It is critical for principal bearing stiffness for actuators to be evaluated for robot applications. The bearing stiffness calculation can be obtained from Hertzian contact theory. From a design standpoint, given the bearing stiffness, the target stiffness of the actuator shell structure and output plate may be an order higher than that of the principal bearing. This paper evaluates and compares the relationship between the stiffness of the shell structure and principal bearing as well as between the stiffness of the output-plate and principal bearing. The principal bearing for the actuator could be the crossed roller bearing (CRB), four-point bearing (FPB), or a tapered roller bearing (TRB) because they best accommodate the combined radial, axial, and moment loads. Also, the bearing weight and assembled weight (shell structure + principal bearing + output-plate) required to obtain appropriate stiffness can be evaluated based on the stiffness of these adjacent structures which hold the principal bearing. Finally, the shortest possible force path passing through the principal bearing from the shell to the output plate makes it possible to maximize actuator stiffness and reduces other negative effects.

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