How Anti-Loosening Nuts (Hard-Lock) Work

Ever since the screw and nut were invented around 1550, engineers have struggled with loosening — countless accidents have been traced back to a single screw that worked loose and fell out.

After plenty of inventive thinking, three mature anti-loosening approaches have emerged: friction locking, mechanical locking and permanent locking.

Each method has its own trade-offs. Permanent locking is non-removable; friction and mechanical locking are removable. Common mechanical methods include stop washers and split (cotter) pins. Common friction methods rely on washers and self-locking (prevailing-torque) nuts. Hard-lock style nuts combine these principles to maintain clamp load even under severe vibration.