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Most rolling-element bearings are either ball or roller bearings. The roller bearing family consists basically of cylindrical, tapered, spherical, and needle bearings. Needle roller bearings are the smallest and lightest of the roller bearing family. That gives them specific advantages for certain applications, particularly those requiring reduced weight and space. The high roller length-to-diameter ratio helped give the bearings their name as well as their operating characteristics. Essentially, needle roller bearings have:
• Higher load capacity than single-row ball or roller bearings of comparable OD.
• The ability to handle a larger, more rigid shaft in a given application.
• Excellent rolling characteristics within a small cross section.
• Generally lower cost, especially for the drawn-cup type compared with machined versions.

Needle rollers

The most economical type of needle roller bearing is a full-complement of loose needle rollers assembled directly between a hardened and ground shaft and housing. Generally, hardened end washers provide axial location. This type appears in many applications such as those where a hardened and ground gear bore serves as the outer raceway.

When application requirements are met and assembly is not difficult, a full complement of rollers forms a bearing of small cross section and high load capacity. It is the lowest-cost rolling bearing per pound of capacity, and is of particular advantage where production rates justify automated assembly equipment.

Needle rollers come in several end configurations. Controlled contours have also been developed to reduce roller-end stress concentrations from misalignment or deflection of one or both raceways under load. Controlled contour refinement brings more uniform stress distribution and optimum bearing performance.

Common applications: U-joints, planetary gears, constant-mesh gears.

Drawn-cup needle roller bearings

For ease of handling and installation, drawn-cup needle roller bearings do as well as loose rollers but in a unitized assembly. The outer shell, which serves as the outer raceway, is accurately drawn from low-carbon strip steel and case hardened. No subsequent machining is needed, making this bearing economical.

The turned-in lips of the cup provide mechanical retention of a full complement of trunnion-end needle rollers (photo). Also, these lips act as close-fitting shields and help retain lubricant and exclude foreign matter.

Full-complement drawn-cup needle bearing load ratings equal or exceed those of ball and roller bearings of comparable OD. Thin cross sections allow larger shaft diameters on which the bearings operate directly. The large number of contact lines formed by the loaded rollers suits this type well for static, slow rotating, or oscillating conditions.

Drawn-cup caged bearings share many characteristics with their full-complement counterparts, such as being wellsuited for use in housings of low hardness. Though they have less load capacity than full-complement bearings, they are better suited for high speed and shaft misalignment or load deflection.

Cage construction is what provides advantages for high speeds and slopes relative to the bearing centerline. The onepiece cage retains and guides the rollers with little effort and produces optimum roller stability.

Open areas of the cage enhance lubricant circulation and provide the grease storage space a prelubricated bearing needs. Optional integral seals help retain grease and exclude contaminants.

Common applications: general gearbox shaft supports, pulley supports, gear pumps, pilot bearings.

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