The functional reliability and the life span of plain bearing mountings depends on, for any specific bearing design, the properties of material pair: bearing - shaft, and on the lubricant properties. According to the operating conditions, either the effect of material of the sliding pair or the effect of lubrication prevails.
Design parameters, operating conditions, and economic demands, are the base for selecting bearing materials. Besides sliding properties, the size and character of the loading force, the range of the sliding speed and operating temperatures induced by them, the quantity and type of lubricant, the lubrication reliability, the effects of the ambient environment, the life span desired and economic factors, all are necessary considerations. It is necessary to consider sliding, mechanical, and physical properties to select the bearing material correctly.
Demands for plain bearings are diverse, so known bearing materials cannot meet them in the full range. The order of most important demands must be determined for any specific case and a partial solution to be selected. For hydrostatic bearings or bearings working in the zone of hydrodynamic lubrication, for the guaranteed strength, the attention must be first devoted to the bearing design and lubricant selection. For bearings working in the zone of limiting lubrication, which were also calculated for the zone of hydrodynamic lubrication, and for the plain nonlubricated bearings, sliding properties of the material must be considered. These sliding properties are the ruling criterion for the zone of mixed friction.
The main criteria for the appreciation of bearing materials are:
The meaning and order of these criteria are different depending on the conditions. According to these conditions, other criteria such as: the running property, emergency properties, wet ability by lubricant, hardness, thermal conductivity, thermal extensibility, density, and so on, can be included.
The most important categories of bearings materials are tin and lead compositions, alloys of copper with tin and lead, aluminum alloys, other metal materials (gray cast iron, porous metals, and so on), plastics, other non-metal materials (graphite, rubber, wood, and so on). The functional criteria of main bearing material categories are presented in the following table:
|
Bearing material |
Resistance against seizing when the steel shaft is used |
Adaptability and absorbability of hard particles |
Corrosion resistance in contact with oil |
Fatigue strength |
|
Tin compositions |
Excellent |
Excellent |
Excellent |
Very bad |
|
Lead compositions |
Excellent |
Excellent |
Average |
Very bad |
|
Cu-Sn alloy |
Very bad |
Very bad |
Good |
Excellent |
|
Cu-Sn-Pb alloy |
Average |
Bad |
Bad |
Good |
|
Al-Sn alloy |
Good |
Good |
Very bad |
Average |
|
Cu-Pb alloy |
Bad |
Average |
Excellent |
Excellent |
|
Gray cast iron |
Very bad |
Very bad |
Average |
Excellent |
|
Mixed plastics |
Excellent |
Excellent |
Excellent |
- |
|
Graphite |
Excellent |
Average |
Excellent |
- |
The higher the hardness of the bearing material, the higher its load capacity and abrasion resistance. The softer the bearing material, the lower is its tendency to harm a shaft surface and the better its adaptability, running property, and absorbency of hard particles, that is its extraordinarily properties. Hence a rule, saying that the difference of both members of sliding pair is authoritative, is true. When using a bearing material, it is necessary to ensure that:
In the following information table, the rough values of the characteristic properties for plain bearing mounting materials are presented:
|
Material |
Average hardness HB at 20 °C |
Average hardness HB at 100 °C |
Tensile strength [MPa] |
Modulus of elasticity [MPa] |
Maximum allowable operation temperature [°C] |
|
Sn-composition |
26 |
9 |
80 |
48 000 |
110 |
|
Pb-composition |
22 |
8 |
77 |
30 000 |
100 |
|
Cu-Sn alloys |
80 |
60 |
250 |
110 000t |
250 |
|
Cu-Pb alloys |
30 |
25 |
60 |
85 000 |
160 |
|
Cu-Sn-Pb alloys |
60 |
50 |
300 |
98 000 |
220 |
|
Al-Cu alloys |
60 |
40 |
400 |
70 000 |
120 |
|
Al-Sn alloys |
35 |
30 |
150 |
65 000 |
120 |
|
Steel |
160 |
150 |
550 |
210 000 |
over 250 |
|
Cast iron |
200 |
190 |
250 |
150 000 |
over 250 |
The main properties, maximum load capacity, and examples of suitable usage are presented in the following paragraphs for some bearing material types.