Professional Guide To Completing An Assignment On Bearings
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Question 1: Why are bearings used on machines?
- Journal bearings support radial loads, which act at right angles to the axis of the shaft as shown in Fig. 17.1(a);
- Thrust bearings support axial loads, which act along the axis of the shaft as shown in Fig. 17.1(b).
Fig 17.1 Plain bearings
Question 2: What is friction and why must its effects be prevented?
Answer: Resistance to sliding of two surfaces in contact is called friction and is caused mainly by very small imperfections on the surfaces, even if machined. When magnified many times, these imperfections appear on both surfaces in contact as 'peaks' and 'valleys' and they tend to interlock and resist motion (Fig. 17.2(a)). But friction exists even between very smooth surfaces, due to pressure and relative motion of the surfaces.
The effects of friction must be reduced in order to prevent
- Loss of energy in overcoming frictional resistance
- Overheating of surfaces in contact, which can result in melting and fusing together of mating parts
- Damage to the surfaces in contact, due to wear
Fig 17.2 Sliding friction
Question 3: Lubrication is a form of reducing friction. Discuss the uses of some of the common types of lubricants
Answer: The effect of dry sliding friction is reduced considerably by introducing a thin film of lubricant - oil or grease - which keeps the two sliding surfaces apart (Fig. 17.2(b)).
Oils are liquid in structure and are used mainly for the following:
- High speeds
- High temperatures
- As coolant to dissipate frictional heat
- In splash and circulation systems of lubrication
Greases are semisolid in structure and are used mainly for the following:
- Low speeds
- Corrosion protection of metal bearings
- Prevention of foreign-matter contamination by partially bearings
- Applications where lubricant leakage must be prevented (food, paper, textile industries, etc.)
- Long-life lubrication for sealed or shielded bearings
Fig 10.3 A method of lubricating bearings
Question 4: Discuss the two types of plain bearings
Answer : Plain bearings consist of a supporting part, called the housing, and a mating part which may be a shaft, pivot, or thrust collar. The ideal plain bearing should be hard, strong, and wear-resistant, with a soft overlay which can be easily deformed, to absorb sudden loads without fracturing, and in which foreign particles can be embedded, thus preventing rapid wear of the mating surfaces.
There are two classes of plain bearings: (a) direct-lined housings and (b) bushes. The materials used for them are discussed in sections 17.3 and 17.4.
In this type of plain bearing, the housing is lined directly with bearing material by means of metallurgical bonding or keying. This construction is mostly confined to low-melting-point white metals attached to ferrous housings.
Applications: Cement-mill machinery, crushing plants, large crankshafts, car engines, etc.
Fig. 17.4 Plain bearings
Bushes are hollow cylindrical pieces which are fitted into housing to accommodate the mating part. When worn, they are removed and replaced.
To prevent the rotation of bushes in their housings, an interference fit is used. Alternatively, pins, screws, or various shapes of bush ends to fit in correspondingly shaped housing bores are utilised. There are two types of bush: solid and lined. In both cases the bearing material might be a white metal, a copper alloy, or an aluminium alloy. Solid bushes these are made entirely of the bearing material. Applications: Earth-moving equipment, gearboxes, crankshaft bearings, steering-gear linkages, diesel-engine small-ends, and general applications.
Lined bushes In these, the bearing material is applied as a lining to a Applications: Plant machinery, turbines, large diesel engines, marine gearboxes, etc.
Question 5: List some of the advantages and disadvantages of plain bearings
Answers: Advantages of plain bearings
- They usually require only a small radial space.
- They are cheap to produce.
- They have vibration-damping properties.
- They are noiseless in operation.
- They can be easily machined.
- They can cope with trapped foreign matter.
Disadvantages of plain bearings
- They require large supplies of lubricant.
- They are suitable only for relatively low temperatures and speeds.
- When starting from rest, the initial resistance to motion is much larger than the running resistance, due to the slow build-up of the lubricant film around the bearing surface.
Question 6: Discuss the qualities considered when developing bearing alloys
Answer: Materials suitable for plain bearings require special properties, as follows:
- Compressive strength, to support loads imposed by components.
- Fatigue strength, to withstand stresses due to forces which are applied repeatedly and which may vary in magnitude and direction.
- Corrosion resistance, to resist corrosion due to oil oxidation products, water, or other contaminants.
- Ductility, to be able to deform very slightly in order to absorb sudden loads without fracturing.
- Thermal conductivity, to conduct heat generated away from bearing contact surfaces.
- Low coefficient of friction, to ensure low friction forces.
- Embeddability, so that all trapped foreign particles can sink in below the bearing surface, thus preventing rapid wear of the mating surfaces.
- Machinability, for economical and easy bearing production.
- Bonding property, to allow the bearing to be attached firmly to a backing material if required.
Bearing alloys have been produced with various combinations of the above properties to suit different applications. The metal on which the alloy is based forms a matrix whose properties are modified by the alloying elements. The harder and stronger the matrix, the higher is its fatigue strength; conversely, the softer it is the better is its embeddability. A soft matrix will accommodate some displacement due to loading and any slight misalignment of a mating part and will allow evenly distributed wear, but it tends to wear more rapidly.