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Bearings Reduce Friction

by Ron Kurtus (revised 16 June 2016)

Bearings are constrained elements used to reduce the friction of certain moving parts in a machine or device. This is done by replacing sliding friction with rolling friction. Bearings are used to provide better linear motion or rotation around a fixed axis.

Roller bearings are typically cylinders similar to free rollers, except that they are constrained in their motion. Ball bearings are spheres that are held in a track.

Roller bearings are used for heavier loads than ball bearings.

Questions you may have include:

What are roller bearings?
What are ball bearings?
How are they used?

This lesson will answer those questions. Useful tool: Units Conversion

Roller bearings

Roller bearings are similar to rollers, except that they are constrained in a track or container.

(See Rollers are Friction-Reducing Machines for more information.)

A common use of roller bearings is the facilitate the turning of a wheel about an axle. The resistive friction is much less than if the shaft was turned with sliding parts.

Roller bearings for an axle

(Wikimedia Commons by Silberwolf)

Ball bearings

Ball bearings are spherical bearings loosely held in a track or groove. They are often used between a wheel and a fixed axle or between a wheel-axle and vehicle shaft.

Ball bearings for an axle

Comparison

Roller bearings are used for high loads or heavy objects. They allow the force to be spread along the length of the roller. Automobile axles usually employ roller bearings. Although roller bearings greatly reduce the friction in rotating a wheel, they do have more friction than ball bearings.

Ball bearings are used when the load is not great. They are used to facilitate opening and closing drawers and in lighter weight vehicles, such as bicycles. Since the point of contact for a ball bearing is very small, the rolling friction is also small, but the pressure at the point of contact can be high.

In both cases, the force mechanical advantage is:

MA_F = μ_S/μ_R

where

MA_F is the force mechanical advantage
μ_S is the sliding coefficient of friction without bearings
μ_R is the total rolling coefficient of friction of the bearings

Summary

Bearings reduce the friction of certain moving parts by replacing sliding friction with rolling friction. They are used to provide better linear motion or rotation around a fixed axis. Roller bearings are typically cylinders similar to free rollers, while ball bearings are spheres that are held in a track.

Roller bearings are used for heavier loads than ball bearings.

Machines can be fascinating