Machine Design

Cylindrical Bearings

Cylindrical roller bearings have high radial capacity. Their low friction permits operation at high speed, and thrust loads of some magnitude can be carried through the flange-roller end contacts.

Although the rolling elements are called "cylindrical," they are not true cylinders. True cylinders would produce stress concentrations at the ends of the roller-race contact, resulting in high wear and high sensitivity to misalignment. Instead, rollers are usually crowned or end relieved.

Because they are used for high speeds, cylindrical roller bearings are commonly made in precision grades such as RBEC5 as well as the lower grades. The close tolerances of the higher grades may be needed in precision spindles as well as in high-speed applications.

Unlike ball bearings, cylindrical roller bearings are generally lubricated with oil; most of the oil serves as a coolant.

A persistent problem in high-speed, lightly loaded, cylindrical roller bearings is roller skidding, which can lead to premature bearing failure. One technique to combat skidding is an out-of-round raceway, which pinches the rollers at two diametrically opposite points. Another technique is the use of two preloaded hollow rollers. Both techniques require precise engineering to eliminate loading without introducing other problems.

Nonlocating bearings have either a separable outer ring or separable inner ring and allow considerable axial movement of the shaft. They must be used with an axially locating bearing such as a ball bearing. Nonlocating bearings have high radial and load capacity, high stiffness, and the highest speed capability of any roller bearing.

One-direction locating bearings also have separable outer or inner rings. But in these bearings the separable ring has a single flange that prevents shaft wandering in one direction. Roller guidance is provided by the double-flanged race. Characteristics are similar to the nonlocating bearings, except for a slightly lower limiting speed caused by the flange.

Two-direction locating bearings may be self-contained or have a two-piece inner ring. The self-contained type has a lower roller complement and lower roller capacity, because it must be filled by displacing the rings and filling the remaining space. The two-piece version has more load capacity, but is more complex.

Full-complement bearings use two snap rings in the nonlocating race to retain the rollers. Usually the outer race is the nonlocating race. Their advantage is high load capacity, their disadvantage is low speed. The speed limitation results from the high rubbing velocity between adjacent rollers and limits the bearing to low-speed or oscillatory applications.

Double-row bearings are used where alignment between shaft and housing is very good and high loads are encountered at moderate speeds. They are nonlocating, and usually have roller-piloted retainers.

Another class of double-row bearings is the journal roller bearing, which usually has two nonlocating races. In some cases, the shaft becomes the inner race. When that is done, the shaft must be hardened and ground to tolerances equivalent to those of a bearing race.

TAGS: Bearings
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