At present, the existing problems in gear volume optimization include: (1) the clearance volume caused by the bottom clearance and addendum modification is not considered in the volume models which causes the error of volume calculation. (2) Gears employ fixed structure and their structural changes caused by the dimension changes are not considered in the process of optimization. In view of the above mentioned problems, the paper firstly deduces the formula of clearance volume. An example is provided to verify that the clearance volume should be considered in the accurate calculation of gear volume. Secondly, volume formulas of different structure of spur gear including solid structure, web-type structure, spoke type structure with cross shape section and with H-shape section are deduced. The design variables, objective function and constrains are briefly introduced. Finally, a single stage spur gear train is as an example, the optimization is carried out. The result shows when the transmission ratio is changed from 3.2 to 2, the structure of gear changes from the web-type to the solid. It means gear with fixed structure is not reasonable in the process of optimization.
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