Engineering Materials – Desirable Bearing Materials Properties

Bearings play an important role in application which is used to support another rotary motion. It is used to give free rotational motion to shaft. It reduces friction and heat. Here, we learn about many different types of materials bearings can be made of.

Properties of Bearing Materials

Bearing is mechanical element which locates two machine parts relative to each other and permits relative motion between them. They reduce friction between two parts and hence it saves the power. We discuss about bearing material properties, covering some of more important relationships among bearing material properties.

Bearing are made of many different materials and these materials function more effectively in certain applications than others. These materials are discussed in brief in this post. Read more about Fluid Mechanics – Reynolds Number

  • Low Coefficient of friction
  • High Compressive Strength
  • High Fatigue Strength
  • Low Thermal Expansion
  • High Thermal Conductivity
  • Elasticity
  • Conformability
  • Embeddability
  • Corrosion Resistance
  • Bonding
  • Relative Hardness of Bearing Material
  • Availability of Material
  • Cost of Material

High Fatigue Strength

Fatigue strength has ability of bearing lining material to bear up stress and strain without cracking, flaking or destroyed by other. In some applications, bearings are high fluctuating loads. So to avoid fatigue failure, this bearing material has enough survival strength. High Fatigue Strength helps in withstanding variable loads while high strength keeps gear safe against static loads.

Corrosion resistance

Where there is high working temperature, this heat oxidizes lubricating oil and forms corrosive acids. This formed acid attacks on bearing material and decay it. So corrosion resistance property is also important in case of bearing material. Also, this corrosion resistance is needed to resist attack by organic acids that are formed in oil at operating conditions.

Compressive Strength

Bearing material must have high compressive strength to avoid extrusion of material from bearing. Since bearing pressure that is acting on bearing is greater than pressure and it is expressed in pounds per square inch of bearing area. In order to withstand this maximum pressure bearing material should have high compressive strength.

Embeddability

Embeddability is bearing material to absorb small particles of dust, dirt or abrasive particles without scoring journal. This dirt particles present in lubrication system. Dirt or dust particles in lubricating oil may cause jam in clearance space. Hence bearing material must have embeddability quality.

High Thermal Conductivity

High thermal conductivity is required to absorb and carry away heat make in bearing. This conductivity is most important, not only in removing frictional heat in oil, but avoids seizure due to hot spots caused by local particles.

Low coefficient of friction

Material combinations of sliding surfaces, along with lubricant must provide low friction coefficient for lower running costs and reducing damage. Also see Fluid Mechanics – Friction Factor and Darcy-Weisbach Equation

Low thermal expansion

During periods of temperature change size remain constant.

Elasticity

Bearing material must be elastic to allow bearing to return to original shape after deformation when external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.

Hardness of Bearing Materials

It is very important property of metals and has wide variety of meanings. It has many different properties such as resistance to wear, scratching, deformation and machinability etc. This means metal has ability to cut another metal. Hardness is expressed in numbers which are dependent on method of making test.

Compatibility

It permits shaft and bearing to get along with each other. It has ability to resist galling or seizing under conditions of metal to metal contact.

This is most truly bearing property because contact between bearing and shaft in good designs occurs only at startup. In rubbing condition shaft and bearing materials will not produce welds leading to scoring or seizure. And good bearing shaft metal combination is necessary.

Conformability

Bearing material has ability to yield and adopt its shape to that of journal. It helps to put up misalignment and increase pressure bearing.

Bondable

This bearing material will form strong bond with other materials. Generally strong bonds are formed with steel. High capacity bearings are made by one or more thin layers with high strength steel. Thus, this strength of bond is an important to select bearing material.

Material availability

Material must be available, not only for initial installation but also to facilitate replacement in event of bearing failure.

Different Bearing Materials and their Uses

There are different types of bearing material and their uses.

Babbitt bearing material

Babbitt is important bearing material is Babbitt or white metal. There are two types of Babbitt like lead-base and tin-base, it depends upon major alloying element. They are used in form of strip or thin lining about 0.125mm thick bonded to steel shells.

Advantages of Babbitt

  • Babbitt has good conformability and embeddability
  • Babbitt will easily bonded to steel shell
  • Babbitt has better corrosion resistance

Disadvantages of Babbitt

  • Babbitt has poor fatigue strength
  • Babbitt strength decreases rapidly with increasing temperature
  • There is shortage of lead and tin, which are main constituents of Babbitt
  • Cost of Babbitt is high
  • This Babbitt bearing is used in petrol and diesel engines, centrifugal pumps, turbines, concrete mixers and conveyors.

Copper Alloys

Copper alloys have high lead content and these alloys put on the market under different trade names. They have low efficient of thin-film lubrication friction about 0.005. Brasses and Bronzes are used in alloys of Cu. Brass is an alloy with Zn. Bronzes contain tin, aluminum, silicon or beryllium. They do not score journal when lubrication fails. This pressure allows 1,000 to 5,000 psi in order to obtain good copper lead bearing alloy.

Plastics bearing

This material is fused together under high pressure at high temperature. Celoron, Micarta and Formica bearings are made from special woven duck. These materials have good mechanical properties and great elasticity. Such bearing can be lubricated with grease, oil or water.

They are used on heavy rolling mills for bearing replacement of bronze or lignum vitae without any changes in roll stand itself. But with water lubrication they can stand pressures up to 5,000 psi and velocities of 2,000 feet per minute with friction coefficient less than 0.007.

Rubber

In hydraulics turbines, where water is available this rubber bearings are used with water. Rubber bearings are suitable for use on shafts running at high speeds.

Cast Iron

Cast iron is very good bearing material in regard to friction if lubrication is in thin-film region. However, this combination is suited for light service only where pressure does not exceed 40 psi.

Aluminum Alloys

Aluminum alloys has great resistance to scuffing, due to their low friction and high wear resistance under conditions of boundary and thin-film lubrication. This high heat helps to carry away heat of friction. However, aluminum begins to lose its strength at about 225 F. This bearing can operate at pressures up to 3,000 psi and with velocities up to 2,000 fpm.

Less Oil Bearings

Oil- less bearing depends on lubricant included in bearing during its manufacture. This is done in many ways.

  • You can insert flaked graphite into metallic surface in shape of spirals or studs.
  • Another development is suspending graphite in Babbitt alloy under high pressure at fusing temperature of Babbitt.
  • In other types, oil is infusing into wood or some other porus or fibrous carrier.

Self lubricating

In metal powder, pulverized graphite and bearing bronze or iron are mixed with binder and they are pressed into molds with heat. This process is called sintering and this bearing is porus. It can absorb an amount of oil up to 30 percent of its own weight and can give soil slowly in application. It maintains thin oil film for long time and this is called self- lubricating bearings.

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