Motion System Design
Scatching the surface

Scatching the surface

These new high-tech bearings may look damaged, but the pits and scratches you see are actually microscopic canals that pump lubricants across contact surfaces

Imagine being able to drill a hole into a human hair or carve your name onto a grain of sand. With such precision, researchers are rewriting the future of bearing technology, creating slippery surfaces on which networks of microscopic holes and grooves function like built-in lubrication systems. Maintenance free, these microfluidic marvels may be the next best thing to floating on air.

Micro moves

Over the past decade, engineers have micromachined their way through just about every manufactured device imaginable. Sensors (as in solid-state accelerometers), optical components, and tiny actuators for hard-disk drives are among the new generation of micromachined parts rolling off assembly lines around the world. But bearings?

As a matter of fact, one of the newest frontiers for micromachining is in spherical plain bearings, like those used in military and aerospace applications. In these demanding environments, speed, precision, and lifetime are everything, and all depend on lubrication.

In plain bearings, the lubricant acts as a separating layer that keeps the sliding surfaces apart. A good lubricant inhibits surface wear, while maintaining the lowest possible friction torque. In other words, the lubricant clings to the bearing, but it's not so viscous that it gums everything up.

Bearings typically have surface finishes of 3μin. What would seem smooth, however, under a microscope, is marked by "hills and valleys." When such surfaces slide across each other under high pressure, the irregularities catch – tribologists actually believe the asperities weld together – then snap off. This sort of "adhesive wear" is quite destructive and often leads to ruin.

One approach to alleviate the problem is to engineer the bearing such that the lubricant forms an elastohydrodynamic film. EHD films act like a cushion, separating contact surfaces, and will develop dynamically under the right conditions if the viscosity of the lubricating fluid increases with pressure. As long as the film is thicker than surface roughness, there will be no contact and, subsequently, no adhesive wear in the bearing.

Getting EHD films to develop consistently is somewhat of an art form, however. But through trial and error, designers have learned they can help the process along through the addition of microgrooves. With the proper geometry, these grooves can achieve a self-priming and pumping action that continuously bathes bearing surfaces with lubricating fluid.

Why now?

If elastohydrodynamic lubrication is so great, promising to extend the dynamic range of bearings beyond anyone's wildest dreams, then why didn't someone think of it earlier? Micromachining has been around since the 1980's, and bearings have been around forever. What was missing, though, was the manufacturing process.

It's one thing to project a microscopic image onto a flat piece of silicon, it's quite another thing to transfer the same features onto a three-dimensional sphere. The heart of the micro-groove manufacturing process is a sevenaxis CNC laser etching system.

The laser, a high-energy ultraviolet excimer source, etches the bearing through a stencil-like mask that defines the geometry of the groove. On its way to the bearing the beam passes through a multi-element lens that shrinks the image to the actual size.

Using a multi-axis controller, the system synchronizes all motion axes (two for the mask, four for the bearing) with the firing of the laser to precisely etch grooves into almost any spherical surface. The process produces channels up to 2 mm in width, with depths of 0.250 mm and tolerances of +0.002 mm.

By controlling energy density, the system can safely process hardened steels, ceramics, and polymers as well as crystalline structures. And unlike etchers that use infrared (CO2 and Nd:YAG) lasers, the UV-based system doesn't subject the bearing to excessive heat. Not even a human hair, when drilled, sustains damage.

Fitting in

Although the new process is capable of putting complex grooves on complex surfaces, it can just as easily accommodate simpler surface geometries, along the lines of spindle, radial, angular contact, and thrust bearings. Thus, ordinary bearings, with micro-grooves and no other modifications, can last longer and run better in everything from cars and industrial equipment to medical devices and chipmaking machines.

Micro-groove technology isn't meant for everything, but for bearings that have to maintain an unusually slow or fast pace, it seems to be ideal. It's also worth considering when tolerances are tight. The tighter the tolerance, the stronger the case for microgrooves. Here, the capillary action produced by the grooves forces lubricants onto bearing surfaces even though they fit tightly together.

A classic example is a turbine engine. Precisely engineered, turbines are typically lubricated by aerosolized oils that come in through pneumatic feed lines or through porous media that retain fluids and release them during operation. Micro-grooves can help by pumping oil directly to the bearing surfaces where it's needed most.

On the other end of the spectrum, in low-speed applications, the challenge is to keep lubricants moving, while making sure they remain in the bearing once it stops. The fact that lubricants are usually thicker and more viscous in slowmoving assemblies means only that micro-grooves have to be a little wider and deeper than normal.

Perhaps the most important factor in determining whether to use the technology is lifetime. In many applications, bearing lifetime must exceed the operational life of the system in which the bearing is applied. This suggests the need for a continuously self-lubricating pumping mechanism, like microgrooves, where lubricants can remain sealed within the bearing assembly.

Todd E. Lizotte is Director of Research & Development for Neuman MicroTechnologies Inc., Concord, N.H.

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