a smooth radius to prevent damaged during insertion and using.
The Piston Rod Gland (item #23) and Valve Gland (item #21) needs to be reviewed to insure that it will not be too tight.