Drive Line Analysis for Tooth Contact Optimization of High-Power Spiral Bevel Gears

摘要

It is common practice in high-power gear design to apply relieves to tooth flanks; they are meant to prevent stress concentration near the tooth edges. Gears with crowning have point contact without load, and when load is applied, instantaneous contact turns from point into a Hertzian contact ellipse. The contact area grows and changes location as load increases. To prevent edge contact, the gear designer has to choose suitable relieves considering contact indentations as well as relative displacements of gear members. In the majority of spiral bevel gears, spherical crowning is used. The contact pattern is set to the center of the active tooth flank and the extent of the crowning is determined by experience. Feedback from service, as well as from full-torque bench tests of complete gear drives, has shown that this conventional design practice leads to loaded contact patterns, which are rarely optimal in location and extent. Oversized relieves lead to small contact area, increased stresses and noise, whereas undersized relieves result in an overly sensitive tooth contact. Today it is possible to use calculative methods to predict the relative displacements of gears under operating load and conditions. Displacements and deformations originating from shafts, bearings and housing are considered. Shafts are modeled based on beam theory. Bearings are modeled as 5-degree-of-freedom supports with non-linear stiffness in all directions. Housing deformations are determined by FEM analysis and taken into account as translations and rotations of bearing outer rings. The effect of temperature differences, bearing preload and clearances are also incorporated. With the help of loaded tooth contact analysis (LTCA), it is possible to compensate for these displacements and determine a special initial contact position that will lead to well-centered, full-torque contact utilizing a reasonably large portion of the available tooth flank area. At the same time, crowning can be scaled to the minimum necessary amount. This systematic approach leads to minimum tooth stressing, lower noise excitation, increased reliability and/or power density as compared to conventional contact design method.