Resumo

ENDRES, W.  e  COUTLAKIS, J.. Cracks starting from Hertzian contact with friction. R&D j. (Matieland, Online) [online]. 1993, vol.9, pp.14-23. ISSN 2309-8988.

Cracks due to repeated contact stresses are found in pitting of gear teeth and in T-heads which are used to attach blades in turbomachinery. In these two examples the directions of the cracks are very different and an explanation was sought. In gears, both the normal and the tangential loads are repeating, whilst in T-heads the normal load is steady, but the tangential friction load is repeating. Crack growth theories were applied which allowed calculation of the direction of the crack path. Good agreement was found with the observed initial directions of cracks in gears leading to pitting, as well as those cracks in T-heads. The crack directions varied strongly with the coefficient of friction in gears, whilst in the case of T-heads the crack direction was the same for all values of the coefficient of friction. Size effects make larger contact areas more susceptible to failure and require lower allowable pressures. This is an important conclusion. The coefficient of friction was the governing parameter in all cases and it should be kept as low as possible.

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