Linear-Bearing Design Considerations

Nov. 15, 2002
Load capacity: Hardness, finish, and conformity of race and rolling elements determine load capacity, which varies as the fourth power of the race Brinell hardness number.

Load capacity: Hardness, finish, and conformity of race and rolling elements determine load capacity, which varies as the fourth power of the race Brinell hardness number. The surface finish must be smooth because grinding marks create ridges that prevent conformity between the race and rolling element.

With a flat race, balls have lower load capacity than rollers and large-diameter balls have to be used to carry the same load. If the mating way is ground with either a Gothic arch or circular groove, the closer conformity to the ball allows the use of smaller balls than with flat ways. The Gothic arch approximates a V-type way with the load effectively split at angles of about 45° with the vertical into two loads on the way. The circular groove has a higher load capacity, but the ball bears on the bottom of the groove, which results in side play under loads normal to the line of motion.

Preloading: Every rolling-element linear-motion bearing has some deflection between race and rolling elements. This deflection can be minimized by preloading, but this requires greater bearing accuracy and more careful installation. Preloading also increases drag friction and can be a drawback in servosystems where additional friction might require a larger servomotor.

When the operating speed approaches one of the natural frequencies of the bearing, vibrations can be set up. Rolling element bearings do not have natural damping. However, additional preloading can damp vibrations if drag friction is not a problem.

Life ratings: Manufacturers' data on life ratings generally give fatigue life based on data previously collected for ball bearings. These ratings are not always accurate for a linear-motion bearing. Recirculating flat-way roller bearings, for example, sometimes fail from skewing rather than from fatigue. Recirculating roller bearings, however, generally fail from fatigue and have a significantly longer life than other types of linear-motion bearings within their lower load limits.

Cleanliness: As with any type of bearing, precautions are recommended to exclude foreign matter, such as chips, grit, plastic dust, fibers, or other airborne material. Hard chips in the path of the balls cause ball spalling or fracture and can damage the races. Any dirt or grit between race and rolling elements produces wear, and the balls or rollers suffer progressive lapping that results in increased play. Plastic dust and fibers are particularly bad in recirculating bearings because the material tends to pack into the recirculating channels and eventually prevents movement of the balls or rollers.

Bellows-type enclosures are best for keeping foreign material away from critical parts of the bearings. Telescoping tubes or cover plates also can be used to protect critical elements.

Some linear-motion bearings have integral rubber-lip seals that avoid the need for extended housings (to accommodate outboard seals) and which also simplify seal retention.

For round bearings, a clock-spring arrangement is sometimes used -- the inner coil is attached to the end of the shaft and the outer coil is attached to the OD of the bearing. Some machine tool installations use a window-shade arrangement with way-covering material of heavy rubber or synthetic material stored on a roller arrangement at the ends of the bed.

If none of these methods are feasible, wipers or scrapers should be used. These are commercially available for flat or round surfaces and are made of felt, leather, or plastic with spring-loaded fingers for positive sealing. Enclosures for ball splines are usually custom designed and are generally more effective at keeping out large chips than dust.

Lubrication: For low-friction operation, a thin film of oil keeps surfaces lubricated. The oil also inhibits rust, which causes problems. If the atmosphere is particularly corrosive, stainless steel or some other special alloy must be used as the bearing material. Surface coating and plating are not recommended. Unusual temperatures are not really a problem, but special high-temperature or low-temperature lubricants are available.

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