I essentially have a piston in a tube that is sealed with an O-Ring and actuated with a solenoid.
The friction of the O-Ring however, is higher than estimated (guessed as far as I can tell) by the designer who doesn't have any calculations to show me as reference. I have searched a little with no luck. The piston is pushed back to it's original position (after the solenoid is deactivated) by a spring, but the 5 Newton force the spring provides is not enough to overcome the friction of the seal.
To get an idea of the force required I attached a bolt and added weight until the piston moved, but theoretically, is there a simple way of calculating the force required to overcome the friction of a rubber O-Ring seal?
A paper or two exists with relatively complex mathematical models, which i don't have the time to reproduce. Given how common rubber O-Rings are I'd be surprised if there weren't a commonly used formula.