Ball Bearings in Micro-Sized Turbines


March 2011

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The rotor and stator of a micromotor developed at the University of Maryland is supported by micrometer-scale ball bearings, visible on a track around the stator. The 285-micrometer stainless steel balls are among the smallest manufactured.

Researchers at University of Maryland’s MEMS Sensors and Actuators Laboratory are using an old technology to make fresh progress in reducing wear in microscale systems. Ball bearings the size of dust particles are now being incorporated into MEMS turbines hoped to power miniature pumps, motors, and generators as an alternative to batteries for handheld electronics.

High speed micro-sized air bearings had been the leading candidates for dealing with friction in MEMS turbines for a decade, replacing electrostatically driven MEMS micromotors with contact bearings of the 1980s. As air bearing projects are ending, ball bearings could have the potential to unseat them.

Air Bearings Weren’t Sensitive or Durable

In the mid-1990s, experimental millimeter-scale turbines were built capable of producing a few watts each by Massachusetts Institute of Technology and others during an intensive decade-long research push. They used microscale air bearings which promised virtually zero friction as the rotating parts were supported on a cushion of air and were spun at more than two million rpm.

But air bearings introduced as many problems as they solved. Design tolerances were incredibly exacting; at such high rotational speeds even the slightest contact carries an enormous force. "Any variation or tapering in the sidewalls could result in the loss of the microturbine," said Reza Ghodssi, the University of Maryland engineering professor leading the ball bearing research who worked on the MIT microengine program.

To maintain the cushion separating the moving parts, a steady stream of air had to be introduced into the device, requiring support apparatus many times the size of the MEMS device itself. And while it was excellent at spinning miniature turbine parts at millions of rpm, it was less effective with slower speeds or when turning a rotating piece by a few degrees, Ghodssi said.

"Some people argue that the reason that MEMS is still not a multi-billion dollar industry after so many years of research is because of problems with the long-term reliability of the devices," said Ghodssi. Engineers have yet to design complex microscale machines with rotating parts that can withstand months or even weeks of wear and tear.

Micromoto

The rotor and stator of a micromotor developed at the University of Maryland is supported by micrometer-scale ball bearings, visible on a track around the stator. The 285-micrometer stainless steel balls are among the smallest manufactured.

Micromotors with Ball Bearings Demonstrated

Ball bearings and other roller-type bearings have been a reliable platform for centuries. Unlike contact bearings (such as bushings), ball bearings minimize the amount of contact between moving parts. When the balls are rolling in their tracks, the small amount of contact area produces minimal friction.

In the early 2000’s, Ghodssi’s research team focused on how to integrate ball bearings with microfabricated devices. They used conventional ball bearings because photolithography and other microscale fabrication techniques can’t make objects smooth enough. The team used 285 micrometer diameter bearings to understand the limitations of their new concept even though smaller bearings were available.

After a couple of false starts, they developed a method and demonstrated working motors using the bearings. The balls are placed by hand onto a track etched into a silicon wafer. Another wafer is bonded over the top of the balls, encapsulating them in a racetrack. The wafer is then etched to cut a central rotor that can turn freely, supported only by the bearings.

Of course, no fabrication method that relies on graduate students is ready for mass production. But Ghodssi said that since this technique is fairly straightforward, it has the potential to scale up cheaply.

Bearing

The stators and rotors of the micromotors are fabricated separately and then bonded together to create a single MEMS device.

Ghodssi estimates that rotors supported by ball bearings could run at speeds between 10,000 to 200,000 rpm, making them suitable for power generation. The higher speed possible with air bearings promises more power, but in Ghodssi’s eyes the ball bearings compensate by offering more simplicity.

In 2008, a six-phase micromotor 14 millimeters across was built, capable of more than 300 microwatts of mechanical power. In proof of concept experiments, the rotor with a 10 micrometer air gap turned at more than 500 rpm —a feat only possible because of the consistency and reliability of the ball bearing supports. The experimental motors were tough enough to survive many hours of continuous operation at low speed. Other experiments have run up to 87,000 rpm for shorter periods.

The hope is to use these turbines where tiny amounts of power can do an enormous amount of good. "You can integrate the ball bearing mechanism in a more compact form, because you don’t require external components to operate it," said Mike Waits, an engineer at the Army Research Laboratory from the University of Maryland team. ARL researchers are pursuing the use of microscale components. Devices like micro-pumps, for instance, could feed hydrogen or alcohol to miniaturized fuel cells, or fingernail-size turbines could directly power micro-generators.

This new approach to MEMS turbines is the kind of thinking that could help turn MEMS into the sort of ubiquitous technology that experts had always predicted it would be.

[Adapted from "Rolling with It," by Jeffrey Winters, Associate Editor, Mechanical Engineering, April 2009.]

You can integrate the ball bearing mechanism in a more compact form, because you don’t require external components to operate it, Mike Waits, an engineer, from the University of Maryland Team.

 
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Jeffery Winters

by Jeffrey Winters