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Plain Bearings

Introduction..

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..

  • 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
Metals and Alloys..

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.

2) Copper based alloys..

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..

  • 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.
3) Aluminium Based materials

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.

4) Cast Iron

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..

5) Steel

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..

6) Silver

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.

7) Porous Bearings

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
1) Carbon-Graphite

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.

2) Wood

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..

2) Rubber

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.

3) Cermets

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..

4) Jewels

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..

4) Plastics

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...
  • 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..

  • Nylon...
  • 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..
  • P.T.F.E...
  • 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..
Material Lubrication
Needed
Min. Shaft Hardness Load carrying Capacity Max. Op. Temp Compatability Comformability Corrosion Resistance Fatigue Strength
Brinell No Mpa oC
Tin Based Babbit Yes 150 or less 5,5 - 10 150 Excellent Excellent Excellent Poor
Lead Based Babbit Yes 150 or less 5,5 - 8,3 150 Excellent Excellent Fair Poor
Cadmium Base Yes 230 or less 10-14 250 Excellent Good Poor Fair
Copper Lead Yes 200-250 10-20 175 Fair Fair Poor Fair
Copper Lead Overplated Yes 300 10-17 150 Excellent Good Poor Fair
Lead Bronze Yes 300 20-28 230 Fair Fair Fair Good
Tin Bronze Yes 300-400 28+ 250+ Poor Poor Good Excellent
Aluminium Alloy (6% Sn) Yes 200-300 28+ 120 Fair Fair Excellent Excellent
Aluminium Alloy (20% Sn) Yes 200 28+ 0 Good Fair Excellent Excellent
Silver (Overplated) Yes 300-400 28+ 250 Good Fair Excellent Excellent
Glacier DX Yes - 140 280 Fair Fair Excellent Excellent
Glacier Du No - 140 100 Fair Fair Excellent Excellent


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.



Links to Bearing Design
  1. MDMetric..Oilite bearings data sheets
  2. Glacier...Glacier Plain/Dry/ Bearings ...
  3. Beemer Precision ...Oilite /Plain Bearing Information
  4. RBC bearings..Brochure -Information on Plain Bearings
  5. INA Plain bearings..Details from suppliers technical including downloadable catalogue
  6. Bearings UK..Wide range of bearings available


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Last Updated 22/01/2013