Bearing materials- desired characteristics

Load capacity-

– The allowable compressive strength the material can withstand without any appreciable change in shape is the primary deciding factor in deciding a bearing material

– Plain bearings are expected to have the following characteristics for the ease of functioning and satisfying the design criteria

– Strength to take care of load-speed combinations

– Fatigue strength, where bearing materials are subjected to stress cycle as in internal combustion engines

– The retention of strength characteristics of softer bearing materials at temperature of operation which may rise within the design limit

– The material must easily conform to shape of the journal and should be soft enough to allow the particulate contaminants to get embedded

Bearing materials- desired characteristicsCompatibility-

–The shaft and bearing materials in rubbing condition should not produce localized welds leading to scoring or seizure. –A good bearing-shaft metal combination is necessary

Corrosion resistance- –The oxidised products of oils corrode many bearing alloys. –Some protection can be provided by forming a thin layer of anti-corrosion materials

on the bearing alloy surface

Conformability- –It helps to accommodate misalignment and increase the pressure bearing area

(reduce the localized forcse). –Relatively softer bearing alloys are better in this respect

Embeddability- –It is the ability of a material to embed dirt and foreign particles to prevent scoring and

wear (decrease 3rd. Body abrasion). –Materials with high hardness values have poor embeddability characteristics

Bearing materials- desired characteristics (contd.)

Low coefficient of friction- the material combinations of sliding surfaces, along with the lubricant should provide a low friction coefficient for reducing damage and lower running costs

Low thermal expansion- The size should remain nearly constant during periods of temperature change

High thermal conductivity- The ability to dissipate heat quickly due to friction

Wettability- An affinity for lubricants so that they adhere and spread to form a protective film over the bearing surface

Relative hardness-– The bearing material should usually be softer than that of the journal to

prevent shaft wear but hard enough to resist adhesive and abrasive wear of its own surface.

– Bearings are more easy to replace than shafts (that require dismantling of the whole engine). If one bearing is worn out only that bearing needs replacement instead of the whole shaft

Elasticity- should be elastic enough to allow the bearing to return to original shape upon relief of stresses that may cause temporary distortion, such as misalignment and overloading

Availability- The material should be readily and sufficiently available, not only for initial installation but also to facilitate replacement in the event of bearing failure

Cost- The economic consideration is the ultimate deciding factor in selecting a bearing material

Bearing materials- desired characteristics (contd.)

Aluminium alloys• Good fatigue strength, load bearing capacity, thermal

conductivity, and corrosion resistance

• Less expensive than babbitt materials

• Most aluminium allows contain tin as an element which remains in the free state to provide a better bearing surface

• The strongest aluminium alloy used is aluminium-silicon

• Thermal expansion is relatively high and this restricts their usage at high temperatures

• Emeddability, conformability, and compatibility are not very good and these are improved by providing a babbitt overlay

Cadmium and silver alloys

Cadmium:•Cadmium alloys offer good fatigue resistance and

excellent compatibility characteristics•Their corrosion resistance is poor and are they are

expensive

Silver:•Used as deposited material on steel with an overlay

of lead•The addition of lead improves the embeddability,

anti-weld and anti-scoring properties

Multilayered bearings• Steel is normally used as a backing material• To obtain good embeddability conformability and

compatibility, a layer or coating of relatively softer material is put on a backing material

• These plain bearings are known as multilayered bearings

• When there is one layer of coating material, it is termed bimetal bearing

• Bearings consisting of a thin layer of soft material electroplated or cast on a layer of a bimetal bearing are known as trimetal bearings

• In bimetal bearings the maximum thickness of the overlay is kept within 800 m

Multilayered bearings (contd.)

• The thickness of overlay can be as low as 120 m for babbitts

• The wall thickness of backing material in bimetal bearings is of he order of 0.3 times the bore with a minimum value of 1.5 mm

• In trimetal bearings the surface layer thickness could be as low as 25 m

• With increase in babbitt thickness, the overall fatigue strength decreases

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Plain bearings:Sliding motion takes place

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Types of plain bearings

Porous bearings:– Made using powder metallurgy techniques by sintering powdered

bronze, iron, brass, graphite etc. and obtaining the requisite bearing housing shape by compressing the powder.

– This yields a porous bearing housing which is then impregnated with oil.

– The quantity of oil depends on load and speed for which the bearing is used.

– The variation of pressure during the operation of the bearing along with the circumferential direction and the temperature variation causes oil to flow through the pores due to capillary action into the clearance space between the bearing housing and journal.

– From the loaded portion bearing, the oil flows back along the pores into the bearing housing.

– Applications are mixers, washing machine, garden equipment etc.

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Rolled or strip bearings– Made by rolling a sheet or strip and due to the nature of the

manufacturing process, the bearing housing is split requiring various joining techniques to close this split.

– Sometimes they are provided with a fiber lined cloth of PTFE/Graphite fibers on the inside of the housing for friction reduction and improved strength.

– Among non-metallic bushes, rubber and graphite have been traditionally used.

– Nylon is a valuable plastic material for bushing because of low friction though it has low strength and is suitable for low speeds due to heating effect.

– These are used in grinders and mixers because of resistance to corrosion and quiet operation.

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Rubber bearings– Used where quiet operation is desired, large clearances and

misalignment encountered.

– Found in bearings for propeller and rudder shafts of boats and ships.

– Found where water acts as a lubricant or likely to be a contaminant.

– Even when sand and gravel are present, the resilience of water is an added advantage.

– Wet rubber is very slippery hence its use in such situations to reduce friction.

Non-metallic bearing materials

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Carbon graphite– Used for food handling equipment and in the textile

industry.– Because of the self lubricating property, no additional

lubricant is required, hence limiting lubricant contamination prospects.

– Since it is resistant to corrosion, it can be used even in water.

Teflon (PTFE)– Another plastic with self-lubricating properties and low

friction.– However, it has poor strength, low wear resistance and a

tendency to deform under load.– Reinforcing with fibers improves the strength.

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Phenolic plastic bearings

– Laminated phenolics are formed by treating sheets of either paper or cotton fabric with asbestos or other filler materials bonded using phenolic resin.

– These are stacked to obtain the desired thickness and subjected to heat and presssure to bond the sheets firmly and later formed into required shapes.

– Used in aircraft landing gears and in several applications where water is a lubricant, such as in rolling mills where water is used for cooling and to lubricate.

– Also used in rudder bearings and centrifugal pumps .

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Grooved bearings

Groove

Grooves are provided in bearing surfaces to enable oil flow into the bearing area and to spread along the surface

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Non-circular journal bearings• Oil lubricated bearings have serious limitations as the surface

speeds increase.• The limit to operation is due to journal whirling in the

clearance space within the housing in such a manner as to cause danger of surfaces coming in contact leading to failure

• For this various non-circular configurations have been devised to accommodate for the whirl and provide better stability