What should you know about ball bearings?
Ball bearings are rolling-element bearings that use rolling spheres held between inner and outer raceways to support radial and axial loads acting on rotating and reciprocating shafts. They can be loosely grouped as Conrad types and maximum-capacity types, and 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 ball bearings take and highlight briefly some general applications. For information on other bearing types, please see our Bearings Buyers Guide.
Construction of Ball Bearings
Balls are generally made from hardened chromium steel but other materials such as plastic, ceramic, etc. are sometimes used. The rings are usually of ground, hardened steel for high-quality bearings with unground, hardened steel used in less-stringent applications. Deep-groove, or Conrad, type bearings are filled with balls by locating the inner ring to one side. Once the balls are in the bearing the inner ring is centered and the carrier, or cage, is riveted in place, spacing the balls out evenly. Maximum-capacity bearings rely on a filler notch through which the balls are installed to the full capacity of the bearing. The notch is then plugged and a carrier may or may not (full-complement) be installed. Maximum-capacity bearings sacrifice thrust capacity and tolerance of misalignment for increased radial load capacity over deep-groove varieties, anywhere from 20 to 40% higher. Cages, or retainers, can be manufactured in steel and other non-metallic materials.
Bearings are offered with several methods of protection and are also available as open designs. Shields are usually metal with a slight clearance left between the edge of the shield and the inner race. Seals are usually made of flexible material which presents a thin lip that contacts the rotating race. Seals add friction to the bearing but in general provide better exclusion of contaminants and better retention of grease. A snap-ring groove may be ordered for the outer race to provide a locating surface for mounting.
While standard radial bearings can withstand small amounts of axial thrust, angular-contact bearings use higher shoulders on the grooves of their inner races to increase thrust capacity. Assembly considerations limit shouldering to one side of the race, so an angular contact bearing can provide increased thrust resistance in one direction only. They can be used back-to-back for situations where thrust loading is expected in both directions. Double-row bearings are also made for this purpose but they are filled through slots so they must be oriented correctly at installation. Ball bearing thrust units are also available.
Both static and dynamic shaft loads tend to deflect the shaft and hence the alignment of the shaft with respect to the bearing. Self-aligning bearings increase the tolerance for misalignment. Two self-aligning styles--external and internal--are used. In the external design, the outer ring is radiused and rides in a similarly spherical shell. In the internal design, the balls are segregated between two grooves on the inner race and ride along the outer race on a single, radiused surface. External self-aligning designs require more radial space; internal designs, more room axially.
Bearing Preload of Ball Bearings
Ball bearings such as angular contact bearings are usually installed and set with what is called a bearing preload. The purpose of a preload is to implement a sustained axial force or load to the bearing assembly. The preload force controls play within the bearing that may result from the different tolerances in the manufacturing and assembly process that can stack up. Too much play within the bearing can result in excessive vibration and mechanical wear during operation. With the addition of a preload, contact between the different elements within the bearing (roller balls, bearing spacers or races, etc.) is maintained constantly. The reduction in play assures proper operation of the bearing assembly and prolongs its operational life.
There are typically two methods of implementing a preload in assemblies with bearings. One method is sometimes called solid preload and involves the use of fixed metal shims or spacers that are inserted to take up the needed amount of play. These spacers are then held in place by the proper tightening of a retaining nut. A second method that is simpler and easier to implement is known as spring preload and involves the use of springs designed in the form of components that are called load rings. Load rings can reduce the assembly time needed, eliminate the need for maintaining and using shim stock, and provide for a simpler, highly repeatable manufacturing process that reduces costs and improves quality. The key characteristic of load rings is their ability to provide a near-constant axial force over a relatively broad range in deflection, thus making the preload process easier to properly implement.
Ratings of Ball Bearings
Ball bearings are manufactured in four standard series: extra extra-light, extra-light, light, medium. Ball bearings in these series are fully interchangeable among manufacturers. While most bearings are fine with an ABEC-1 rating, some bearings, classified as super-precision are rated ABEC-7 or ABEC-9 and might be used for machine tool spindles.The operating life of a single ball 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 Ball 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.
Ball bearings are available as special designs as well as in a variety of configurations such as mounted units, duplex bearings, thin-section bearings, etc. They are also tailored to the requirements of specific industries such as aerospace, food, and pharmaceutical, etc. as well as specific applications such as instrument bearings.
Mounted units include pillow blocks, flange bearings, rod end bearings, cam followers, etc. These include housings 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.
Duplex bearings employ matched bearing pairs with their adjoining faces ground such that the bearings may be preloaded during installation. This provides a means of reducing the internal bearing clearance to nearly zero to provide absolute radial shaft location and/or increase system stiffness. Standard bearings are made with enough clearance that the inner ring can be lightly pressed onto a shaft without affecting bearing operation; sometimes bearings are heated prior to installation or shafts are cooled to provide room enough to shrink the bearing into position—these methods must be applied judiciously. When two bearings are mounted on the same shaft, one needs the ability to move axially to allow for thermal expansion of the shaft relative to any housing. A so-called floating installation requires that the unrestrained bearing be installed with a slip fit; the bearing must not be allowed to spin on the shaft or within the housing.
Thin-section bearings are used in applications that demand lightweight components. Unlike traditional bearing designs, thin-section bearings retain the same cross-sectional dimension as bore size increases.
Ball bearings are sold as units and are replaced rather than repaired in all but some special applications, such as cup-and-cone bearings used on bicycle wheels.
Unground ball bearings are used where the precision and cost of ground bearings is unwarranted, as with idler pulleys, casters, etc. Often the supported component itself provides one of the races.
Applications of Ball Bearings
As a general rule, ball bearings are used at higher speeds and lighter loads than are roller 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.
Ball 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.
Ball 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 conditions 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.