The issue of preloading

Positioning systems use linear guideways to control straight motion and a drive mechanism to produce motion. "Unless using hydrostatic or air bearing components, friction will be prominent in these systems," says Jaffe. "Linear drive and guideways components that use recirculating rolling elements are normally preloaded to eliminate play and achieve a higher degree of accuracy." Preloading does increase friction though, which causes wear. The degree of wear, and eventual loss of accurate movement, depends on speed and duty cycle.

On the other hand, the practice of interference fitting of smaller bearings should be avoided at all costs. Usually the contiguous components (specifically, the shafts and housings) have geometrical accuracies measured in tenths of thousandths of an inch at best. In contrast, geometrical accuracy of bearings is measured in millionths of an inch. "So if interference fits are used to mount bearings, the bearings will conform to the inaccuracies of the shaft or housing — and cause excessive torque and heat," comments Howe. What is the proper way to mount these bearings? "Smaller bearings in particular should be mounted with a slip fit and secured with an adhesive," recommends Howe. "If the shafts are rotated as the adhesive sets, the rolling elements and inner and outer rings settle into the paths of least resistance, which in turn decreases torque and heat — and extends bearing life."

Some high-efficiency ballscrews substantially reduce friction, and thus heat and wear, while maintaining system stiffness. But before components are selected for a design, Jaffe recommends that designers first determine whether or not the desired system accuracy requires play-free (that is, preloaded) components. "If not, friction can be greatly minimized, if not almost eliminated, without resorting to more expensive solutions such as air bearings," he says. Jaffe also points out that designers can keep the completely nonfunctional friction of misalignment to a minimum by ensuring that elements are properly mounted.

Heat is the word

Several things can make mechanical components fail. One is overheating — from contamination, misalignment, excessive loads, and excessive speed. Of course, a high ambient temperature or a chemical or mechanical breakdown of lubricant can also make systems overheat in a hurry. As a general guideline: "Use 1570°F as a benchmark. For every 200°F increase in temperature you can cut the expected life of the lubricant in half," says Howe.

Lubricants do two things: They provide a film that separates wear surfaces, and they carry away heat to cool down mechanical components. Howe elaborates. As temperature increases, the film strength of lubricant diminishes, and mating surfaces come into contact with more aggressive surface asperities. It is then that these asperities micro-weld to the surface of the mating component. "In ball bearings, these weldments eventually hit the raceways and break off, making an abrasive slurry of the lubricant." At this point bearings become very noisy.

There is one caveat. Explains Bryson: "Wear in components in which sliding friction is present is greater than that of components experiencing viscous friction. But viscous friction can be improved, too." In a few cases, it has been reduced by the application of a coating to the surfaces of fluid moving equipment. "The coating's relatively rough surface has the same effect on fluid flow as dimples on golf balls, extending laminar flow near the surface and reducing energy wasting turbulent flow."

Bryson was involved in coating pistons of diesel engines with a polyimide/amide/PTFE low-friction material. "In this application, the coating increased the lubricity at the piston-cylinder interface, and prevented piston slap." Efficiency of the engines was increased by almost 15%, as demonstrated in a 200,000-mile test. Dynamometer trials showed a corresponding increase in power of 16%.

Heat also causes rolling elements to experience thermal growth — expansion of the metal — if the bearing is not stopped. What's more, "this thermal growth is very seldom uniform. Rolling elements can grow enough to bind between the raceways. The free rolling elements will then drive the element's separator into the stalled element, causing the separator to explode," warns Howe. With a few more revolutions the bearing fails totally.

Cool-running components also benefit from the accuracy of larger systems. "We work with a major U.S. machine tool builder with a reputation for axis accuracy on water-cooling ballscrews," says Jaffe. They recently introduced a new line of milling machines without any cooling, using ballscrews that run about 67% cooler than conventional designs. "The reduced friction also makes the machine run significantly quieter."

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