Linear Bearings and Lubricants
Linear Bearings - Design and Considerations
Linear Bearings - 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.
Linear Bearings - Preloading: All rolling-element linear bearings have 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 linear bearing, vibrations can be set up. Rolling element linear bearings do not have natural damping. However, additional preloading can damp vibrations if drag friction is not a problem.
Linear Bearings - 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 linear bearings. Recirculating flat-way linear bearings, for example, sometimes fail from skewing rather than from fatigue. Recirculating linear bearings, however, generally fail from fatigue and have a significantly longer life than other types of linear bearings within their lower load limits.