Failure investigation of a crane gear damage

Spiral bevel gear, used in a crane drivetrain, experienced wear, damage and fracture during standard operation. Experimental and numerical procedures were combined to determine the origins of the failure. Experimental procedures consisted of visual examination of the gear, optical and electron microscopy of the teeth, analysis of chemical composition of the material, testing the mechanical properties of the material and hardness testing. Numerical procedures employed finite element analysis to determine stress, strain and contact stresses of the gear pair. Based on this, time-varying mesh stiffness is calculated for undamaged and damaged gear. Obtained results suggest that subsurface material inclusions, detected below gear surface, served as initiation points for cracks leading to failure mechanism that corresponds with tooth flank fracture. Excessive stress that provoked crack propagation was probably due to gear shafts misalignment resulting in teeth mismatch. Numerical analysis offered acceptable values of stress distributions, but extensive surface pressure in the region of the pitch line, generates the collapse of the ligament between subsurface crack tip and contact surface.