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
to take care of load-speed combinations
– Fatigue, strength where bearing materials are subjected to stress
cycle as in internal combustion
– The retention of strength characteristics of softer bearing materials
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
–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
–It helps to
accommodate misalignment and increase area the pressure
the localized forcse).
bearing alloys are better in this
–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
along with the lubricant should provide a low friction coefficient for reducing
damage and lower running costs
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
for lubricants so that they adhere and spread to form a protective film over the bearing surface.
– 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
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
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
• Most aluminium allows contain
tin as an element which remains in the free state to provide a better bearing
• 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
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
– 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.
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