On the redesign of a shear pin under cyclic bending loads

An integrated motor and gearbox package drives a 3.9 kN flocculator train at a water filtration plant. The 4–20 rpm output shaft turns the flocculators at 1–5 rpm. Mounted on this shaft is a torque limiting coupling, utilizing a SAE 1040 steel shear pin. Analogous to a fuse in an electrical circuit, this shear pin fails if the flocculator paddles jam, thus protecting the equipment from overload. With the current design, a shear pin only last about one week. Analysis suggests that cyclic fatigue, caused by misalignment between driving and driven coupling elements, causes premature pin failure. A redesigned shear pin made from PH 13-8 Mo stainless steel, processed to improve fatigue resistance, is described.