When the surfaces of bearing are separated by a fluid film, the bearing materials do not have any effect on its operation so long as the parts have sufficient strength to withstand the imposed loads and sufficient rigidity to maintain alignment.
On account of strength and rigidity requirements, journals are generally made of steel. For the bushing, however, there are other requirements, as discussed below, that must be considered in selecting the best material for bearing.
Low Coefficient of friction:
The bearing materials combination of sliding surfaces, along with the lubricant should provide a low friction coefficient for reducing damage and lower running costs.
High Compressive Strength:
The bearing material should have high compressive strength to prevent extrusion of the material from the bearing, Since the maximum bearing pressure may be considerably greater than the pressure expressed in pounds per square inch of projected bearing area, yielding may occur in localized regions of the bearing to the extent that clearance in materially changed.
High Fatigue Strength:
Under severe conditions of loading and temperature, surface fatigue cracks may develop. Those cracks may cause checking and pitting of the surface, thus leading to its deterioration to the extent that operation of the bearing is affected. Thus a bearing material having high fatigue strength is desirable.
For severe operating conditions a very thin bearing metal bonded to a steel back has been found to prevent the spreading of the fatigue cracks. Such a combination also prevents extrusion of the bearing material because of the restraining effect of the hard backing. In some automotive engines a tin-base Babbitt lining 0.003 inch in thickness is used.
Low Thermal Expansion:
The size of bearing should remain nearly constant during periods of temperature change.
High Thermal Conductivity:
The ability of bearing material to dissipate heat quickly due to friction.
Bearing material should be elastic enough to allow the bearing to return to original shape upon relief of stresses that may cause temporary distortion, such as misalignment and overloading.
Because of small inaccuracies in the form of the journal and its deflection under the imposed loads, the material of the bearing should adapt its shape to that of the journal. this change of shape may be accomplished by plastic flow, by wearing away, or by local melting. Plastics flow is most desirable to achieve conformability, since wearing away and local melting are accompanied by excessive heat, which may burn out the bearing.
All bearing should be designed so that foreign particles will be executed, but this is difficult to accomplish in plain bearings lubricated with oil, since grit, sand and metal particles may be introduced with the lubricant or ventilating air. if the bearing materials are hard, the particles may score the surfaces and produce undue wear. But if the bearing lining is soft so that the particles are completely embedded, this trouble may not be serious.
Bearing materials have various degrees of resistance to corrosion; this should be considered where corrosive oils must be used.
A bearing material that will bond readily and permanently with steel or bronze back is necessary for long bearing life. For the thin linings mentioned above, an intermediate layer of sintered alloy is used to secure a good bond.
Relative Hardness of Bearing Material:
- The bearing material should usually be softer than that of the journal to prevent shaft wear but hard enough to resist adhesive and abrasive wear of its own surface.
- Bearings are more easy to replace than shafts (that require dismantling of the whole engine). If one bearing is worn out only that bearing needs replacement instead of the whole shaft.
Availability of Material:
The material should be readily and sufficiently available, not only for initial installation but also to facilitate replacement in the event of bearing failure.
Cost of Material:
The economic consideration is the ultimate deciding factor in for selecting a bearing material
About Different Bearing Materials and their Uses:
SAE 11 Babbitt is used for bearings which are subjected to heavy pressures. SAE 10 Babbitt is also suited for heavy pressures; it is very fluid and can be applied for thin linings of bronze-backed or steel-backed bearing shells like those used in automotive and aircraft engines.
Lead-base Babbitt may be used for larger bearings when maximum pressures are below 500 psi. however, a lead-base alloy with the trade name Magnolia Metal, the analysis of which is about the same as that of SAE 14 Babbitt, seems to give good service even in heavy-duty bearings if they are subjected to pounding.
Brass is used where the pressure is too high for Babbitt but where the service is not severe enough to call for a more expensive bearing metal. Bronzes are used where the pressures are so high that thin-film lubrication may occur.
Copper alloys having a high lead content 20 to 50% are of special interest. These alloys, put on the the market under different trades names. have a low efficient of thin-film-lubrication friction, about 0.005; and like Babbitt, they do not score a journal when lubrication fails. The allowable pressure is 1,000 to 5,000 psi. In order to obtain a good cooper-lead bearing alloy, it must be casted centrifugally.
Celoron, Formica and Micarta bearings are made from a special woven duck impregnated with phenolic resin. The materials are fused together under a very high pressure and at a high temperature. These materials have good mechanical properties and great resilience. Such bearings can be lubricated with oil, grease or water. they are used on heavy rolling mills as a replacement for bearings of bronze or lignum vitae without any changes in the roll stand itself. With water lubrication they can stand pressures up to 5,000 psi and pheripheral velocities of 2,000 feet per minute (fpm), with a friction coefficient (μ) less than 0.007.
In hydraulics turbines, in stern bearings of ships and in other machines where water is available, rubber bearings lubricated with water are commonly used. Rubber bearings are particularly suitable for use on shafts running at high speeds.
For hardened steel journals, cast iron is a very good bearing material in regard to friction an wear, even if lubrication is in the thin-film region. However, this combination is suited only for light service where the pressure does not exceed 40 psi.
Aluminium alloys are remarkable for their great resistance to scuffing, their low friction and their high wear resistance under conditions of boundary and thin-film lubrication. the high coefficient of heat conductivity (k) helps to carry away the heat of friction. However, aluminium begins to lose its strength at about 225 F. aluminium bearings can operate with pressures up to 3,000 psi, or even 4,000 psi and with peripheral velocities up to 2,000 fpm.
Oil Less Bearings:
Oil less bearing depends on a lubricant incorporated in the bearing during its manufacture. This is done in several ways:
- In some types, flaked graphite is inserted into the metallic (usually bronze) surface in the shape of spirals or studs.
- Another development consists in suspending graphite in a Babbitt alloy under a high pressure at the fusing temperature of Babbitt.
- In other types, oil is impregnated into wood or some other porus or fibrous carrier.
Self Lubricating Bearings:
In powder-metal bearings, pulverized graphite and bearing bronze or iron are mixed with a binder and are pressed into molds with the application of heat. This process is called sintering. A sintered bearing is porus. It can absorb an amount of lubricating oil up to 30 percent of its own weight and can give the soil up very slowly in operation. Sintered bearings maintain a thin oil film for a long time and are correctly called self-lubricating bearings.