Rotary micro-ball bearing designs for MEMS applications

• Main Author: Hergert, Robert J.
• Other Authors: Holmes, Andrew
• Published: Imperial College London 2013
• Subjects: 621.381
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.693955

Micro-electro-mechanical systems (MEMS) technology allows the fabrication of small mechanical systems in silicon using standard micro-fabrication pro- cesses. MEMS techniques have found wide acceptance in such devices as ac- celerometers, micro-mirrors, resonators, probes, and micro-tweezers to name a few. Though small linear motions are common in MEMS applications, few devices exhibit reliable rotary motion. This work explores several methods of fabricating rotary bearings using micro-balls as the support mechanism. Micro-ball bearings have several advantages over other MEMS bearing tech- nologies in that they provide robust mechanical support, require no external control systems, and basic designs require very few fabrication steps. Ball cages or retainers are common in macro-scale bearings, providing uniform spacing between the balls. Several cage designs are proposed and explored in this work: a radial ball bearing with an integrated ball cage, a dual-row style cage, and ve unique cage geometries integrated into silicon micro-turbines (SMTs.) Also, an example of a curved or angular contact race- way is presented as an example of this type of raceway geometry in MEMS devices. Each is presented with a discussion of the design considerations and fabrication process. This is followed by a characterization of the performance of each design. These studies found that the integrated cage in the radial ball bearing performs well at speeds ranging up to 20 000RPM. Minimal wear was ob- served after 6 hours of continuous testing. However, the solder bond in the cage was a common failure point in these devices, limiting the reliability and longevity. The dual groove style cage was designed to eliminate the solder bond. However, the higher frictional forces between the ball and the cage in this design resulted in higher losses during operation. Taking into account the higher losses and the added complexity of the design, it seems unlikely that this approach would be appropriate for further study. However, the design does represent a novel approach for releasing multi-wafer rotary structures and is presented here as example of this technique. Testing of the cage de- signs for the SMTs indicated that a full ring design (a full annulus with holes for the balls) performed the best of the 5 cage geometries. However, these devices do not perform as well as cage-less designs for high speed applications due to higher ctional forces and increased raceway wear at the interface be- tween the ball and the raceway edge. Finally, the curved raceway has shown excellent performance up to 2500RPM with normal loads up to 40mN in tribometer testing. SMTs with this raceway desing were also tested for over 10 million revolutions and at speeds over 70 000RPM. The test results for all of the bearings designs presented here show that the devices exhibit stable operation at low to moderately high speeds.