Root cause analysis of bowl-mill pinion shaft failures

• This article describes in detail the approach and root cause for failure of pinion shafts used in bowl mills of thermal power plants.
• This analysis clearly indicated that ceasing of the movement of the bowl mill segment all of a sudden due to hard, rocky lumps was the main cause for failure.
• Based on the evidence available, this failure was concluded to be a consequential, overload failure.

Pinion shafts made of 18CrNiMo7-6 material, are used for transmitting torque from motor to gear box used in bowl mills of fossil fuel fired power plants. This work elucidates the metallurgical investigation that was carried out on a failed pinion shaft for analyzing the cause for failure. Fractography revealed the initiation of a crack from the keyway corner. Mechanical testing indicated that the yield strength of the material was lower than the specified value. Observation of the bowl mill at site after failure indicated that hard lumps were present in the bull ring segment, which clearly made it evident that there was sudden jamming of it which in turn led to overloading of the pinion shaft leading to the initiation of crack. A small overload fracture zone was also observed in the interior of the shaft suggesting low stress but high stress concentration torsional failure. Hence, this investigation concluded that this was a consequential failure.