In this paper, the new driving structure of three cycloid gears at eccentric angle of 120 with one another is adopted so as to get rid of the weakness for the bearing of rotating arm. Thus, under the condition of that the basic dimension is not changed, by conducting optimization of profile modification and parameters optimization on traditional structure, the axial dimension is increased by width of one cycloid gear transmitting torque is increased about 40%, and the goal of large capacity with small size is realized. Three main forces are stressed upon the cycloid gear in actual work: the meshing force between gear and cycloid gear, the force upon the cycloid gear by the pin of output mechanism and the force upon the cycloid gear by the bearing of rotating arm. The paper mainly aims to work out the force upon the cycloid gear by the pin of output mechanism.

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