![if !IE]> <![endif]>
Bearing –desired materials characteristics
– 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 or 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 characteristics
–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
–The oxidized products of oils corrode many bearing alloys
–Some protection can be provided-by forming a thin layer o anti corrosion material on the bearing alloy surface
–It helps to accommodate misalignment and increase area the pressure bearing (reduce the localized forcse).
–Relatively softer bearing alloys are better in this respect
–It is the ability of a material to embed dirt and oreign particles to prevent scoring and wear (decreaserd.Body abrasion)3.
–Materials with high hardness values have poor embeddability characteristics
q 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
q Low thermal-expansion The size should remain nearly constant during periods of temperature change
q High thermal-conductivity The ability to dissipate heat quickly due to friction
q Wettability- An affinity for lubricants so that they adhere and spread to form a protective film over the bearing surface.
q 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 shats (that require dismantling of the whole engine). If one bearing is worn out only that bearing needs replacement instead of the whole shaft
q Elasticity-should be elastic enough to allow the bearing to return to original shape upon relief o stresses that may causes temporary distortion, such as misalignment and overloading
q Availability- The material should be readily and suiciently available, not only for initial installation but also to facilitate replacement in the event of bearing failure
q Cost-The economic consideration is the ultimate deciding factor in selecting a bearing material.
• 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 alloys offer good fatigue resistance and excellent compatibility characteristics
• Their corrosion resistance is poor and are they are expensive
•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 (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
Types of plain 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.
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 techniquesto 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.
Non-metallic bearing materials
– 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.
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.
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 .
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
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.