Plain bearings may be classified as follows: Plain bearings
Direct lined bearings
(a) Solid inserts (b) Lined inserts (c) Wrapped bushes
(i) Thick walled bearings
(ii) Medium walled insert liners
(iii) Thin walled insert liners.
The bearing metal should have a low coefficient of sliding friction, be able to conduct heat generated away from the bearing surfaces, resist wear in use and be tough enough to withstand shock loading in service. In the event of breakdown due to lack of lubrication, it may be desirable when overheating occurs for the bearing material to run, preventing seizure and possible severe damage to associated mechanical parts.
These housings are lined directly with bearing materials and the choice of material is limited by the practicality of keying or bonding the bearing material to the housing surface.
The dimensions of the housings, casting temperatures and bonding characteristics of the bearing materials will determine whether metallurgical bonding is possible without unacceptable distortion of the housing. Generally this technique is limited to ferrous housings with low melting point whitemetal bearing surfaces. Light alloy and zinc base housings are difficult to line directly with whitemetal.
These are bearing elements which consist of a liner inserted into a previously machined housing and they can be divided into separate classes:
(a) Solid insert liners.
(b) Lined inserts.
(c) Wrapped bushes.
Solid insert liners Manufactured wholly from suitable bearing materials such as aluminium alloy, copper alloy or whitemetal, these liners consist of machined bushes, half bearings and thrust washers.
The housings are machined to relatively close tolerances. An insert may be finished machined after assembly or a prefinished standard precision liner added as a final operation and this has the added advantage of spares replacement.
Typical applications of insert liners are to be found in diesel engine small bores, crank shaft main bearings, bushes for gearboxes, steering gear and vehicle suspensions.
Lined inserts These consist of a backing material such as cast iron, steel or a coper alloy which has been lined with a suitable bearing surface of aluminium or copper alloy, or of whitemetal. This type can also be supplied as a solid insert, a split bush, half bearing or thrust washer.
Insert bearing half liners are manufactured as;
(a) Rigid or thick walled bearings.
(b) Medium walled bearings.
(c) Thin walled bearings.
Thick walled bearings These are backing shells of cast iron, steel pressings and copper base alloys generally lined with whitemetal and copper alloys are used to produce bearings which are manufactured as pairs and used in turbines, large diesel engines and heavy plant machinery. Usually more economic than direct lined housings, these bearings may be provided with a finishing allowance for the bore and length which is adjusted during assembly.
Medium walled insert liners Normally a steel backing is used with a wide range of lining materials. Bearings are prefinished in bore and length and manufactured as interchangeable halves.
Thin walled insert liners These are high precision components with steel backing and whitemetal or copper and aluminium base alloy surfaces, and are suitable for universal application in large production products such as high speed diesel engines and compressors. Wrapped bushes These are pressed from flat strip of rolled bronze, or steel lined with whitemetal, lead bronze, copper-lead, or aluminium alloys. They are supplied as a standard range of prefinished bushes or with a bore allowance for finishing in situ by fine boring, reaming, broaching or burnishing. These are suitable for all bushing applications in which the tolerable wear will not exceed the thickness of the lining material.
Plain bearing lubrication
The requirements of a lubricant can be summarized as follows:
(1) To support the bearing when static and under all speed and load conditions.
(2) To have a low coefficient of friction.
(3) To be non-corrosive to the materials used in the bearings.
(4) To maintain viscosity over the operating range of temperature.
(5) Able to provide an effective bearing seal.
(6) Have the ability to adhere as a film to the bearing.
(7) Be able to conduct heat rapidly.
No single lubricant can satisfy all of these properties and the design of the equipment will determine which aspect needs priority before a choice from available types can be made.
The application of the bearing, the bearing material and the lubricant used are all interdependent, but four basic requirements are necessary for the material:
(1) Strong enough to resist failure by fatigue or overheating.
(2) Good wear resistance and running properties.
(3) Good lubricant retention.
(4) High corrosion resistance which may arise due to temperature, the environment and lubricants used.
A wide range of materials consists of metallic, metallic backings with various bearing surfaces, reinforced synthetic resin, graphitic and sintered metallic. Various surface treatments are also available to improve wear resistance and reduce friction.
These are a large range of either lead base or tin base alloys and are covered by British Standards. Antimony is used as a hardening agent since tin and lead are soft. Whitemetal is a low melting point alloy which is compatible with virtually any type of mating surface. Bearing materials should not be subject to corrosion due to water or the products of oil oxidation and the resistance of tin base whitemetals is high but lead base alloys are susceptible to acidic corrosion from oil oxidation products. Whitemetals are nearly always lubricated under pressure. Loss of lubricant for a short period may cause the bearing to soften and 'wipe'. It loses its compressive strength at elevated temperatures.
Other bearing materials
Other materials for plain bearings include copper lead alloys, lead bronzes, tin bronzes (phosphor bronze), gun metals and aluminium base alloys.
Before concluding this section it should be stated that metallic porous metal bearings are widely used which are manufactured by powder metallurgy where very fine metal powders are mixed and compressed in moulds to the correct form and sintered at high temperature in a reducing atmosphere. The product is in effect a metal sponge which can be impregnated with lubricating oil. The porosity depends on the initial compression and these products are designed for suitable applications where high volume is required. Self lubricating materials are also available in tube and bar form for individual manufacture.
Figures 28.1 to 28.9 show a selection of different types of bearing from the range manufactured by The Glacier Metal Company Limited. It is generally the case that for small quantities a design, for economic reasons, should incorporate a standard bearing as first choice if possible. Bearing manufacturers employ applications engineers to advise and ensure the correct application and use of their products.
Plain bearings are also used in structural work and bridge construction to permit expansion and movement and a selection are shown below with an indication of possible motions. This type of bearing utilizes the low friction properties of polytetrafluoroethylene (PTFE) and in the applications shown can withstand a maximum live loading up to 45 N/mm2. Illustrations of bearings in Figs 28.1-28.9 are reproduced by kind permission of the manufacturers, The Glacier Metal Company Limited Argyle House, Joel Street, Northwood Hills, Middx. HA6 1LN.
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