Gerald Gambs, Jr., PE, MME ::::
Case Synopsis: A large, rubber-tiered straddle crane, 50 feet tall and 75 feet wheelbase, was being operated in a rail yard, loading and unloading 40 foot containers. At the end of the graveyard shift, the operator executed a "swing" turn across a triple track, in order that the only operational cab would be on the optimum side for the next shift. During the swing turn, one of the wheels buckled under the frame and the crane collapsed. During the collapse, the turntable bearing at the wheel yoke-to-frame interface ruptured.
Expert Analysis: While investigating the scene, on behalf of the Bearing Company, it was determined that the turntable bearing in question had been secured to the frame by 24 screws (3/4 inch UNC grade 8 hex-head screws). Twelve (12) of the screws had been pulled apart. Exemplar screws, manufactured by the same company, were procured and sent to a mechanical testing lab for tensile testing. It took nearly 60,000 pounds to pull one screw apart. Since 12 screws failed, the load experienced by the bearing was in excess of 150,000 pounds. Additionally, the load from the most heavily loaded screw would be shared by adjacent screws to some degree, based on the gap that opened between the bearing and the frame as each screw stretched apart. The bearing was only rated for 22,000 pounds.
Result: The Bearing Company was excused from the case due to the fact that the bearing was grossly overloaded, well beyond the design load.Categories: Case Studies