Metallic surfaces sliding together under load have a tendency to adhere causing tearing or scoring of surfaces,
heat generation results and finally seizure. This factor is of primary importance in all bearing design.
To counter this problem the designer aim is to use materials with suitable lubricants to minimise this effect..A bearing material
should , if possible provide the following characteristics..
Metals and Alloys..
- Have a good resistance to wear, fatigue and corrosion
- Have sufficient strength to support the load
- Have a fairly high melting point- to reduce the tendency for creep in use
- Have suitable thermal properties to enable heat to be conducted away
- As metal to metal contact will be unavoidable in service the material
should be selected to minimise seizure, fretting, scoring and welding
- The bearing should be tolerant to dirt and foreign matter- e.g. soft surface
- Should be tolerant to misalignment
- Should be compatible to lubricant used- e.g should not corrode if water is used
1) Whitemetals or Babbit Metals..
2) Copper based alloys..
These are typically tin based (88% Sn, 4% Cu, 8% Sb, or lead based 80% Pb, 14% Sb, 6% Sn.
The materials have properties that include hardness combined with ductility,
a structure that holds lubricant , little tendency to cause wear to journals,
they embed dirt easily and are easily cast. The two types are generally
interchangeable but the tin based ones are usually more expensive , have better wear
resistance, stand higher loads and are not as brittle and are more corrosion resisting
than the lead based bearings.
Copper based alloys are considerably harder and strong, have better high temperature
characteristics, have greater resistance but poorer anti- scoring properties than
the white metals.
There are four main classes of these alloys..
3) Aluminium Based materials
- Copper lead alloys...
These have less resistance to seizure than the white metal but more than twice
the fatigue resistance even at high loads and temperatures. Hardened journals or lead-tin
or lead indium plating of the bearing surface can help to reduce the wear..
- Leaded bronze alloys...
These combine good compatibility characteristics with excellent coating and easy machining
properties and have good structural properties and high load capacity.
The are inexpensive and are useful as a single material without the need for a separate overlay
or steel backing.
- Tin bronze ...
These usually contain small percentages of tin and lead to aid machining and small amounts of zinc and nickel are often added
to improve strength. They are restricted to low speed applications but will carry heavy loads.
- Aluminium bronze bearings...
Bearings great strength can be produced using iron, silicon, and nickel as alloying elements.
The have excellent shock, wear and corrosion resistance. Their strength is retained at elevated
temperatures so they can be used in equipment operating above 260 deg C.
This alloy however has poor compatibility, poor embedding properties and poor conformability and so
is best suited to heavy duty low speed applications with good lubrication.
These materials were developed as an improvement on the white metal and copper based
alloys and to provide bearings that carry high loads. Special features are
their good resistance to corrosion, high thermal conductivity and high fatigue
strength, high thermal conductivity and high fatigue strength but they have the
disadvantages of only moderate embedding properties, poor compatibility and high
coefficients of thermal expansion. If used as solid unbacked bearings
this type of alloy were usually too weak to maintain an interference fit and too hard to run
satisfactorily against an unhardened shaft.
4) Cast Iron
As a bearing material unalloyed aluminium has a tendency to seize to a steel mating
surface. It was found that a 20% of tin added to the aluminium improved seizure
resistance and that cold working and annealing helped to prevent brittleness. The difference in coefficients between
aluminium and steel necessitates work hardening the bearing before use to prevent loosening in service.
Cast iron is an inexpensive bearing material for operation under relatively light loads.
Grey cast iron is widely used for machine tool beds due to its damping characteristics. The presence
of graphite in the iron improve running properties..
The bearing surface is often machined directly into the cast iron structure.
Generous lubrication and large clearances are necessary to avoid scoring. A speed of 0.8 m / s and a pressure
of no greater than 3.5 MPa are the maximum duty for cast iron bearings .
Because of poor comformability it is essential that the bearings have good alignment and freedom from contamination..
As with cast iron, steel bearings required lots of lubrication and generous clearances. Nickel steel
bearings are best operated with intermittent loads rather than continuously and using low journal speeds
and temperatures less than 40o C
6) Cadmium Alloys
These alloys have greater high temperature lives than the white metal materials but are more
subject to corrosion. This can be overcome by plating with indium.
Cadmium has a low affinity for steel and so does not seize easily. Cadmium
materials are not widely used because of their high costs..
Silver bearings are highly resistant to fatigue but their anti-friction qualities are inferiour to
the whitemetal bearings . A disadvantage is that they become readily
welded to the shaft even if the oil film breaks down for only and instant. They also do not possess the embedding properties
of other softer bearing metals. Silver bearings are often overlaid with
lead and indium or lead and tin, to provide better resistance to seizure. The corrosion resistance,
temperature strength ratio and thermal conductivity are all good. Hard
shafts (300 Brinell) are necessary with silver bearings and bearing loads of above 28.0 MPa may be carried
at speed of 10 m/s.
7) Porous Bearings
Silver bearings have an excellent record in heavy-duty applications such as aircraft master rod and diesal engine main bearings.
