Thin Section Bearing

Many Sizes and Configurations
Thin section bearings are offered in a variety of standard cross-sections. Designers can choose from radial, angular contact and four point designs. Angular contact bearings have an open design, while radial and four-point contact bearings can be open or have seals to retain lubrication and keep out contaminants. Radial contact bearings are intended primarily for pure radial loads, but with deep ball grooves can accept some thrust load in either direction. If an increased diametral clearance is specified, the contact angle under axial load increases to give greater thrust capacity. Angular contact bearings can support radial and unidirectional thrust load. They are usually mounted in opposed pairs because they require a thrust load to establish the proper contact angle between ball and races. Four-point contact bearings can support any combination of radial, thrust and moment loading. A single four-point contact bearing can often do the job of two angular-contact bearings.

Separators
Radial contact and four-point-contact ball bearings typically have snap-over, continuous ring separators, while angular contact bearings use circular-pocket, continuous ring separators. Standard materials are brass, low-carbon steel, and glass-fiber-reinforced nylon. For applications requiring corrosion resistance or high-speed operation, stainless steel or phenolic laminate separators are often used. Formed wire separators or segmental cages are an alternative when more balls than usual are needed — e.g., high load capacity at low speed. In low speed positioning applications where torque uniformity is a major requirement, ball separation can be accomplished with specially designed helical compression springs, toroid ball spacers.

Seals and Shields
Like other bearings, a thin section bearing performs best when the rolling elements are kept clean and lubricated. This can be accomplished with a seal, which makes positive contact with the inner race, or a non-contacting shield. Speed, torque, and lubricant must all be considered when choosing a seal or shield. A seal is generally more effective, but generates more torque. This results in more heat and reduces the bearing's limiting speed. Integral seals – often chosen to minimize the overall design – are typically molded of nitrile rubber and are reinforced with steel. Where required, separate ring-type seals may be used.

Design Considerations
Thin section bearings can be used to particularly good advantage where bearing cross-section can be specified early in the design process. (And it's important to consider them early, to take advantage of the ripple effect savings noted earlier.) They are best-known for saving space and weight, but have many other advantages. Thin section bearings are excellent choices for applications requiring high stiffness, accurate positioning at low shaft speed, or a combination of radial, thrust, and moment loads. They permit designers to reduce housing sizes without changing shaft size, and use larger shafts to increase stiffness and/or provide space for other components.

Assuring Proper Fit
Changes in external fit can impose significant bearing loads that increase running torque and decrease life. To minimize this effect, the run out tolerances specified for the housing and shaft should be identical to those specified for the bearing. All tolerances – for shaft and housing-bore concentricity, shaft and housing shoulders, and corner squareness of all mounting surfaces – should be specified with care. Thermal expansion, although easily overlooked, is another consideration. Since thin section bearings are often mounted in lightweight structures with thermal expansion rates different from that of bearing steel, particular attention should be given to temperature differentials between shaft and housing. Steel liners between the bearing and its seats are another way to control fit, and special diametral clearances can be provided to allow for changes in internal clearances. As a rule of thumb, it's a good idea to consult the bearing manufacturer when a change in fit creates an interference greater than or equal to the bearing diametral clearance, or a looseness greater than twice the diameter tolerance of the bearing race.

Lubrication and Maintenance
Lubrication requirements for thin section bearings are substantially the same as for conventional bearings. In general, grease is used for pitch-line velocities up to 2,000 FPM and oil at higher speeds. The oil is delivered by a drip feed, flood, mist, or jet system, depending on the speed, operating environment and other conditions. Some high-speed applications do use grease; usually a special type. Maintaining a clean supply of lubricant should be the only maintenance needed, either by filtering the oil or purging and replenishing the grease (if needed). Thin section bearings, when properly selected, mounted and lubricated, perform as well as any other precision bearing, large or small.