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..

1) Whitemetals or Babbit Metals..
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..

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.

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 40^o C

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.

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..

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.

Notes:
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.