In rolling-element bearings (also called “antifriction bearings”), rolling elements such as balls or rollers interposed between the housing and the shaft produce rolling motion instead of sliding motion. The shape of the rolling elements as in Figure 1, such as ball, tapered roller, needle roller, or spherical roller, usually describes the modern rolling-element bearings. European companies took the lead in much early bearing production, so the metric system of measurement has been widely adopted, ensuring high interchangeability of many popular bearing types and sizes.
Ball bearings are the most common rolling-element bearings and are produced in a variety of sizes and types for applications from miniature high-precision equipment such as dental drills, to huge industrial machines and jet engines.
Bearings using rollers that are tapered, cylindrical, or spherical give the machinery designer greater load-carrying capacity than ball bearings, and the geometric shapes of their rolling elements offer unique capabilities.
Making it work with misalignment
A bearing’s basic function is to reduce friction while carrying radial load or thrust load, or both. It may also have to accommodate either dynamic or static misalignment.
An example of dynamic misalignment is a bent shaft rotating in a bearing, which causes constant deflection or wobble of the shaft where it sits in the bearing. A shaft may deflect elastically under load, rather than bend permanently, but the effect on the bearing is the same.
Also, the head shaft on a heavily loaded belt conveyor might deflect, or the support structure could also move slightly under load, increasing potential misalignment at the bearing. Spherical roller bearings are designed to rotate while constantly accommodating this “wobble” and yet carry full system load.
Self-aligning spherical bearings can also compensate for manufacturing tolerances in machined housings and misalignments common in cast or fabricated equipment structures. If both the shafting and structure are rigid, the initial manufacturing tolerances of bearing housings make it difficult to align shafting perfectly perpendicular to the bearing housing, as some types of bearings require. Most spherical roller bearings can accommodate ±2-deg misalignment. On bearings set at 10-ft centers, this can equate to as much as 8.3 in. of misalignment, Figure 2. For either dynamic or static misalignment, spherical bearings provide the “forgiveness” necessary where economics or the real-world environment compromise the perfect alignment that ball, tapered, or cylindrical bearings may require.
Types of spherical bearings
Spherical roller bearings come in two types of self-aligning rollers, Figure 1. The Swedish-designed “barrel-shaped” roller is common and adjusts for misalignment as the roller operates around the spherical surfaces of the inner and outer ring. Self-alignment can be accomplished also by hourglass-shaped rollers, a unique United States design patented by Julius Shafer. As shown, the hourglass roller misaligns relative to the inner and outer spherical raceway surfaces in a fashion similar to the barrel shape.
Spherical design characteristics
Figure 3 shows the relationship between diameter and width on standard spherical bearing series for a given bore. Load capacity increases as the boundary envelope enlarges. The increasing mean diameter of the boundary envelope permits larger rollers.
For the best dynamic ratings, designers tend to use as large and as few rollers as possible. For the best static load-carrying capability, more rollers, and therefore smaller diameters, are best. ID, OD, and width of these units are standardized metric dimensions, with the last two digits in the bearing’s nomenclature representing the bore size. For example, if the last two digits are 12 (say the bearing number is 22212) then multiplying 12 by 5 indicates the bore is 60 mm. The smallest spherical bearing bore diameter interval that has evolved is 5 mm. Thus, standard spherical designs are offered in even multiples of 5 mm, and it has become industry convention to use a bearing nomenclature where the bore is listed as the quantity of 5-mm increments. This permits a two-digit value to span a bore range from 20 mm (04) to 480 mm (96). This numbering system is used also on other types of rolling-element bearings, such as ball bearings and cylindrical roller bearings.
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Spherical roller bearings are usually designed with two rows of rollers tilted at opposite angles to resist thrust load in either direction. Thrust loads are common when deflections and misalignments are sufficient to require a spherical roller bearing.
For the special situation with only radial load, bearing designers can use the same boundary to contain only one row of even-larger-diameter rollers. This maximizes the bearing’s radial load-capacity but minimizes thrust capacity.
