BEARING PROPERTIES

A good bearing alloy must possess certain desirable characteristics and the minimum of undesirable properties, the essentials being :

1.   Mechanical Strength - Here a compromise must be adopted as too soft a bearing metal, while possessing, other desirable properties, tends to extrude or flatten under heavy loads. On the other hand, a harder alloy capable of withstanding high loads, may possess high frictional characteristics and of greater importance, it may be brittle and have poor fatigue characteristics.

2.   Softness and melting point - The softness.and modulus of elasticity of a bearing alloy should be as low as possible but hard enough to withstand the heaviest continous or shock loading to which it is likely to be subjected, without plastic deformation.

Low melting point coustituents will under boundary conditions with high local asperity contact temperatures, enable the softer metal to melt and flow locally and/or deform plastically, without 'running' or damage to the harder steel. This property of the softer bearing alloys such as tin - base or lead - base whitemetal is frequently termed 'Conformability'.

When there is dirt, rust or other foreign matter entering the bearing with the oil, it is advantageous if the bearing alloy has the ability to absorb such contaminents, thus avoiding damage or scoring of the journal. Again white metals or babbits are good in this respect, particularly if a thick lining is used. This property of bearing alloys is commonly termed 'embeddability'.

3.   Compatiability - This is an indication of the anti - weld or anti - score characteristics of a bearing material with a given bearing and journal combination under boundary lubrication conditions. In general the softer alloys are beneficial.

4.         Corrosion Resistance - Under high temperature conditions, some metals used in bearing alloys are attacked by decomposition products of lubricating oil, such as weak organic acids and even peroxides. Others may be attacked severely by extreme pressure additives under very high temperature condition and some by highly alkaline additives. In the case of diesel engine, the attack may be caused by mineral acids (H₂SO₄) formed due to high sulphur content of fuel oil.

Alloys containing lead, cadmium, silver, zinc, copper and phosphorus are most susceptible to corrosive attack, specially at higher temperature.

A successful method of protecting both steel and bearing alloys is to add oxidation inhibitor and corrosion inhibitor in the oil.

Provided that the loads and temperatures are not excessive, good tin - base or even lead - base whitemetal alloys are most suitable for journal and thrust bearings.

THIN SHELL BEARINGS

In order to improve the relatively poor compressive strength at high operating temperatures, of thick whitemetal lining bearings, it is now common to use steel bearing shells with a relatively thin lining of whitemetal, which has a high load capacity and high fatigue strength. For example, in small high speed diesel engines, the fatigue strength of a 0.3 mm whitemetal lining is about 141 kg/cm² whereas if this is reduced to 0.08 mm, the fatigue strength rises to 211 kg/cm² of course, the thinner the lining, the poorer the conformability and embeddability.

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