Dolphin guides, from Pacific Bearing Co., consist of only two pieces. The rail is an extruded piece of ceramiccoated aluminum. The carriage is a one-piece extrusion with FrelonGold bearing material bonded to the running surface. During the first few cycles of operation, the material deposits onto the mating part, ensuring lubrication. The guides can reach speeds of 300 sfpm, dry.

When mechanical components make contact, there can be trouble unless there's sufficient lubrication between them. But ensuring the right amount of lubrication - reliably - has confounded industry for years.

Automated and forced lubrication systems, and scheduled maintenance aren't the answer because at some point, someone is bound to forget to refill the lubricant reservoir with oil or grease.

Fortunately, recent developments in polymers are providing much needed independence from oil replenishment. In some cases, they eliminate the need for oil or grease altogether.

For years, polymers have been used to form plain bearings. "Now, with the development of some new materials, self-lubricating technology is making its way into other areas," says Bob Schroeder, CEO, Pacific Bearing Co., Rockford, Ill.

The polymers are being used as bearings in a carriage, or as a coating specially bonded to linear rails or slides. Not only is lubrication assured for the life of the slide or guide, the linear system's design is often simpler because there are fewer parts. And without oil or grease, they have the added benefit of being environmentally friendly.

Oil change

Initially, self-lubrication referred to systems that automatically deposited a lubricant onto a sliding surface. More recent systems, however, are totally oil-free, resulting in some confusion about what is meant by that description.

"You need to evaluate alternatives carefully today because the term selflubricating does not define a system as oil-free," says Scott MacGillivray, product manager, igus inc., East Providence, R.I.

The self-lubricating systems without oil come in two basic versions. One uses a plain bearing or a carriage riding on sliding contact bearings made from the special polymer. The other version has no bearings at all. Instead, mating parts are coated with the polymer that functions as both bearing surface and lubricant.

In both cases, the polymer is a proprietary low-friction material coated or bonded onto a metallic part, such as a shaft or rail. Functioning as a wear surface during operation, it deposits bits of itself onto the mating surface, ensuring lubrication.

In play

Self-lube systems apply to virtually the same applications as forced lube or automatic lubrication systems. Some versions, though, require an analysis of the force exerted on the transported load and the force that moves the slide. Going beyond a given range of force will quickly raise friction levels, which can increase the power requirements of associated components, such as belt-drives, ball screws, or cylinders powering the guide.

Positioning accuracy is another factor. "There will be a slight increase in drag due to the additional contact of the self-lubricating material," says Scott Schuler, director of linear guides, Thomson Industries Inc., Port Washington, N.Y. Though it's not huge, it needs to be considered in stroke-length calculations because it will increase the overall guide length by about 10 to 20%.

It's possible to increase the position accuracy of adjustable polymer bearings, even to a level that approaches ultra precision. "But the tradeoff is an increase in friction," MacGillivray points out.

The way to increase accuracy is by reducing the normal clearance between the bearings under the carriage and the rail they ride on. Some designs allow such adjustment. How much they allow, though, depends on the application. At some point, the accuracy increase results in the system needing larger components to drive it.

Clearance also plays a role in acceleration rates. In general, sliding bearings have no restrictions unless they're under high force.

More than convenience

Because self-lube designs removed the necessity to oil or grease systems in hard-to-reach places, the automotive industry is a big user. However, automotive applications are not the only ones where these guides and slides make sense.

Most of these guides are immune to problems from dirt, dust, and other particulate contaminants prevalent in such applications as packaging, woodworking, and printing. The clearance between sliding parts also serves to let contaminants simply pass through. Without oil, the particles can't form that sludgy mix that gums up mechanical parts.

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