Premature Failure of an Industrial Mixer Timing Gears

A systematic and practical methodology was adopted to determine the root cause(s) of the premature failure of a pelletizer mixer timing gear. The investigation activities covered all possible causes of failure and included field examination, interview of engineers and operators, lubrication analysis, metallurgical examination. Fracture surfaces and microstructure of gear material were examined, and hardness profiles were developed. Analyses of shaft misalignment and teeth profiles were performed and found to be within acceptable limits. Results clearly indicate the surface hardness deficiencies in many locations specifically at center of the driven gear, area of severe pitting. The developed hardness profile for all locations is lower than that specified by the manufacturer. Fractographic analysis revealed that failure occurred by pitting followed by crack propagation. A number of cracks are seen to branch in different directions indicating the presence of high contact stresses combined with weak surface strength. Bending fatigue and pitting fatigue stress calculations revealed that the safety factor under contact is well below the desired value.