Two spherical roller bearing pillow blocks during setup on large fan drive at Owens-Corning Fiberglas plant, Newark, Ohio. This is typical of good applications for spherical roller bearings: high loads, misalignment potential, tough environments.

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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