Failure analysis of a pole gin

Abstract

Lugs on a cast aluminum/fiberglass pole gin failed while raising an electrical transformer on a power pole. This same system had just lowered a heavier transformer. The pole gin consisted of a cast aluminum base that was strapped to a utility pole by a nylon belt-and-ratchet mechanism. A fiberglass pole was mounted in the base, and a pulley was attached to the other end through another aluminum casting. Rigging for the lift was complex and required a physical simulation to estimate actual lug hole loads and to determine that overall loading was within the manufacturer’s published limits. Possible abuse by hammer blows was evaluated by dynamic testing to measure force attenuation in the system. Results ruled out abuse as a factor. Literature revealed that the heat treatable Precedent 71A, or A771-T7 alloy, used for the base casting was very susceptible to stress-corrosion cracking (SCC). Evidence was observed for features indicative of creep-rupture damage on the fracture surface. Evaluation of all of the evidence led to the conclusion that time-dependent crack growth, most likely by both SCC and creep-rupture, plus the effect of bolt hole loading on crack growth could best explain the failure of the gin under a less severe condition than had just occurred earlier in the day.