What should you konw about roller bearings?
Roller bearings are rolling-element bearings which use rolling cylinders held between inner and outer raceways to support radial and axial loads acting on rotating and reciprocating shafts. They can be loosely divided among bearings that support mainly radial loads, bearings that support mainly axial loads, and bearings that support combined radial and axial loads. This article will discuss the distinct forms that roller bearings take and highlight briefly some general applications. For information on other bearing types, please see our Bearings Buyers Guide.
Construction of Roller Bearings
Rollers can be cylindrical, tapered, spherical, or needle-like, depending on the intended application of the bearing. They are generally made from carburized alloy steel as carburization leaves beneficial residual compressive stresses on the roller surface. Unlike ball bearings, which are usually purchased and installed already assembled, roller bearings may be separable, with the races pressed onto shafts and into housings individually, with the caged rollers then installed between them. Cages can be made from stamped steel, machined bronze, polymer, etc., depending on the application and volume requirements of a given bearing. Pin-type roller cages are sometimes used in very large bearings.
Cylindrical rollers are designed to handle large radial loads and are available as single, double, and multi-row varieties. With length-to-diameter ratios from 1:1 to 3:1, they come both as caged and cageless (full complement) versions. Needle rollers (L:D > 4:1) are of the same cylindrical shape but proportionally longer and in cageless versions handle very high loads. They are especially useful for reciprocating motion in the cageless versions as the needles can skew. Oscillation thus helps the needles to realign. For rotational motion, the cage-type needle bearing is preferred.
Sometimes needle bearings are used without inner races and the needles ride directly on the surfaces of hardened shafts. Their outer races can be made quite thin too, called drawn-cup, and, in such cases, depend on the strength of the backing material in which they are mounted to manage the load. A typical application of full-complement needle bearings is in universal joints, where the motion is reciprocating, the needles ride directly on the shaft, and the joint housing provides material to back up the thin-walled outer case.
As radial bearings, neither cylindrical nor needle rollers handle axial loads particularly well, although light thrust loads may be accommodated by cylindrical roller bearings if they are designed as locating units. Both cylindrical and needle rollers are used in making dedicated roller thrust bearings. Rollers are kept short and often used in multiples, to keep sliding to a minimum. The crossed-roller bearing is a variant of the cylindrical bearing designed to handle radial and axial loads.
For combined radial and axial loads, tapered rollers are preferred. The geometry of these bearings is such that the apexes of the inner and outer races (sometimes called cones and cups) and the tapered rollers all meet at a common point along the axis of rotation, eliminating any sliding motion between the roller and the race. The steeper the taper of the rollers, the more axial load the bearing will manage. Radial loads will also apply an axial component because of bearing geometry. These bearings need to be installed with careful attention to preload, as too much preload can cause the rollers to bind.
Finally, spherical roller bearings are used for their capacity to tolerate misalignment between the shaft and the bearing, a common situation on shafts due to deflection caused by static and dynamic loads. Roller bearings generally tolerate misalignment poorly as compared to ball bearings, and the spherical roller addresses this deficiency. When used as single-row versions, spherical roller bearings have little thrust capacity but as double-row designs can handle axial loads upwards of 30% of the radial loads.
Sealing and shielding of roller bearings are not as straightforward as it is for ball bearings. Seals for roller bearings are available in many engineered designs. All-metal shields are sometimes used in high-temperature applications that preclude the use of elastomer lip seals. Seals frequently combine metal and elastomer which provides for a rigid fit in the bore coupled to a flexible lip seal on the rotating element.
Ratings of Roller Bearings
As it did for ball bearings, ABMA established an RBEC rating system for roller bearings. These ratings define the permissible tolerances within cylindrical and spherical roller bearings and are covered by ABMA standard 20. Standard 21 covers tapered roller bearings.
Roller bearings are less standardized than ball bearings with mainly their envelope dimensions codified. Many internal dimensions, tolerances, etc. will vary among manufacturers. Bearing selection should take into consideration the manufacturer’s catalog data.
The operating life of a single roller bearing is difficult to predict, so bearings are rated based on the number of revolutions a group of them will complete before 10% are showing signs of failure, as evidenced by fatigue in the balls or the races. The so-called basic load rating is defined as the radial load a group of bearings will sustain for a certain number of revolutions. The basis for rated capacities may differ among manufacturers.
Selection of Roller Bearings
In choosing a bearing, consider the type, grade, lubricant, any shielding/sealing, and the basic load rating. If the bearing will be subject to shock while stationary, consider its static load rating as well. Shock loading while operating will also be a factor in the bearing life. Bearing bores and ODs match basic shaft sizes and housing bores, and bearings are available both in millimeter and inch dimensions corresponding to these basic sizes.
Roller bearings are available as special designs as well as in a variety of configurations such as mounted units, multi-row bearings, etc. They are also tailored to the requirements of specific industries such as aerospace, food, and pharmaceutical, etc.
Mounted units include pillow blocks, take-ups, and flanged bearings. These include housings, seals, and, often, shaft adapters, in addition to the bearings themselves. Pillow blocks are often used to support fan shafts and flange units are often found on conveyors. Shaft adapters using set screws are limited to slow speeds; higher speeds require that the shaft be supported more fully such as through the use of taper-lock bushings. Replacement bearings for housed units are supplied as cartridges.
For installations in which shaft removal is difficult, split-type bearings are available in which the inner and outer rings and the rolling-element cages are made in two pieces.
Applications of Roller Bearings
As a general rule, roller bearings are used at lower speeds and higher loads than are ball bearings. Roller bearings perform better under shock and impact loading. Ball bearings tolerate misalignment better than roller bearings do. Roller bearings can handle heavy combined radial and thrust loads.
Roller bearings may be grease- or oil-lubricated. Advances in sealing technologies have enabled the development of sealed bearings that do not require grease replenishment over their lifetimes. Although many factors will cause a bearing to fail, even those that are properly specified, correctly installed and aligned, kept free of debris, and sufficiently lubricated will eventually fail due to fatigue. Various charts are available to help designers determine the appropriate bearing for a given application based on the criticalness of the operation and the nature of a given machine’s operational cycle.
Roller bearings are routinely monitored as part of predictive-maintenance programs. Bearings may be monitored continuously for machines in critical service or periodically for balance-of-plant equipment. Bearings will produce characteristic tones in the frequency domain that can be attributed to specific bearing geometries. These tones can be trended and used to predict bearing condition and how soon a bearing might fail. Predictive maintenance thus allows repairs to be scheduled during outages, etc. rather than simply letting a machine run to failure.