MICRO ROTARY BALL BEARING WITH INTEGRATED

   BALL CAGE: FABRICATION AND CHARACTERIZATION

        Robert Hergert, Ingrid S.Y. Ku, Tom Reddyhoff, and Andrew S. Holmes

                                    Imperial College London, UK

Journal:- IEEE 23 rd INTERNATIONAL CONFERENCE ON MEMS (Micro Electro Mechanical Systems), 2010

v ABSTRACT

This paper presents a rotary MEMS ball bearing with an integrated silicon ball cage. The device is a deep groove radial ball bearing consisting of steel balls encapsulated between two micromachined silicon wafers. The silicon ball cage is released from the bulk silicon substrate during fabrication. The objective was to show that a simple caged bearing design provides reliable motion at both high and low speeds. 

v INTRODUCTION

Reliable rotary MEMS devices require a long lasting and mechanically stable bearing.

They can provide greater longevity and higher speed operation than pin-joint bearings while being more stable and easier to control than magnetic levitation bearings. Ball bearings can also operate reliably at lower speeds and have a less complex design than air bearings such as those described in. Ghodssi et al. have recently demonstrated the effectiveness of micro ball bearings in MEMS devices. As discussed in, one of the problems encountered in current designs is the tendency for the balls to co-locate or jam in the bearing raceway. We have addressed this problem by integrating a ball cage in our design.

v DESIGN

Figure 1 - CAD rendering of an assembled bearing with ball cage (left) and exploded diagram showing all of the individual parts (right).

In the exploded diagram on the right the top part is the bearing rotor. The middle part represents the ball cage with the steel balls in the cage openings. Both of these parts fit into the stator, at the bottom, which is designed to fit into a torque testing device. All of the pieces, with the exception of the steel balls, are fabricated from a silicon substrate. The bearing is fabricated in two halves, which are held together with solder bonds.

                       

Figure 2 - Depiction of the bearing design parameters: g -cage release gap (40μm), C – cage width (100μm), t –raceway tolerance (10μm), Dr – rotor diameter (1.18mm), Db – bearing raceway diameter (2.2mm).

v FABRICATION

Figure 4 - SEM images of the micromachined bearing parts: ball cage (top left), rotor (top right), stator (bottom left) and fully assembled device (bottom right).

Figure 5 - SEM image of DRIE damage on the stator. Similar damage is present on the rotor.

v TEST SETUP

Prototype ball bearings have been characterized in a custom microtribology test rig developed at Imperial College. A full description of the measurement setup can be found in [7]. The rig allows the measurement of the frictional torque of the ball bearing under varying normal loads and over a range of rotational speeds. The outer part of the bearing is mounted in a silicon micromachined test platform incorporating an elastic suspension (see Figure 6), while the inner part is coupled to the shaft of a precision motor, which provides both normal force and rotation.

Figure 6 - Micromachined test platform. Outer springs are used to determine normal load; inner platform beams are used to measure applied torque.

The limitation of this testing arrangement is that we can only measure the axial normal load and not the radial load on the bearing. However, this does provide the opportunity to stress test the device using the least ideal load configuration for a radial deep groove bearing.

v RESULTS

Initial testing shows that the bearings perform well at both high and low speeds. Testing was carried out on two devices at speeds ranging from 10 to 20,000 rpm with a 60mN axial load. The motor used during testing limited the maximum speed. The torque profile for the devices conforms to the expected torque curve. At lower speeds the torque will decrease as the balls move from sliding and stick/slip to rolling friction. Figure 8 shows the average of three low speed torque measurements for both tested bearings.

Torque measurements from three high speed tests on the second device.

After 5 hours of continuous testing, we debonded the second device to evaluate the wear at the contact points between the steel balls and the silicon cage and raceway. Figures 9 to 11 show that there was relatively little wear of the silicon structures during testing. The first device failed after 3 hours of testing due to poor adhesion between the solder and the silicon arms of the cage.

SEM image showing the wear on one half of the ball cage. Wear areas enclosed in the circles are the lateral contact points of the steel balls.

v CONCLUSION

We have shown that an encapsulated ball bearing with an integrated ball cage can perform reliably between 10 and 20,000 rpm. The current design shows relatively minimal silicon wear after several hours of continuous operation. A closer evaluation of the fabrication damage and the testing wear will need to be carried out determine the causes of the raceway damage. These initial results seem to indicate that a micro ball bearing with an integrated

cage could provide a stable platform for rotary MEMS devices at both high and low speeds.

However, longevity and higher speed testing would provide a clearer picture of the durability and applicability of the design.


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