An Analytical Comparison Between Ball and Crossed Roller Bearings for Utilization in Actuator Modules for Precision Modular Robots

A modular robot system is a collection of actuators, links, and connections that can be arbitrarily assembled into a number of different robot configurations and sequences. High performance modular robots require more than just sophisticated controls. They also need top-quality mechanical components. Bearings in particular must operate well at low speed, have high rotational accuracy, be compact for low weight, and especially be stiff for high positional accuracy. To ensure the successful use of bearings in precision modular robots, knowledge of the bearing properties and requirements are investigated. Background information on various topics such as modular robots, precision modular actuators, and their error sources are described with respect to precision engineering. Extensive literature on thin section bearings is reviewed to examine their use in precision robotic applications. Theoretical studies are performed to calculate bearing stiffness adopting a methodology based on Hertzian theory. This approach is applied to analyze two proposed designs of equivalent-sized crossed roller and four-point bearings, principal bearings used for transmitting all the payload and mass of the robot structure. The maximum deflections and contact stresses for the proposed actuator assembly and loading conditions are estimated and compared including a range of general bearing properties such as friction, cost, and shock resistance.