The influence of bearing misalignment on the performance of helicopter gear boxes

The misalignments in the bearings of a gear box affect the life of the bearings and the deflection of the tooth contact points of the gears and consequently the performance of the gear box as a whole. Owing to the interaction of the deflections of all component parts of a gear box such as gears, shafts, bearings, spacers etc. it is necessary to consider these in combination rather than individually. A computer program was developed for analysing typical helicopter gear boxes with cantilevered housings in which the gear shafts are supported by taper roller bearings. In this program such factors as the misalignment of the bearings, the torque transmitted, the bearing preload, the rigidity of the casing and shaft complete with the bearing spacer, the spacing between the bearings, the location of the external load and the elements of elastic deformations in the bearings are considered. The particular assembly used in this paper is based on a Wessex tail rotor gear box, although the program developed is of a general nature.The influence of the misalignments between the bearing races that may be present before the application of an external load is investigated in this paper. The effect of crowning of the rollers or race tracks, to reduce the edge loading, on the deflection components of the gear tooth contact points is also included in the investigation.