Bearing Metals
The most important properties of a bearing metal are that it should be hard and wear-resistant, and
have a low coefficient of friction. At the same time, however, it must be tough, shock-resistant, and
sufficiently ductile to allow for 'running in' processes made necessary by slight misalignments. Such
a contrasting set of properties is almost impossible to obtain in single metallic phase. Thus, whilst
pure metals and solid solutions are soft, tough and ductile, they invariably have a high coefficient of
friction, and consequently a poor resistance to wear. Conversely, intermetallic compounds are hard
and wear-resistant, but are also brittle, so that they have a negligible resistance to mechanical shock.
For these reasons, bearing metals are generally compounded so as to give a suitable blend of phases,
and generally contain small particles of a hard compound embedded in the tough, ductile background
of a solid solution. During service, the latter tend to wear away slightly, thus providing channels
through which lubricants can flow, whilst the particles of intermetallic compound are left standing
'proud', so that the load is carried with a minimum of frictional loss. cheap
.with cheap
1- White bearing metals
White bearing metals are either tin-base or lead-base. The former, which represent the better-quality
high-duty white metals, are known as 'Babbitt' metals, after Isaac Babbitt their originator.
All white bearing metals contain between 3.5 and 15 per cent antimony, and much of this combines
chemically with some of the tin, giving rise to an intermetallic compound SbSn. This forms cubic
crystals ('cuboids'), which are easily identified in the microstructure (Figure18.1). These cuboids are
hard, and have low-friction properties; consequently, they constitute the necessary bearing surface
in white metals.
The background (or 'matrix') of the alloy is a tough ductile solid solution, consisting of tin with a
little antimony dissolved in it. In the interest of economy, some of the tin is generally replaced by
lead. This forms a eutectic structure with the tin- antimony solid solution. The lead rich white
metals are intended for lower duty, since they can withstand only limited pressures. Some white
bearing metals are detailed in Table 18.10.
2-Aluminium-tin alloys
Aluminium-tin alloys containing 20 per cent tin are now used as main and big-end bearings in
automobile design. Aluminium and tin form a eutectic containing only 0.5 per cent aluminium so
that the final structure of these bearings consists of an aluminium network containing small areas of
soft eutectic tin which wears and so assist lubricant flow.
Because of the long freezing range of this alloy, segregation is a danger so that the cast material is
usually cold-rolled and annealed to break up the eutectic leaving small islands of tin in an
aluminium matrix. Bearing shells of this type are usually carried on a steel backing strip.
3-Copper-base bearing metals
Copper-base bearing metals include plain tin bronzes (10-15 per cent tin) and phosphor bronzes
(10-13 per cent tin, 0.3-10 per cent phosphorus). Both of these alloys follow the above mentioned
structural pattern of bearing metals. Some of the tin and copper combine to form particles of a very
hard intermetallic compound Cu31Sn7, whilst the remainder of the tin dissolves in the copper to
form a tough solid-solution matrix. These alloys are very widely used for bearings when heavy
loads are to be carried. For many small bearings in standard sizes, sintered bronzes are often used.
These are usually of the self-lubricating type, and are made by mixing copper powder and tin
powder in the proportions of a 90-10 bronze. Sometimes some graphite is added. The mixture is
then 'compacted' at high-pressure in a suitably shaped die, and is then sintered at a temperature
which causes the tin to melt and so alloy with the copper, forming a continuous structure - but
without wholesale melting of the copper taking place. The sintered bronze retains its porosity, and
this is made use of in storing lubricant. The bearing is immersed in lubricating oil, which is then
'depressurised' by vacuum treatment, so that oil will be forced into the pores. In many cases,
sufficient oil is absorbed to last for the lifetime of the machine. Selflubricating sintered bearings are
used widely in the automobile industries
and in other applications where long service with a minimum of maintenance is required.
Consequently, many are used in domestic equipment such as vacuum cleaners, washing-machines,
extractor fans and audio equipment. Leaded-bronzes (see Section 16.6) are used in the manufacture
of main bearings in aero-engines and for automobile and diesel crankshaft bearings. They have a
very high wear-resistance and a good thermal conductivity, which helps in cooling them during
operation. Brasses are sometimes used as low-cost bearing materials. They are generally of a lowquality 60-40 type, containing up to 1.0 per cent each of aluminium, iron and manganese.
Plastics bearing materials
Plastics bearing materials are also used, particularly where oil lubrication isimpossible or
undesirable. The best-known substances are nylon and polytetrafluoroethylene both of which have
low coefficients of friction. Polytetrafluoroethylene (PTFE, of 'Teflon') is very good in this respect,
and in fact feels greasy to the touch. It is also used to impregnate some sintered-bronze bearings
(Figure 18.2).