Official Answer
Answer
As a wave spring is compressed, its diameter increases. If it is tightly fitted in a bore, the resulting friction force $F_f = \mu F_n$ (where $\mu$ is the friction coefficient and $F_n$ is the normal force against the bore wall) opposes the spring's motion. This results in 'hysteresis' in the load-deflection curve, where the loading force is higher than the unloading force. For sensitive pressure relief valves, the bore should be lubricated or the spring diameter adjusted to ensure that the friction does not cause a 'stick-slip' condition, which would result in inconsistent valve cracking pressures.
