Bearing selection and lubrication guide

A bearing supports a moving load. The earliest bearings were probably logs placed under heavy building stones so they could be rolled into position. Lubricated axles in holes that elongated under load led to replaceable plain bushings.

04/01/1999


A bearing supports a moving load. The earliest bearings were probably logs placed under heavy building stones so they could be rolled into position. Lubricated axles in holes that elongated under load led to replaceable plain bushings. About 1500 AD, Leonardo DaVinci invented and developed the modern antifriction bearing. Widespread use of rolling element bearings began about the time of the Civil War.

Since then, rolling element or antifriction bearings have developed into a variety of designs, including balls and various rollers such as cylindrical, barrel shaped, tapered, and needle. Despite the different shapes of rolling elements, bearings have several components in common: inner ring, outer ring, rolling elements, and cage to keep the rolling elements apart.

Balls geometrically contact raceway surfaces at "points" while the contact surface of rollers is "line." Theoretically, rolling bearings are constructed to allow the rolling elements to rotate orbitally while rotating on their own axis.

Antifriction bearings are characterized by low frictional losses. Starting friction is low so savings in driving power for machinery can be effected. Other features are long service life, low lubricant consumption, little wear, rapid fitting and dismantling, and interchangeability. Bearings are normally supplied as units ready for installing.

For commercially available bearings the external dimensions and tolerances are internationally standardized. Internal dimensions and designs are not standardized, though as a rule there are no significant differences between the bearings of a certain class and quality supplied by different manufacturers. Most bearings have a cylindrical bore, but in some cases a tapered bore is provided to fit on a tapered shaft.

Can a bearing be reused?

Carefully clean and inspect a used bearing to determine if it can be reused. Use a small mirror and dental-type probe, with a rounded point, to inspect raceways, cage, and rolling elements. Be alert for scratches, marks, streaks, cracks, discolorations, and mirror-like surfaces. Spin the bearing and listen to the sound. An undamaged bearing runs evenly without abnormally large radial internal clearance. Before a large bearing is remounted, have the manufacturer examine it. The cost of such inspection can actually save money.

Bearing facts

* Ball bearings are the least expensive in smaller sizes under light loads.

* Ball bearings are suited for light to moderate radial loads, limited thrust loads, and high speeds.

* Roller bearings are the least expensive in larger sizes under heavy loads.

* Roller bearings are suited for heavy radial and thrust loads and lower speeds.

* Roller bearings handle shock or impact loading better than ball bearings.

* Roller bearings are more sensitive to misalignment than ball bearings.

* Ball-thrust bearings are for pure thrust loading only. At high speeds, a deep-groove or angular contact ball bearing is a better choice, even for pure thrust loads.

* Self-aligning ball bearings and cylindrical roller bearings have very low coefficients of friction.

* Deep-groove ball bearings are available with built-in seals and lubrication to operate for long periods of time without attention.

Bearing characteristics

Bearings support two types of loads, alone or in combination: radial (perpendicular to the shaft) and axial (thrust along the shaft).

Ball bearings have a small contact area between the ball and raceway that allows them to run faster and generate less heat than roller bearings. They accept moderate radial loads and light thrust loads. Some designs compensate for shaft misalignment.

Angular contact bearings accept high thrust loads in combination with radial loads. The amount of thrust load depends on the angle of contact between the races and the bearing axis.

Thrust bearings support high thrust loads. They use balls or cylindrical, spherical, or tapered rollers sandwiched between flat races.

Spherical bearings have spherical rollers and races that withstand both radial and thrust loads. Their spherical shape makes these bearings self-aligning.

Cylindrical roller bearings have more contact area between rollers and races and support heavier loads than ball bearings. Three main types of rollers are cylindrical, spherical, and tapered. Cylindrical bearings support the highest radial loads. Some designs carry thrust loads. Similar to cylindrical types, needle bearings have smaller diameter rollers, no inner races, and require less space.

Tapered roller bearings carry both radial and thrust loads. Their components include tapered rollers contained by a cage and outer and inner rings called cups and cones. Internal clearances accommodate misalignment.

Bearing selection guide

Bearing Sealed (Se) Load capability Allowable Coefficient of

Type of bearing bore or shielded (S) Radial Axial misalignment, deg friction (est)

Single row deep 1 or 2 Se Light and Light and

groove ball Cylindrical 1 or 2 S medium medium 0.01-0.05 0.0015

Single row angular Medium

contact ball Cylindrical -- Medium and heavy -- 0.0020

Double row angular

contact ball Cylindrical -- Medium Medium -- 0.0024

Double row Cylindrical Light and

self-aligning ball or tapered 2 S medium Light 2-3 0.0010

Single row

cylindrical roller Cylindrical -- Heavy -- 0.03-0.10 0.0011

Double row

cylindrical roller Cylindrical -- Heavy -- -- 0.0011

Needle roller Cylindrical -- Heavy -- -- 0.0025

Single row Medium

tapered roller Cylindrical -- Heavy and heavy -- 0.0018

Double row Cylindrical Very Light and

spherical roller or tapered -- heavy medium 1.5-3.0 0.0018

Single direction Light and

thrust ball Cylindrical -- -- medium -- 0.0013

Double direction

angular contact Cylindrical -- -- Medium -- 0.0013

thrust ball

Not to exceed Refer

55% of to

Spherical roller thrust Cylindrical -- simultaneously Heavy 1.5-3.0 manufacturer

acting axial load

Information given in this chart is for general guidance only. The degree of misalignment permitted is dependent on internal design.

Manufacturers should be consulted.





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