Bearings
Introduction:
The term “bearing” typically refers to contacting surfaces though which load is transmitted. Bearings may roll or slide or do both simultaneously. The range of bearing types available is extensive, although they can be broadly split into two categories: sliding bearings also known as plain surface bearings, where the motion is facilitated by a thin layer or film of lubricant, and rolling element bearings, where the motion is aided by a combination of rolling motion and lubrication is often required in a bearing to reduce friction between surfaces and to remove heat.
The optimal choice of bearings will depend on a number of factors such as load capacity, speed, life, cost, size
Sliding bearings:
The term ‘sliding bearing’ refers to bearings where two surfaces move relative to each other without the benefit of rolling contact.
Rolling contact bearings:
The term ‘rolling contact bearings’ encompasses the wide variety of bearings that use spherical balls or some type of roller between the stationary and moving elements. Most common type of bearing supports a rotating shaft resisting a combination of radial and axial (or thrust) loads. Some bearings are designed to carry only radial or only thrust loads.
Bearing life and selection:
The load on a rolling contact bearing is exerted on a very small area. The resulting contact stresses are very high and of the order of 2000 MPa. Despite very strong steels, all bearings have finite life and will eventually fail due to fatigue.
For two groups of apparently identical bearings tested under loads P1 and P2, the respective lives L1 and L2 are related by:
………………..(1)
Where L1, life at load P1 (number of revolutions); L2 life at load P2 (number of revolutions); P1, load (N); P2, load (N); and k, 3 for ball bearings; and k, 3.33 for cylindrical roller bearings.
The dimensions for rolling element bearings have been standardized and can be purchased as stock items from specialist manufacturers and suppliers. The selection of a bearing from a manufacturer’s catalogue involves consideration of the bearing load carrying capacity and the bearing geometry. For a given bearing the load carrying capacity is given in terms of the basic dynamic load rating and the basic static load rating. The various commonly used definitions for rolling element bearing life specifications are outlined and illustrated by the example below.
The basic dynamic load rating, C, is the constant radial load which a bearing can endure for 1 x 106 revolutions without evidence of the development of fatigue in any of the bearing components.
The life of a ball bearing, L, is the number of revolutions (or hours at some constant speed), which the bearing runs before the development of fatigue in any of the bearing components.
The rated life, L10, of a group of apparently identical bearings is defined as the number of revolutions (or hours at some constant speed) that 90 per cent of the group of bearings will complete before the first evidence of fatigue develops.
If in Eq. (1), P2 = C and the corresponding life L2 = 1 x 106, then the life of a bearing L, with basic dynamic load rating C with a load P is given by
million revolutions ………………………. (2)
Where L, life (millions of revolutions); C, basic dynamic load rating (N); P, load (N).
When selecting a particular bearing from a manufacturer’s catalogue, it is useful to know the required basic dynamic load rating C for a given load P and life L, which is given by:
……………………………………… (3)
Example 1:
A straight cylindrical roller bearing operates with a load of 7.5 kN. The required life is 8760 hours at 1000 rpm. What load rating should be used for selection from the catalogue?
Solution:
Using Eq. (3)
The equivalent load, P, is defined as the constant radial load which if applied to a bearing would attain under the actual radial and thrust load and rotation. When both radial and thrust loads are exerted on a bearing the equivalent load is the constant radial load that would produce the same rated life for the bearing as the combined loading.
Normally;
……………………………………………(4)
where P, equivalent load (N), V=1.2 if mounting rotates is recommended, V=1.0 if shaft rotates, X, radial factor (giving in bearing catalogues); R, applied radial load (N); Y, thrust factor (giving in bearing catalogues); T, applied thrust load (N).
X = 0.44:0.56, Y=1.0:2.0
Example 2:
A bearing is required to carry a radial load of 2.8 kN and provide axial location for a shaft of 30 mm diameter rotating at 1500 rpm. An L10 life of 10 000 hours is required. Select and specify an appropriate bearing.
Solution:
Axial shaft location is required, so a deep groove ball bearing, which provides axial location capability in both directions, would be suitable.
The total number of revolutions in life is
So
L = 900. The load is purely radial, so P= 2800 N. The required dynamic loading is given by:
Referring to the deep groove bearing chart show a suitable bearing could be:
· ISO designation 6306
· Bore diameter 30 mm, outer diameter 72 mm
· Width 19 mm
· C = 28 200 N, Co = 16 000N
· Speed limit (using grease) 9 000 rpm
· Speed limit (using oil) 11 000 rpm
The basic static load, Co, is the load the bearing can withstand without any permanent deformation of any component. If this load is exceeded it is likely the bearing races will be indented by the rolling elements (called Brinelling).
