Some things to consider when selecting bearings

Load on bearings

The load may either be radial (journal) or axial (thrust) or may be a combination of the two.

Radial Load is applied at Right Angles to the axis of the shaft

Axial Load is applied Parallel to this axis of the shaft

In order to calculate the load on a bearing the origin of the load must be known; this can be one or a combination of the following :-

1. Torque reactions in power transmission
2. Reactions due to centrifugal force, out of balance and inertia loads
3. Weight of machinery components supported by the bearing
4. Reaction due to tension in belts, etc

These loads must resolved into one radial load and the larger the axial load (if there is one in each direction) : this then gives the dynamic loading of the bearing.

The static loading of the bearing must also be calculated, this usually being the only weight of components supported by the bearing.

Speed and Life

The speed and required life of the bearing must be determined, the speed quoted in rev/min and the life in hours or millions of revolutions.

The manufacturer's catalogue should be consulted and the load, speed and life related (by the formulae given) to the dynamic capacity quoted in the catalogue. Each manufacturer uses a different optimum speed and life on which the dynamic capacity is based; therefore no general formulae can be given.

Diametral Clearance

This is the total distance through which the inner race can move relative to the outer race in a radial direction. Bearings are usually manufactured with what is known as normal of two dot (00) diametral clearance but bearings with larger or smaller diametral clearance can be supplied at extra cost.

Fit of Bearings on Shaft and in Housings

The fit of a bearing in a housing or on a shaft can affect the diametral clearance; therefore it is necessary when dimensioning a shaft or housing to consult the manufacturer's catalogue which will usually quote limits to be applied for various fits.

Friction

The friction coefficient will very considerably with load, speed and lubrication but the following values give a guide to values applicable to normal operating conditions and favourable lubrication :-

Self-aligning ball bearings - - - - - - - - - - - - - - - - - - - - - - - - - 0.0010 microns
Cylindrical roller bearings with flange guided rollers - - - - - - - - - 0.0011 microns
Thrust ball bearings - - - - - - - - - - - - - - - - - - - - - - - - - - - - 0.0013 microns
Single row deep groove ball bearings - - - - - - - - - - - - - - - - - 0.0015 microns
Taper and Spherical roller bearings - - - - - - - - - - - - - - - - - - 0.0018 microns

The values quoted are for running friction and it can be expected that friction during starting will be higher.

Lubrication

Provision must be made for lubrication of all rolling element bearings. Lubrication can be either grease or oil. Grease must usually fed to the bearing by grease cup or grease gun injection to provide periodical replenishment, or pump feed injection to supply a large number of bearings simultaneously at intervals. A drain should be provided in order that contaminated gases can be ejected; this should be fitted on the opposite side to that of the filling cup or nipple. Grease cannot be used to dissipate heat from a bearing.

Oil lubrication can be provided by :-

• Hand Feed - Into a ball sealed nipple for machinery operating at high speed for short and infrequent periods.
• Splash - e.g. gearboxes up to medium size in which the bottom rims of the lowest gears run in oil and splash the remaining components.
• Oil Bath - Suitable for moderate speeds, oil level maintained at a predetermined level. Usually provided with a sight glass and a drain hole to prevent overfilling.
• Drip Feed - Can be used for higher speeds than the oil bath method. Metered supply of oil drips into housing, must be provided with a drain hole.
• Wick Feed - Alternative to drip feed for high speed applications. Wick bears on some part which will throw oil into the bearing.
• Oil Thrower - Thrower rotates in oil bath (maintained at a constant level), picks up oil and throws it onto the bearing.
• Circulatory System - For high speed and high temperature operation where heat dissipation is essential. Oil is circulated by pump in closed circuit.
• Forced Lubrication - Oil is fed under pressure through system by pump, providing a generous flow and good heat dissipation.
• Oil Mist - For very high speeds: small quantities of oil are blown into the bearing housing by compressed air. The oil is atomized before entering the housing.

Seals

All bearings on rotating machinery must be provided with adequate sealing arrangements. These can take the form of external shields and seals in which the devices are incorporated in the assembly external to the bearings, or built-in shields and seals incorporated in the bearings. Any seal which bears on rotating component will increase friction considerably.

Seals used when grease, but not oil is employed as lubricant are : felt washers, grease grooves and labyrinths; seals for oil lubrication are synthetic rubber (also suitable for grease); piston ring seals and labyrinths: the last two only for horizontal applications.

Mechanical Engineering Design Topics Page - Bearing Selection Page

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Pages created by David Price - 9/08/95
This Information is an educational resource and is not to be reproduced (other than for private study) without written consent of the University of Southampton. copyright © Dave Whatley 1995
GlobalSpec.com is presenting this material with the consent of Dave Whatley.

Disclaimer Whilst every care has been taken to ensure the accuracy and completeness of this information, neither the author nor the University of Southampton will accept responsibility for loss, damage etc. however arising occasioned by the use of this information.