When space is at a premium

Sept. 1, 2005
Needle-roller bearings go places others can't.

Needle-roller bearings come in a variety of sizes and configurations. Applications include hydraulic gear pumps and motors, appliances, office equipment, two-cycle engines, power tools, gearboxes, steering, automotive accessories, and rear axles.

Cage-and-roller-needle bearings are made with either steel or engineered-polymer cages to accommodate a wide range of temperature and loading conditions.

Needle-roller bearings are typified by an ability to handle high loads in a compact envelope. This quality makes them a staple in a variety of industries, including power transmission, construction, agriculture, consumer products, automotive, and industrial goods.

Needle-roller bearings come in four main types: drawn cup, machined race, cage and roller, and thrust.

Drawn-Cup Bearings
Drawn-cup bearings, so named for their mechanically drawn cup of strip-steel stock, support radial loads. Needle rollers go between the cup bore (outer raceway) and shaft OD (inner raceway). Most applications depend on a housing press-fit to both size and locate the bearing. The design can accommodate seals, additional oil holes, and hold grease, if needed. Drawncup bearings include full complement, caged, heavy wall, balanced stress and inverted cup as well as roller clutches and drawn sleeves.

Full-complement types employ the maximum number of rollers, which are typically retained by the formed lips of the drawn outer shell. Caged bearings instead use a one-piece steel or engineering-polymer cage to guide and retain the rollers. Full-complement bearings support greater loads than caged versions, though the latter can operate at higher rotational speeds. The use of heavy wall cups raises load capacity.

Engineers may want to consider balancedstress, drawn-cup bearings for demanding applications with high shaft stresses. The patented technology from The Timken Co., Canton, Ohio, extends bearing life by balancing the stresses between the cup and roller, and the roller and shaft. Applications include gear-pump shafts and axle bearings.

Rollers in inverted-cup bearings ride on the O.D. of a drawn sleeve. The arrangement can replace ground raceways on shafting in planetary gear sets, for example. Eliminating shaft heat treating and grinding may lower manufacturing costs. Drawn sleeves can also be used to transfer oil or act as spacers and valve-body sleeves.

Drawn-cup roller clutches lock in one rotational direction and overrun in the other. Clutch-only units and clutch-and-bearing assemblies are compact, lightweight, and work directly on hardened and ground shafts.

Machined-Race Bearings
Machined-race needleroller bearings also support radial loads and come in full complement, caged, or balanced-stress versions. The bearings are made with through-hardened outer rings, which are machined and ground from seamless tube or bar stock. Rollers (either single or double path) typically ride directly on a shaft, as opposed to a separate inner raceway. A machined, inner ring serves as the inner raceway should shafting not meet bearing-raceway specs. Machined-race bearings generally have a clearance fit in the housing and mount against shoulders or washers. They are made to tighter diameter and running tolerances and do not need the housing for sizing as do drawn-cup bearings.

Machined-race bearings can incorporate flanges, grooves, or holes for better lubrication, and come in larger sizes and capacities than drawn-cup types to support high radial loads. Bearings with a split outer and two-piece cage and roller ease installation in certain designs. A snap ring secures the outer-ring halves. The split design is used in the center main bearing position on a one-piece engine crankshaft, for example. Machined-race bearings also come packaged with seals, inner rings, or grease. Typical applications include hydraulic pumps, steering gears, outboard engines, and automatic and manual transmissions.

Cage-And-Roller Bearings
Cage-and-roller-needle bearing assemblies support radial loads at high rotational speeds. Here, a cage both guides and retains the rollers. Cage-androller assemblies come with single or double-row bearings, and with steel or engineered-polymer cages. Multiple-row bearings boost load capacity while flanges axially locate the bearing and handle limited thrust loads. Scalloped flanges improve lubrication flow in such applications as drives and speed reducers. Cages can be made in one continuous piece, with a single split (polymer only), or a pair of split halves (steel or polymer). Polymer cages can incorporate lubrication passages and other customdesign features.

