Fall-Prevention Device Shoulder Bolt Failure

Fall-prevention devices are ubiquitous in both recreational climbing and occupations where falling is an identified hazard, including tree care and tower climbing. One such device commonly used by arborists, called a “rope-grab,” is a simple cam-loaded device permanently affixed to a rope that allows the user to shorten or lengthen their rope lifeline. Rope-grab devices are typically constructed of three main components: the body that houses the rope, the cam that pinches the rope, and a fastener that connects the two and allows the cam to pivot in the body.

We investigated the failure of a rope-grab device shoulder bolt fastener that allegedly occurred during normal operation and use of the device and resulted in the user falling from a roof, several stories in the air. Fractography indicated a ductile overload failure of the bolt. The loading scenario during the alleged use of the device was considered and the stresses on the bolt were analyzed using a finite element model. Even with a factor of 10 applied to the assumed loads and a less than favorable loading condition applied to the bolt, the stress analysis indicated that the stresses on the bolt at the location of the crack initiation were well below the fatigue threshold stress and the yield strength of the bolt. Based on our analysis, failure of the shoulder bolt did not occur due to normal use of the rope-grab device.