Abstract
A general theory is developed whereby the effect of the individual component stiffnesses of an epicyclic gear train can be considered in order to determine the misalignment in the planet journal bearings and the load distribution at the sun gear and the ring gear meshes. The epicyclic gear train considered consists of a free sun gear, a free carrier and a fixed ring gear. The twist in the planets, which contain the journal bearings, is governed by the stiffnesses of the sun gear and the ring gear. The twist in the journals, which are supported by the carrier and which effectively act as a cantilever at the carrier output plate, is dependent on the stiffness of the carrier itself and the stiffness of the hydrodynamic oil film in the journal bearing. The relative twist of the journal with respect to the bearing constitutes the misalignment in the bearing. This misalignment generates non-uniform pressure distribution along the journal causing moments in the hydrodynamic film both in the tangential and the radial planes and it significantly increases the peak pressure and decreases the minimum oil film thickness. These moments are balanced by the generation of reacting moments at the sun gear/ring gear meshes and this causes the load distribution to peak towards one end which consequently augments the gear tooth stresses. In this paper the various gear train components are idealized by rotational springs. Compatibility and force equilibrium equations are used to develop the expressions for the misalignment moments. A computer program has been written and general curves developed to show the variation of the misalignment moment as a function of individual component stiffnesses. To cover a practical range of stiffnesses the component stiffnesses for a small turbo-prop gearbox were determined by using finite element computer programs based on isoparametric shell and solid three-dimensional elements. This study showed the changes to the carrier stiffness to be most effective and the changes to the ring gear stiffness to be the least effective in achieving a reduction in the misalignment in the range and the ratios of the parameters considered. The use of the method developed will result in a more realistic analysis of both the journal bearing and the gear stresses.
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