Typical applications for cage-and-roller bearings include gearboxes, transmissions, two and four-cycle engines (main, crank-pin, and wristpin positions), planetary gear sets, and air compressors.

Thrust Bearings
Needle-roller thrust bearings carry only axial loads and consist of equally spaced, caged rollers arranged in a radial, spokelike pattern. Thrust bearings can use as raceways the shaft and housing shoulders, separate thrust washers, or both in a unitized assembly. Unitized designs simplify installation and eliminate the need for heattreated and precision-finished shafts and housing shoulders. A washing machine, for example, uses a unitized needle thrust bearing assembly between the transmission and brake. Compared with the original multipart design, the unitized bearing better retains grease for longer life and solves a piloting problem. In addition, the design assembles only one way, lowering the number of installation errors.

In any case, engineers considering needle roller bearings should look at the size/envelope, type of load, load capacity, speed, temperature, misalignment, housing and shaft specs, service life and lubrication needs. In addition, it's probably a good idea to involve a bearing engineer, preferably early in the design phase.

Selecting a needleroller bearing

Given the design variables: Bearing nominal bore = 1.5000 in. Constant speed, n = 3,000 rpm Constant radial load, P = 4,000 lbf

Select an appropriate needle-roller bearing. In this case, cage-androller assemblies have the highest limiting speed and full-complement types, the lowest. Generally, limiting speed is a function of the friction generated by the line contact between the rollers, shaft and housing. Dynamic load rating is a function of the number of rollers, roller diameter, and length. Other factors that influence bearing life include housing and shaft materials, lubrication, installation and assembly procedures, application temperature range, environmental cleanliness and sealing needs, and duty cycle.

Cage & roller assembly
Caged drawn cup
Full-complement drawn cup
Caged machined race
Full-complement machined race
Bearing bore, in.
Bearing OD, in.
Bearing width, in.
Housing specs
Surface finish: ≤8 μin. Ra Tolerance: 0.0006 in. Hardness: ≥HRC 58 Roundness: 0.0003 in.
Surface finish: ≤63 μin. Ra Tolerance: 0.0010 in. Hardness: not applicable Roundness: 0.0005 in.
Surface finish: ≤63 μin. Ra Tolerance: 0.0010 in. Hardness: not applicable Roundness: 0.0005 in.
Surface finish: ≤63 μin. Ra Tolerance: 0.0012 in. Hardness: not applicable Roundness: 0.0006 in.
Surface finish: ≤63 μin. Ra Tolerance: 0.0012 in. Hardness: not applicable Roundness: 0.0006 in.
Shaft specs
Surface finish: ≤8 μin. Ra Tolerance: 0.0006 in. Hardness: ≥HRC 58 Roundness: 0.0003 in.
Surface finish: ≤ 8 μin. Ra Tolerance: 0.0005 in Hardness: ≥HRC 58 Roundness: ≤ 0.0002 in.
Surface finish: ≤ 8 μin. Ra Tolerance: 0.0005 in. Hardness: ≥HRC 58 Roundness: ≤0.0002 in.
Surface finish: ≤ 8 μin. Ra Tolerance: 0.0006 in. Hardness: ≥HRC 58 Roundness: ≤ 0.0002 in.
Surface finish: ≤ 8 μin. Ra Tolerance: 0.0006 in. Hardness: ≥HRC 58 Roundness: ≤ 0.0002 in.
Dynamic load rating1, Cr, lbf
Static load rating2, Cro, lbf
Limiting speed, rpm
1 per ISO 281 2 per ISO 76 * Basic rating life; Assume oil lubrication for each bearing type.

Reducing Stress

Needle rollers under load experience contact stresses that are concentrated toward the roller ends. Misalignment between the shaft and bearing bore raises roller contact stress as well. A combination of high radial loads and shaft misalignment can cause excessive shaft stresses that trigger fatigue spalling. That is why rollers are often designed with a special profile that relieves the high stress concentration. A balance-stress bearing design further lowers shaft stress and extends fatigue life by 20% compared with standard designs under the same loading and shaft-misalignment conditions.

The Timken Co., www.timken.com

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