(i) Lead or tin-based alloys. (ii) Cadmium-based alloys.
(iii) Aluminium based alloys. (iv) Silver-based alloys.
(v) Copper-based alloys. (vi) Sintered bearing materials,
(vii) Non-metallic bearing materials.
(i) Lead or tin based alloys (Babbitt metals)
— They may be divided as
(a) The high tin alloys with more than 80% tin and little or no lead.
(b) The high lead alloys with about 80% lead and 1-12% tin.
(c) The alloys with intermediate percentages of tin and lead.
In addition to lead and tin, these bearing alloys contain antimony and copper also.
— Typical compositions of
A lead based alloy
A tin based alloy
— Lead base alloys are softer and brittle than the tin base alloys.
— Lead base alloys are cheaper than tin base alloys.
— Tin base alloys have a low coefficient of friction as compared to lead base alloys.
— Lead base alloys are suitable for light and medium loads, whereas tin base alloys are preferred for higher loads and speeds.
— Whereas tin base alloys find applications in high speed engines, steam turbines, lead base alloys are used in rail road freight cars.
— Solidus temperature of Tin base alloys —Approx. 222°C
— Solidus temperature of Lead base alloys — Approx. 240°C
— Besides, both these alloys possess
Good ability to embed dirt
Good conformability to journal
Good corrosion resistance
Very good seizure resistance, etc.
— White metals are tin base or lead base bearing alloys and are usually referred to as babbitts.
— Chemical composition:
— These bearing alloys have a structure consisting of a soft matrix containing harder crystals of intermetallic compounds.
— These alloys are not very popular because of high price of cadmium.
— These bearing alloys possess greater compressive strength than tin base alloys.
— Cadmium-based alloys possess :
(a) low coefficient of friction,
(b) high fatigue strength,
(c) high load carrying capacity,
(d) low wear,
(e) good seizure resistance,
(f) fair ability to embed dirt, and
(g) poor corrosion resistance (using ordinary lubricants).
— Cadmium-based alloys were tried in automobile and aircraft industries and good results were obtained.
(iii) Aluminium-based alloys
— Chemical composition:
— These are two-phase alloys, for use in bulk form or as linings on a steel base.
— These alloys possess :
(a) excellent corrosion resistance;
(b) fair conformability to journal;
(c) good ability to embed dirt;
(d) good seizure resistance;
(e) good thermal conductivity; but
(f) high coefficient of expansion.
— These alloys find applications as bearings in diesel engines and tractors.
(iv) Silver-based alloys
— Silver bearings are produced by the electro-deposition of a 0.3-0.5 mm layer of silver on a steel support shell, with an intermediate layer of Cu or Ni.
A 0.02-0.03 mm film of lead and indium is then deposited on top of the silver, and the indium diffused into the lead by heart-treatment at 180°C.
This covering layer aids in improving the running-in properties and the corrosion-resistance of the silver layer.
— These are highest priced bearing alloys.
— They are employed where other materials do not produce satisfactory results.
— These alloys are used on the connecting rod bearings of aircraft engines.
(v) Copper-based alloys
— The term bronze covers a large number of copper alloys with varying percentages of Sn, Zn and Pb.
— Bronze is one of the oldest known bearing materials.
(a) is easily worked;
(b) has good corrosion resistance; and
(c) is reasonably hard.
— Typical compositions of bearing bronze are :
— Tin bronze (10 to 14% tin, remainder copper) is used in the machine and engine industry for bearing bushes made from thin-walled drawn tubes.
Copper-based alloys are employed for making bearings required to resist heavier pressures such as in railways.
(vi) Sintered bearing materials
— Copper powder with 8 to 10% tin and sometimes with 1 to 3% graphite is used for making porous sintered bronze bearings.
— Besides using bronze powder, iron powder has also been tried for making sintered iron bearings. Such bearings possess greater strength.
— Table 9.1 gives properties of some sintered bearings.
Iron & Lead
|Iron & Copper|
|of oil (%)|
(vii) Non-metallic bearing materials
(a) Teflon (polytetrafluoroethylene)
— It has coefficient of friction <= 0.04, without lubrication.
— It has good stability at high temperatures.
— It is chemically inert to water and many chemicals and solvents.
— Fillers like glass and graphite increase the resistance to deformation.
— Nylon bearings have coefficient of friction
0.15-0.33 for dry friction
0.14-0.18 with water lubrication
0.09-0.14 for oil lubrication, with a load of 5-25 N and a relative velocity of 2.5 to 110 m/min.