The bearings are made by electodepositing silver on a steel backing with and overlay of lead 0,025-0,127 mm. thick . A thin layer of indium is them deposited over the lead to provide corrosion protection.
Porous bearings of sintered metals, usually plain or leaded bronze or iron are moulded
to shape under pressure and this process results in a sponge like structure with from 10-35%
of the metal volume as voids. This allows for impregnating with oil or graphite.
In operation the oil feeds through the interconnecting pores to the bearing surface.
The overall loss of oil is low although from time to time the bearing has to be re-impregnated.
Methods of available for continuously feed oil to porous bearing using force feeding or using
very simple wick feeds.
Porous bearings are very useful in locations with limited access and /or where regular
lubrication or engineered lubrication systems are difficult to implement.
Porous bearings i.e. Oilite are widely used throughout industry..
Non- Metal Bearings
These materials are self-lubricating , stable at temperatures up to 400 oC
and resistant to attack by chemicals and solvents. Bearings are moulded or machined from solid.
This material is used for applications where lubrication with grease of oil is not practical.
In some cases metal alloys may be added to the composition to carbon-graphite alloy
to improve the compressive strength.
Wood especially lignum vitae has been used for large low pressure, low speed bearings.
the hardwoods absorb oil and grease and so little attention is needed for maintenance. They
do not score or seize their shaft but due to uneven expansion and contraction large clearances are required..
Lignum vitae has an inherent oiliness which makes it practically self-lubricating.
Due to poor availability and uneven quality , it is being largely replaced by laminated plastics..
Natural rubber, and some synthetic types such as butatiene , acrynitrile, can be be used
as bearings materials where resilience is needed with water as a lubricant. The bearings
usually consist of a fluted lining to a metal shell so that the shaft is carried on a series of rubber
strips running the length of the bearing. A flow of water is provided to cool the bearing and to
flush through any dirt collecting in the channels between the rubber bearing strips. This type of
bearing is used in marine applications and may be loaded up to specific pressures of 0,4Mpa.
Certain hard carbides such as pressed and sintered titanium carbide or tungsten carbides
in a cobalt matrix can be used for high temperature applications for sliding components
in nuclear reactors and for other difficult duties. The cermet is very rigid, highly
resistant to corrosion and capable of taking a fine finish. Cermets have poor conformability , poor
impact resistance, are difficult to machine and are expensive..
The outstanding property of cermets is their resistance to wear..
Jewel baring are usually made of saphire or hard borosilicate glass and are used in low
torque instruments and control devices where low coefficient of friction , non magetic properties
and long life are required..
The wide use of plastic bearings results from their freedom from corrosion, quiet operation,
availability in shapes and their good compatibility. Plastic bearings need
little lubrication and water can often be used for lubrication. Plastics are
often resistant to most chemicals including acids.
Phenolics or polyester resins reinforced with either cotton fabric, asbestos, glass fibre
are widely used and are very strong - up to 300Mpa in yield strength in compression.
They can be water lubricated and are useful where good electrical insulation is needed.
The thermal conductivity is low so the heat generated by friction cannot easily be transmitted away through
the bearing. As a result heavier loaded bearings must have a feed of cooling
or lubricating fluid to remove the heat..
Bearings made of nylon are very satisfactory for light loads at high speeds.
Nylon has low friction characteristics and can be used with no lubrication.
It is quiet in operation, wears at a low rate when lubricated, is easily moulded and is inexpensive..
PTFE has and exceptionally low coefficient of friction , is self lubricating and is resistant
the most chemicals and operates at temperatures of up to 250 deg.C. However
it has poor mechanical properties, has low thermal conductivity and has high coefficient of
thermal expansion. In its pure form it is only useful at low speeds and loads.
The mechanical properties of PTFE can be improved by filling with glass fibre.
PTFE is often made into a composite in porous metals such as bronze or steel which
results in a low friction material with good mechanical properties. A widely
used bearing design is based on porous bronze filled with a ptfe /lead mixture supported
by a steel backing strip. PTFE filled with glass fibre and graphite or other inert materials
are very popular in mechanical and civil engineering..
|Min. Shaft Hardness
||Load carrying Capacity
||Max. Op. Temp
|Tin Based Babbit
||150 or less
||5,5 - 10
|Lead Based Babbit
||150 or less
||5,5 - 8,3
||230 or less
|Copper Lead Overplated
|Aluminium Alloy (6% Sn)
|Aluminium Alloy (20% Sn)
Temperatures are in practice subject to lubrication limitations.
Any bearing not lubricate with thick film lubricatin will be subject to significant
wear related to loading and type of motion...
Glacier DX consists of three bonded layers: a steel backing strip and a
sintered porous bronze matrix, impregna-
ted and overlaid with a pigmented acetal
copolymer bearing material.
Glacier DU consists of three bonded layers: a steel
backing strip and a porous bronze matrix, impregnated
and overlaid with the PTFE/lead bearing material.