Characterization of dynamic friction in MEMS-based microball bearings

Rolling element bearing is a well-known concept in macroscale machinery applications. They are prospective candidates for friction reduction in microelectromechanical system (MEMS), as well as for providing stable, robust support for moving micromechanisms. The characteristics of rolling element bearings need to be investigated to facilitate their applications in MEMS. It is well understood that the measured data on the macroscale cannot be directly applied to the microscale. This paper presents an in-situ noncontact experimental system to characterize the friction behavior of microball bearings on the microscale. The methodology presented in this paper provides a useful template to study the dynamical behavior of linear microball bearings with a variety of materials, geometries, and surface qualities. The system, actuated by a motor, affords wide ranges of motion for measuring the dynamic friction using a vision system. It allows the determination of the coefficient of friction (COF) without any interference due to the measurement system. With careful optimization, the error in measurement has been reduced to 2%. Different designs of microball bearings are proposed to achieve lower friction. The studied microball bearings demonstrated an average static COF of 0.01 and an average dynamic COF of 0.007 between stainless-steel and silicon-micromachined contacting surfaces at 27°C and 40% relative humidity.