BEARING MATERIALS

INTRODUCTION TO BEARINGS

Bearings support moving parts, such as shafts and spindles, of a machine or mechanism.

Bearings may be classed as(i) Rolling contact (Ball and roller) bearings.(ii) Plain bearings.Rolling contact bearings are almost invariably made

of steel that can be hardened after machining. Both plain carbon and alloy (Ni, Cr, Mo) steels are

employed for different applications.

PROPERTIES OF BEARING MATERIALS

A bearing material should:Possess low coefficient of friction.Provide hard, wear resistant surface with a tough

core.Have high compressive strength.Have high fatigue strength.Be able to bear shocks and vibrations.Possess high thermal conductivity to dissipate heat

generated due to friction between the bearing and the rotating shaft.

• Possess adequate plasticity under bearing load.• Possess adequate strength at high temperatures.• Be such that it can be easily fabricated.• Possess resistance to corrosion.• Be such that it does not cause excessive wear of the

shaft rotating in it, i.e., bearing material should be softer than the shaft material.

• Be having small pieces of a comparatively hard metal embeded in a soft metal.

• Maintain a continuous film of oil between shaft and bearing.

TYPES OF BEARING MATERIALSLead or tin based alloys (Babbitt metals)Cadmium-based alloysAluminium based alloysCopper based alloysSilver-based alloysNon-metallic bearing meterials

Lead or Tin Based Alloys (Babbitt Metals)The high tin alloys with more than 80% tin and

little or no lead.The high lead alloys with about 80% lead and 1—

12% tin.The alloys with intermediate percentages of tin and

lead.

Typical compositions of

A lead based alloy A tin based alloy

Pb 75% Sn 88%

Sb 15% Sb 8%

Sn 10% Cu 4%

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 possessGood ability to embed dirtGood conformability to journalGood corrosion resistanceVery good seizure resistance, etc.

Cadmium-based alloysChemical composition

cd 97%

Ni 2%

Ag, Cu and zn are added in small percentage. These bearing alloys have a structure consisting of a soft matrix containing harder crystals of intermetallic compounds. These alloys aren't very popular because of high price of cadmium. These bearing alloy possess greater compressive strength than tin bearing alloys.

Cadmium-based alloys possess

(a) low coefficient of friction,

(b) high fatigue strength,

(c) high load carrying capacity,

(d) low wear, good seizure resistance,

(e) fair ability to embed dirt, and

(f) poor corrosion resistance (using ordinary lubricants).

• Cadmium-based alloys were tried in automobile and aircraft industries and good results were obtained.

Aluminium based alloys

Chemical composition

Al 91.5%

Sn 6%

Cu 1%

Ni 1%

Small amount of silicon is used along with these. The microstructure consist of NiAl and CuAl2 in the matrix of aluminium solid solution.

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

(J) high coefficient of expansion

These alloys find applications as bearings in diesel engines and tractors.

Copper based alloysChemical composition

Cu 80-85%

Sn 10-15%

Pb 10%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.Bronze,(a) is easily worked(b) has good corrosion resistance(c) is reasonably hard

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.

Silver-based alloys

Silver bearings are produced by electro deposition of a 0.3 to 0.5 mm layer of silver on a steel support shell ,with an intermediate layer of Cu and Ni. A 0.02-0.03mm of lead is then deposited on top of the silver and the indium diffuse into the lead by heat treatment at 180oc. This covering layer aid in improving the running in properties and the corrosion resistance of the silver layer.

These are highest prized alloysThey are employed where other materials don’t

produce satisfactory resultsThese alloys are used on the connecting rod bearings

of aircraft engines

Non-metallic bearing materials(a)Teflon (poly tetra flouro ethylene)

It has co-efficient of friction <0.004 with out lubrication. It has good stability at high temperature. It is chemically inert to water and many chemicals and solvents, fillers like glass and graphite increases the resistance to deformation.

(b)Nylon

Nylon bearings have co-efficient of friction 0.15-0.33 for dry friction, 0.14-0.18 with water lubrication. 0.09-0.14 for oil lubrication with load of 5-25Nand a relative velocity of 2.5-110m/min

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