Roller bearings are used to replace sliding movement with low friction, rolling motion in rotary applications. The principal types of roller bearings are cylindrical, spherical, and tapered. In general, roller bearings offer higher load capacities than ball bearings of the same size.

Types of Roller Bearings

There are five main types of roller bearings:

Cylindrical Roller Bearings have high radial-load capacity and moderate thrust loads. They contain rollers which are cylindrically-shaped, but crowned or end-relieved to reduce stress concentrations. Cylindrical roller bearings are similar in design to needle roller bearings but the dimensions of diameter and roller length are closer in magnitude.

Spherical Roller Bearings are self-aligning, double row, combination radial and thrust bearings. They use a spherical or crowned roller as the rolling element

Tapered Roller Bearings consist of an inner ring (cone), an outer ring (cup), a cage and rollers, which are profiled to distribute the load evenly across the roller. During operation, tapered roller bearings create a line contact between the raceway and rolling element, distributing loads across a larger area.

Needle Roller Bearings are a type of cylindrical roller bearing where the length of the roller is much larger than then the diameter. Needle roller bearings are designed for radial load applications where a low profile is desired.

Thrust Bearings are designed for pure thrust loads, and can handle little or no radial load. Roller thrust bearings use rollers similar to other types of roller bearings

Components

Radial type roller bearings (cylindrical, tapered, spherical, and needle) consist of four basic components, an inner ring, an outer ring, rollers, and a cage (roller retainer). Under normal operating conditions, bearing rings and rollers carry the load while the cage spaces and retains the rollers on the cone.

Comparison of Cylindrical Roller Bearing and Ball Bearing Components

Image Credit: bridgat

Roller thrust bearings are designed to carry pure thrust loads. Like radial roller bearings, roller thrust bearings also consist of two rings, rollers, and a cage (roller retainer). However, instead of an inner and outer ring concentric to the axis of rotation, they have two rings or thrust washers on either side of the roller.

Tapered roller thrust bearings

Image Credit: Shandong HRT Bearing Manufacturer Co., Ltd.

Specifications

Key specifications for roller bearings include dimensions, rated speed abd various rated load types.

Dimensions

Important dimensions to consider when specifying bearings include:

Bore - The bearing industry uses a standard number system for radial bearings with metric diameter bores. For bore sizes 04 and up multiply by 5 to obtain the bore in millimeters. If the bore is a hex this refers to the dimension across the flats. If the bore is tapered this refers to the smaller diameter.

Outside diameter - The outside diameter of the bearing includes the housing if a housed unit, but excludes the flange if a flanged bearing. The outer ring width is the overall width of the outside of the bearing.

Overall width - The overall width of the bearing or bearing assembly includes the locking collar, if present.

Operating Specifications

Important operating specifications to consider when searching for bearings include rated speed, dynamic axial or thrust load, and dynamic radial load.

• The rated speed for a bearing running with grease lubrication is lower than a bearing with oil lubrication.
• The static axial or thrust load is the maximum load a bearing can endure parallel to the axis of rotation without excessive, permanent deformation.
• The static radial load is the maximum radial load a bearing can endure without excessive permanent deformation.
• The dynamic axial or thrust load is the calculated constant axial load, which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of the inner ring.
• The dynamic radial load is the calculated constant radial load, which a group of identical bearings with stationary outer rings can theoretically endure for a rating life of 1 million revolutions of the inner ring.

More information on bearing load, fatigue, and lifespan can be found on the How to Select Ball Bearings page.

Features

• Flanged - The bearing has a flange for mounting or locating.
• Spherical outside diameter - The bearing is tolerant of misalignment and has a greater load capacity than internal self-alignment, but requires more radial space.

Materials

Materials used for roller bearings are typically alloy or low carbon steels. Some applications require case or thoroughly hardened high carbon bearing quality steel. Depending on the size of the bearing to be produced, appropriate quantities of alloying elements are added to the steel melt to assure optimum properties in the finished product.

When low carbon carburized grades of steel are used, carbon is often introduced into the surfaces of the roller bearing components after machining, to a depth sufficient to produce a hardened case that will sustain bearing loads. The carbon and alloys added earlier ensure a proper combination of hard fatigue resistant case and a tough ductile core. High carbon grades of steel do not require carburizing and are either case-hardened, normally by induction heating, or through-hardened by conventional heating methods.

Another benefit derived from case carburizing tapered roller bearings is the development of residual compressive stresses in the surface layers. These residual stresses retard the propagation of fatigue cracks that initiate close to the bearing raceway and roller surfaces. This helps to improve the bending fatigue resistance at the large rib undercut and ensure the ability to endure heavy shock loads without damage. The hardened case of roller bearing components provides fatigue resistance and the ductile core provides toughness for the roller bearing.

Split Bearing Solutions -  Video Credit: Emerson Bearing via YouTube

Bearing Setting

The bearing setting of a roller bearing is defined as the specific amount of either endplay or preload.

Tapered roller bearings have the inherent advantage of being adjustable; therefore, they can be set to approach optimum performance in almost any application. They may be set manually, supplied as a preset assembly, or set by using an automated setting technique.

Automated roller bearing setting techniques offer many advantages such as reduced setting time, decreased assembly cost, and consistent and reliable setting with minimal skill requirements. They may be applied to both assembly line and field repair applications.

Selection

Selection of roller bearings consists of two steps. First, one must determine the desired bearing life and then select the roller bearing with a sufficient basic dynamic load rating to meet that life requirement.

Applications

Roller bearings are used for heavy-duty moderate-speed applications. Potential applications for spherical and cylindrical roller bearings include power generation, oil field, mining and aggregate processing, wind turbines, gear drives, rolling mills. Single-row tapered roller bearings are used in such applications as machine tool spindles, gear reduction units, automotive transaxles, transmissions, vehicle front wheels, differential and pinion configurations, conveyor rolls, machine tool spindles, and trailer wheels.

Image Credit: NSKF-Bearings (Ningbo) Co., Ltd.

Standards

Roller bearings are subject to standards which indicate their precision and efficiency. Bearing quality is rated by RBEC (Roller Bearing Engineering Committee). These ratings classify different accuracy and tolerance ranges for roller bearings. The higher the RBEC number, the tighter the bearing tolerances. Very-high-speed applications will see the greatest benefit from a more precise bearing.

A manufacturer does not have to follow these industrial guidelines. North American roller bearings are under edict of the RBEC scale, while other ball bearings adhere to ISO or its regional equivalent (DIN, KS, etc.) There are five accepted levels of the RBEC scale and the level is not related to the size of the bearing.

There are over 1000 standards related to roller bearings and the related products. Below is a brief list of standards to reference.

A-A-52414 - Roller Bearing Thrust

ABMA STD 11 - Load ratings and fatigue life for roller bearings

BS EN 1337-4 - Structural Bearings - Part 4 Roller bearings

DIN 5402-1 - Roller bearing components; cylindrical rollers

Resources

Introduction to Antifriction Bearings - Online lecture from Timken

Bearing Basics - Emerson Industrial

Image credits:

SKF | Victor