For the special situation with only extremely high thrust load such as a rock crusher, you can use a spherical thrust bearing. It consists of one row of rollers positioned between special bearing races so that high axial shaft load can be transmitted through the bearing to the mounting structure and yet accommodate the types of misalignment described previously.
You can get spherical roller bearings with selected inner and outer ring tolerance variances for special shaft or housing fits, and manufacturers’ catalogs usually provide details. Many factors influence proper shaft or housing fit, such as magnitude of the load, shock, vibration, and temperature. Furthermore, housing fits must consider axial movement requirements for shaft expansion.
Internal operating clearance of a spherical roller bearing — the space between the rollers and the outer ring, Figure 4 — can also be critical to the application. Internal operating clearance is also encoded in the nomenclature. For example, a suffix could be shown as C2, C0, or C3, with C0 representing normal tolerance; C2, less clearance than normal; and C3, more. Be sure to check nomenclature designations on a given bearing with a catalog from the maker of that bearing to assure meanings of all suffixes in the nomenclature. There are some variations among manufacturers.
As an example of the usefulness of operating clearance suffixes in bearing nomenclature, high-temperature or highspeed applications provide better service if additional clearance (C3) is provided. Lower clearance (C2) may increase service life in a heavily loaded, low-speed application, because more roller surface is in contact with raceways.
There are three general mounting arrangements for spherical roller bearings. When a manufacturer provides the bearing housing, such as a pillow block or a flange block, the bearing is often mounted with a collar and setscrews. These lock the inner ring to the shaft. Proper shaft diameter tolerance and mounting collar setscrew torque are important and let these factory-assembled and prelubricated units be mounted and dismounted quickly.
Setscrew-mounted bearings are ordinarily designed to accommodate light to medium loads, but not heavy loads. Most bearing makers define a heavy load as one greater than about 25% of the bearing rating. (Bearing dynamic capacities are reference values for longevity calculations and do not represent allowable load limits for normal use.) Bearing manufacturers have established shaft-to-inner- ring fits that allow the convenience of slip-fit installation, while still providing adequate support for the expected loads. With setscrew mounting, the “bite” of the screw is a function of its diameter and tightening torque. Bearing makers select screw sizes that resist the expected loads if the screws are fully tightened. Over-designing the screw size has a detrimental effect: excessive ring bending. Especially with larger loads or speeds, it is important to follow manufacturers’ recommendations for shaft fit and setscrew tightening to achieve the desired bearing performance.
A second mounting arrangement common in split pillow blocks is the tapered adapter sleeve, Figure 5. In this arrangement, the tapered sleeve is drawn into the bearing, clamping it tightly to the shaft. When a locknut is tightened, the tapered adapter compensates for shaft clearance, but the mounting procedure requires more skill and care to ensure that the bearing adapter is tightened enough and the operating clearance of the bearing has not been altered. Manufacturers’ instructions usually outline proper mounting procedures — be sure they are followed.
Direct shaftmounted spherical roller bearings may be either slip-fit or press-fit, depending on loading considerations and service life expectation. As with tapered sleeve mountings, housing and shafting tolerances determine resultant fit — make sure manufacturer’s suggestions are followed.
As with ball and cylindrical roller bearings, spherical bearing bore and outside diameter tolerances are in accordance with the system of tolerancing established by the International Organization for Standardization and adopted by the American Bearing Manufacturers Association and the American National Standards Institute. Housing fits, for example, are designated by a capital letter and a number. The letter indicates the location of the housing bore tolerance limits with respect to the nominal bearing OD. The number indicates the size of the tolerance zone. A similar system, using a lower case letter and a number, applies to shaft bearing seat diameters.*
Most modern ball and roller bearings provide exceptional value for their price. Unusual problems in application or machinery design — especially misalignment problems — may be better fulfilled by the unique capabilities of spherical roller bearings, When properly selected and mounted, they can solve problems and boost service life.
*For more about tolerancing of bearing bores and ODs, see PTD, “Shafts and Housings for Radial Bearings: Picking the Right Fit,” 3/90, p. 47.
William D. Schroeder is Marketing Manager, Link-Belt Bearing Div., Rexnord Corp., Indianapolis. When this article was written, he was Product Manager.