Patent ID: 7172381

Claim:
An unfastening prevention device attachable to a screwed up fastener, comprising: a cap covering at least a torque-receiving portion of the fastener axially from one side, the cap having a groove on an inner surface and a widened part on a portion of the inner surface close to an open end, an inner diameter of the widened part gradually as it goes toward the open end; a spring biased outwardly so as to engage an outer periphery thereof into the groove formed on the inner surface of the cap, the spring being able to slide along the inner surface of the cap except for an area of the groove and being able to decrease its diameter due to compression from outside by the widened part: a restriction device for the spring axially restricting movement of the spring; and a first elastic body located inside of the spring restricted by the restriction device, the first elastic body biasing an inner surface of the spring radially outward.