Bearing Pressure

The initial step in the selection and sizing of a bearing involves determination of the operation bearing pressure,. Bearing pressure is defined as the load divided by the projected area :

P = W / d x L

where :

P = Bearing pressure, MPa
W = Load, N
d = Shaft diameter, mm
L = Bearing length, mm

This gives the average pressure, MPa, that the bearing supports. Elevated temperature reduces load capacity ; lower temperature generally increases static load capacity.

V = π x d x N

where :

V = Bearing speed, m/s
d = Shaft diameter, m
N = Revolution per second

This gives the sliding velocity of the bearing. Lubrication or liquid cooling can extend these limits significantly.

Bearing Proportions

Optimum performance can be achieved by specifying a length to inside diameter ratio (L/d) ranging from 0.5 to 2.0.  Values of L/d less than 1.0 result in easier escape for wear debris and less sensitivity to shaft deflection and misalignment. There may also be some cost advantage in using a bearing with a small L/d ratio.

If the L/d is higher than 2.0, distortions or misalignment may cause stress concentrations and excessive localized heating. When a long bearing is required, it is advisable to consider using two bearings with a small gap between them or to increase the inside diameter, d, and re-estimate the bearing geometry.

PV Limits

In addition to the individual consideration of load capacity, P, and speed of operation, V, the product PV is an important performance parameter for bearing design when boundary lubrication occurs. The PV value is a measure of the ability of the bearing material to accommodate the frictional energy generated in the bearing. At the limiting PV value, the bearing will not achieve a stable temperature limit, and wear will increase rapidly as a consequence of thermal effects or of stresses approaching the elastic limit.
Frictional heating can be reduced by material configurations or liquid lubrication. Many CDS bearing materials are offered with internal dry lubrication such as graphite, MoS2 PTFE, or oil impregnation. Bearing performance can also be enhanced with regular lubrication in the form of oil or grease to reduce friction and remove heat from the bearing interface.

Plastic-based materials have comparatively high coefficients of thermal expansion and as a consequence bearing clearance tends to decrease with rising temperature.

Moisture absorption is a characteristic of polymer-based material which must be considered when deciding running clearance, as the bore of the bush will close slightly due to swelling of the material.

Mating Metal Surface

A material that is hard compared with the bearing material is chosen to ensure that the bearing wears in preference to the mating material. For CDS bearing materials, stainless steel and hard chromium plated steel are often satisfactory. The counterface finish can substantially affect the wear of the bearing bedding-in and during running. Values of 0.2~0.4μmRa with an upper limit of 0.8μmRa are usually specified for the surface roughness.
However reductions down to 0.05μmRa will almost always further reduce wear rate. For the same roughness, the type of surface finish also affects wear rate. In general a ground surface is preferable to a turned surface but in either case a fine polishing operation is often beneficial. The finishing operation should preferably be in the same direction as the bearing motion relative to the mating surface.

Wall Thickness and Bore Closure

Due to the resilience of CDS bearing materials, an allowance must be made for bore contraction which occurs when a bush is pressed into its housing and is dependent on the ratio of bore diameter to wall thickness. A bearing employing the optimum wall thickness for CDS bearing materials, 2.5 + 0.05d , will experience a contraction in the bore directly proportional to the degree of interference on the outside diameter.

Environmental Suitability

Selection of appropriate materials for bearing application is to eliminate those that are unsuitable for the environmental conditions. Polymer and their composites are particularly resistant to attack by many chemicals, and can therefore be used in many applications where chemical media are present.
The majority of polymer materials can be used in inert gas or vacuum environments, the important exception being composites that contain graphite. These should not be used in a vacuum or in very dry gases because, in order to act as a lubricant, graphite requires the presence of a condensable fluid such as water vapour.
The environmental suitability of the CDS materials are summarized briefly in the below table :

The grading in this table gives an approximate indication of the proportion of suitable materials. "A" indicates that the majority of materials in this group will be suitable while "E" indicates that few, if any, will be unsuitable.

Fitting

The normal method of retaining a bush in its housing by an interference fit is only satisfactory for applications where operating temperature changes are modest. The level of interference required depends upon the type of material used. For applications involving large temperature changes, some positive method of location, such as a suitable adhesive or mechanical device, is essential.