Encapsulated Ball Bearings for Rotary Micro Machines

August 1, 2012

Army Research Laboratory, Adelphi, Maryland

Microelectromechanical systems (MEMS)-fabricated silicon rotary elements for micro-motors, micro-generators, and micro-turbomachinery have received growing attention with applications in power conversion and actuation. Within these technologies, the bearing mechanism is the primary determinant of device performance and reliability. Active bearings, such as magnetic or electrostatic, have the advantage of being controlled during the operation, but at the cost of the accompanying circuitry. Passive bearings span a large range of velocities that include center-pin bushings with low revolution rates possible, and hydrostatic or hydrodynamic bearings with high revolution rates possible.

A rotary ball bearing mechanism has been developed in which the rolling elements are encapsulated at the periphery of the rotor to enable high-speed rotation without relying on any attractive force between the rotor and stator. An experimental test stand demonstrated the encapsulated bearing operation at rotational speeds up to 16 krpm. Results for both dry and lubricated full-complement bearings lead toward the development of micro-turbomachinery. A qualitative analysis is given for the wear seen on the silicon race and the stainless steel ball, and correlated to the start-up behavior of the bearings.

The design and fabrication of the rotary ball bearing is based on commercially available 440C stainless steel balls with a diameter, dball, of 285 μm and a lot diameter variation of 0.254 μm. The design features balls housed at the periphery of the rotor to enable a two-layer fabrication sequence for encapsulation via bonding. At the same time, this scheme of encapsulation allows features to be patterned on either side of the rotor while having minimal influence from the bearings.

A square groove race was designed to encase the micro ball bearings. Alter n a tive designs may improve the performance and fatigue characteristics of the bearings. Housing the balls using a square groove race fabricated by a dry anisotropic etch process, such as deep reactive ion etching (DRIE), allows control of the contact points and better repeatability when compared to other race designs and fabrication methods.

The bearing mechanism is fabricated in three major steps: (1) silicon races are fabricated on the wafer level, (2) balls are placed into the race and an identical race is bonded on top to encapsulate them, and (3) silicon DRIE is used to release the rotor.

Demonstration of the encapsulated ball bearing was accomplished using a setup in which standoffs are placed on both the top and bottom of the rotor, and the stack is held in place using a mechanical vice. A nitrogen line is placed within 2 mm of the die corner using a second mechanical vice. The flow from the nitrogen line causes the outer portion of the die to spin about the clamped center. A Philtec D6 Fiberoptic displacement sensor is used to measure the angular velocity of the spinning square die.

Initial movement of the square die about the center required a line pressure greater than 5 psi for most of the devices tested. Lower pressures were not able to produce enough force on the edge of this square die to counteract static friction, which is much higher than the dynamic friction once it is rotating. In some cases, bearings have to be manually started to overcome this initial friction. Once they are spinning, line pressures as low as 1 psi have been used to maintain operation.

The testing method induces a high load onto the bearing device. This, as well as the square groove race design and the brittle silicon material, lead to high wear. Scanning electron micros copy (SEM) images were used to investigate the wear of the silicon race and of the stainless steel balls. The device, which operated for 39 minutes before jamming, was separated on a hot plate at 400 °C.

Ball jamming that occurred for both bearing devices is an inherent problem when full-complement-type bearings are used. The race dimensions increase as wear ensues, giving the balls more play and a greater chance to seize. Im ple - mentation of a low-wear material on top of the silicon race, along with tighter fabrication tolerances, can reduce the probability of jamming, but not eliminate it completely. Instead, a retainer ring (or cage) could be used to isolate the balls from one another. In addition, to eliminating ball jamming, the use of a retainer ring can greatly reduce the friction since a much smaller number of balls can be used. A retainer ring maintains a separation distance between the balls allowing for a minimum number of balls to be used. Reduction in the friction by decreasing the number of balls will similarly lead to much higher speeds as well as better reliability.

This work was done by C. Mike Waits and Reza Ghodssi of the Institute for Systems Research, and Bruce Geil of the Army Research Laboratory. ARL-0133

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