Design and development of strain wave generating cam for a new concept ‘harmonic drive’

A split cam design is proposed to solve the problem of assembly of the single piece cam in the flexible raced bearing of an earlier proposed novel harmonic drive system, which shows better torque characteristics and capacities in comparison to the conventional one of same size with oval-shaped strain wave generating cam. The cam profile has circular arcs at two working zones at 180° phases. The proposed profile shape is identified as the cause of trouble in assembly if the cam is made single piece. The split cam is made of two identical pieces having circular arc edges. These pieces constitute the cam in assembly after putting it inside the inner race of the flex bearing and adjusted by an adjuster. The design, kinematics, and the assembly method of the proposed split cam are presented in this article. The split cam arrangement not only solves the assembly problem but also gives a scope of fine adjustment of center distance (eccentricity). Such an adjustment is not possible in conventional oval wave generating cam. Stresses in flex gear cups assembled with both type cams at load and no-load conditions are estimated using finite element method. Some results are verified experimentally. Although the flex gear cup with the proposed split cam experiences lower stresses at load transmitting active gear contact zones, it shows higher stresses at some non-active zones (where teeth are free of load). It is apparent from results that stresses at those non-active zones do not increase substantially with the increase in torque, as they are away from active zones.