Predicting and confirming the lifetime of o-rings

We derive an empirical method for estimating the equilibrium sealing force appropriate to o-rings under their application (e.g., squeezed) conditions. Another empirical approach allows us to estimate the equilibrium compression set for o-rings once they have been released from their compressed state. Comparing the two equilibrium values for three different butyl o-ring materials aged under both laboratory accelerated aging conditions and under long-term field aging conditions indicates an approximately linear relationship between equilibrium set and equilibrium sealing force. When the results are combined with modulus measurements, the approximate linearity is shown to be consistent with a 60-year-old theory derived by Tobolsky and co-workers. These results allow us to use ∼20-year field aging results to quantitatively confirm accelerated aging predictions derived in an earlier publication. In addition, we describe experiments that allow us to quantitatively estimate that the sealing force per unit sealing length must drop below ∼1 N/cm before seal leakage is indicated. Combining this result with the accelerated aging predictions allows us to make lifetime predictions versus aging temperature for the butyl o-rings.