U-Cup Friction Calculation

[magusinp]

I am designing a product using U-cups in a double-acting piston seal. The piston diameter is 1.5625" and the pressure on the face of the piston is about 3 psig. The only force activating the piston is the 3 psig on the face. The device is operating in SAE 30 oil so there is enough lubrication. The prototype pistons will not run so therefore the U-cup manufacturer's recommended clearances are too tight.

Does anyone know what the running and break-out friction formulas are for U-cups? If I can't calculate the friction I'll have to just loosen the clearances by guess-and-by-golly. Maybe 10% at a time. I found formulas for O-rings but none for U-cups.

 

[CESSNA1]

magusinp: The force on the piston @ 3psi is about 5.75 pounds. The pressure will also push the sides of the u cups against the wall of the cylinder and offer a restaining force of uN. If uN is around 5.75 pounds the piston will not move under pressure. do not count on the oil to make things slippery. In this instance with close clearances and thin oil films it may offer substantial drag. pull on teh cylinder with a scale so you can measure the actual force. Then go from there

Regards
Dave