Machinedesign 2019 Dolphin Guides0999 0 0

Easy glider

Sept. 1, 2000
With the aid of new polymers, linear slides are born to glide.

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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Self-lube designs are also quiet, making them suitable for medical applications. For example, they can move imaging tables and beds without further stressing already nervous patients. And in manufacturing applications, they don't contribute to the overall noise level.

Going green

There's another reason to use selflubricating slides and guides, however, that's becoming increasingly important. "There's a continuing push to remove oils from the workplace because of environmental concerns," says Schroeder. Many design decisions today reflect these concerns, which self-lubricating systems neatly address.

Because self-lubricating slides are environmentally friendly, they can help meet the goals of many international companies that are moving to follow ISO 14000 guidelines. ISO 14000 is a collection of documents that offers guidance on determining which products and manufacturing methods affect the environment and how companies can develop programs to manage their non-pollution efforts.

Adherence to this standard is growing because it's becoming a trade issue. But it is not a typical standard because it doesn't set specific targets or goals. Because of this, many members have expressed concern that companies can claim compliance without actually altering their design or manufacturing methods. Efforts are underway to bring clarity as to what will qualify as compliance.

In the meantime, you probably won't go wrong if you steer clear of oil and grease.

Don't forget rolling element

Some of the newer recirculating slides and guides are also turning to self-lubricating systems. They typically use an oil-impregnated polymer that's attached to the carriage's exterior. The part is either a fibrous or microencapsulated sponge-like material filled with enough oil to last the life of the guide. As the slide moves, oil weeps onto the rail in measured amounts.

These attached polymer parts do not supply all the oil needed for lubrication, however. Grease still lubricates the balls and races within the bearing. "The polymer parts, though, better ensure proper lubrication, which translates to longer bearing life and smoother motion," says Scott Schuler, director of linear guides, Thomson Industries Inc. "The shorter you make the path of lubrication, the more reliable the system."

Like plain bearings, these designs are also kind to the environment. The oil-filled polymer part deposits oil exactly where it's needed, instead of on the floor, or in the product, or misting into the air. For example, a typical polymer-part design contains about 10 cc of lubricant. That's all that's needed for a normal size block and rail. By contrast, forced lubrication systems often use about 600 cc, much of which doesn't even get onto the rails.

What's more, as you replace traditionally lubricated recirculating guides with self-lube versions you can have the same speeds, accuracies, and load handling abilities, plus a few benefits. For example, the polymer part damps vibrations on the slide. Not only does this make motion a bit smoother, it reduces noise. The polymer part also seals the carriage or rail from most contaminants.

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