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Drive Line Analysis for Tooth Contact Optimization of High Power Spiral Bevel Gears
It is a 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 crownings have point contact without load. 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, 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 crownings are used. The contact pattern is set to the center of active tooth flank and the extent of crownings is determined by experience. Feedback from service, as well as from full torque bench tests of complete gear drives have shown that this conventional design practice leads to loaded contact patterns, which are rarely optimal in location and extent. Too large relieves lead to small contact area and increased stresses and noise; whereas too small relieves result in a too 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-DOF 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, crownings can be scaled to the minimum necessary amount. This systematic approach leads to minimum tooth stressing, lower noise excitation as well as increased reliability and/or power density as compared to conventional contact design method.
During recent years ATA Gears Ltd. has gained comprehensive know-how and experience in such analyses and advanced contact pattern optimization. The methodology and calculation models have been verified in numerous customer projects and case studies.
ISBN: 978-1-55589-990-5 Pages: 14 Authors: J. Rontu, G. Szanti and E. Mäsä
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