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A spring washer is widely used as a method for preventing loosening. However, experimental results presented by Sakai and Yamamoto et al., show that the performance of a spring washer as a loosening prevention mechanism is at best equal to that of a washer-less joint, if not worse. In this paper we analyzed loosening under shear loading and as a result were able to explain the mechanism that accelerates loosening in the framework of the three-dimensional finite element method (FEM). A spring washer causes non-uniformity of contact pressure at the washer interfaces, because of its asymmetric shape. When a bolted joint with a spring washer is subjected to shear loading, sticking area on the contact surfaces of the spring washer is limited to two corner points before the bearing surface undergoes gross slip. One of these points is on the upper surface of the spring washer and the other is on the lower surface. In this situation the nut rotation around these sticking points results in drastic loosening. We also conducted comparative simulation with Sakai's experimental results. Compared with experimental results, the bolted joint with the spring washer is easy to loosen according to the simulation results. It is believed that the difference comes from the spring washer's edge cutting into the contact surface of the nut, something not included in this simulation.

Self-loosening Behaviour of a Spring Washer: Three-dimensional Finite Element Method Study