Lock Washers

Lock washers are used to resist loosening of bolted joints. These washers, placed under the nut or screw head, compress as the fastener is tightened and the spring-back tension deters loosening. Also, because of their spring-like take-up action, lock washers can compensate for slight material wear. But because spring types flatten when tightened, their "locking" action doesn't come into play until the fastener has slightly loosened.