Vision-Based Microtribological Characterization of Linear Microball Bearings

Microball bearings can potentially provide robust and low friction support in micromachines such as micromotors and microgenerators. Their microtribological behavior needs to be investigated for design and control of such micromachines. In this paper a vision-based, non-intrusive measurement method is presented for characterization of friction in linear microball bearings. Infrared imaging is used to directly observe the dynamics of microballs and track the motion of bearing components. It is verified that microballs roll most of the time with occasional sliding or bumping resulting from fabrication nonuniformity. The friction-velocity curve demonstrates evident hysteresis. The dependence of frictional behavior on several factors is studied. INTRODUCTION Microball bearings have potential to offer robust and low friction support in micromachines, such as micromotors and microengines. A careful study on their microtribological behavior is necessary for proper design and effective control of micromachines based on such bearings. A linear microball bearing structure was proposed and its static coefficient of friction (COF) measured by Ghodssi et al [1]. A vision-based experimental setup was developed to study the dynamic friction of linear ball bearings by Lin et al [2], where the underlying friction model consisted of the Coulomb friction only. As a continuation of the work in [2], this paper contains indepth characterization and understanding of the microtribological behavior using infrared imaging, scanning electron microscopy (SEM) imaging, and analysis. The contributions of this paper include: a) verification of rolling motions of microballs by directly observing trajectories of all bearing elements (stator, slider, balls); b) observation of the hysteresis between the friction and the relative velocity; and c) investigation of the influences of the ball number and oxide formation on the frictional behavior. EXPERIMENTAL SETUP A schematic of linear microball bearings is shown in Fig.1. The bearing consists of two silicon plates (slider and stator) and stainless steel microballs of diameter 285 μm. Two parallel Vgrooves, which house the balls, are etched on the plates using potassium hydroxide (KOH). In the experiments the stator of the bearing is mounted on an oscillating platform driven by a servo motor. The only force acting on the slider is the friction applied by the microballs. By collecting motion data of the bearing elements through a CCD camera, one can infer the friction dynamics inside the system. For a more detailed description of the basic setup, see [2]. Several improvements have been made to upgrade the system reported in [2]. A camera (Sony DCR-TRV22) with night-shot mode, a finepositioning stage, a 24 X magnification lens, and a halogen bulb as an infrared source enable one to see through the slider and observe the microballs closely. Figure 2 shows a picture of the experimental setup. Fig. 1. A schematic diagram of linear microball bearings [2]. * Corresponding authors. X. Tan is currently affiliated with the Department of Electrical and Computer Engineering, Michigan State University, East Lansing, MI, 48824. Email: [email protected] (X. Tan), [email protected] (R. Ghodssi). Proceedings of TRIB2004 2004 ASME/STLE International Joint Tribology Conference Long Beach, California USA, October 24-27, 2004