TSME Conference System, TSME International Conference on Mechanical Engineering 2010

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Failure Analysis of a Helical Gear

Samroeng Netpu

Last modified: 2010-09-27


This paper reports the result of an investigation of a premature failure of a helical gear in a reducer gearbox used in a continuous hot rolling steel re-bars mill in Thailand. Standard investigative procedures were employed in the analysis. It was found that the gear failed by fatigue fracture. Beach marks on the fracture surfaces were clearly visible. Detail examination of the surface of the gear revealed that extensive surface damage had occurred in the form of pitting. Sub-surface damages in the form of spalling were also observed. Such observations indicated that the gear was under excessive contact stress during operation. Stress analysis did, in fact, confirm such hypothesis. These surface and sub-surface damages lead to fatigue crack initiation followed by crack growth and eventual
fracture. Excessive contact stress resulted from the replacement of the original 300 kW motor by a new,more powerful 600 kW motor in order to roll thicker billets. It is concluded that the helical gear failed by fatigue fracture initiated by surface and sub-surface damages resulting from excessive contact stress.
The lesson learned from this case is that one must be careful when replacing key components of machines or other engineering systems. The effects of such replacement must be thoroughly analysed.

Keywords: Helical gear, Helical gear failure, Failure analysis, Fatigue failure