Sizing of Profile Modifications for Asymmetric Gears
Today, the benefits of asymmetric gears are being extensively discussed. They have yet to be widely adopted, but investigations are ongoing as to whether gear reducers can be improved with such techniques. The geometric definition of asymmetric gears is shortly described. The strength calculation of such gears can be calculated according to ISO standards, but the method for the bending stress must be adapted. This permits the power capacity of asymmetric gears to be calculated and compared with symmetric gears. In specific cases, when using asymmetric gears, the power capacity of a gear pair can be increased by up to 30%.
The power capacity of a gear pair is critical, but the noise and vibration behavior is also highly relevant. With appropriate profile modifications, gears can be significantly improved. A modification of the gear profile will change the load distribution during a meshing cycle, therefore changing transmission errors, contact pressure and power losses. Additionally, with tip and/or root relief, the contact shock at the beginning and the end of the meshing can be removed, substantially reducing the vibration and noise of the gear mesh.
The layout of profile modifications must be verified by a loaded tooth contact analysis (LTCA), which permits the analysis of the contact during a meshing cycle step by step. LTCA can be performed based on a finite element method (FEM) or with semi-analytical method (usually based on the Weber-Banaschek approach ). To get an optimum solution for a profile modification, it is very convenient to use a parameter variation technique. For example, tip relief and tip modification length are varied to find the best solution. For such a task with some hundreds of variants to check, the calculation time for an LTCA analysis becomes an issue, therefore, the use of the Weber-Banaschek approach is preferred. This approach was recently adapted for asymmetric gears to allow for efficient analysis.
The selection process of a steel gear with asymmetric teeth is discussed in detail. With a well selected profile modification, the noise excitation and Hertzian pressure can be reduced. The behavior of the critical parameters was verified for different torque levels and for helix and profile errors due to manufacturing tolerances.
Author: Ulrich Kissling
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