Abstract
This paper presents the failure analysis of cooler fan drive gear system of helicopter. The work involves failure analysis of two components of main gear box assembly i.e. intermediate gear and drive fan pinion. The materials of the gears are made up of case carburizing steel. The microstructure of both the components is martensite. The microstructure of un-failed teeth is finer than that of the failed teeth. The crack is observed near the root of the failed teeth. It seems that cracks are formed only at the roots of the teeth and propagated inside the material. It has been observed that the fatigue cracks are formed at the roots of many teeth during service. These cracks have been sharply revealed by etching. The large crack length introduces simultaneous failure of several teeth as observed in intermediate gear while the short crack of drive pinion results in successive failure of the same. Both the teeth of intermediate gear and drive fan pinion fail by bending fatigue due to over/shock loads although the associated mechanisms are quite different.
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