The stress distribution in the holed web of a gear was studied in order to evaluate the effects of overstress caused by a reduction in weight in comparison with a gear without lightening holes. The analysed gear is a component of a helicopter principal transmission: geometry and dimensions (web with eight circular holes) are usually employed.

A 3-D mesh of the gears was made using PATRAN and a FEM analysis was carried out using NASTRAN 65a2.

Moreover the principal stress history in one revolution of the gear was reconstructed at the critical points of the web. Thus it was possible to evaluate the actual fatigue cycle and the consequent safety margin for the different load conditions under which helicopter transmissions are generally checked.

It was found that the stress variations in one revolution of the gear are significant even in constant power conditions and that the stresses at each point on the border of the holes always have the same sign in a complete revolution.

Safety margins in the web were evaluated and compared using different formulas.

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