A plain bearing is formed when a shaft rotates in a bush, liner or the bore of a housing. Plain bearings are used when simplicity and cheapness with less precision are the requirements. The plain bearings may be journal bearing, thrust bearing or combination of journal and thrust bearing.
Journal bearing is capable of carrying radial loads whereas the thrust bearing is capable of carrying axial load. The friction loss is more in plain bearings compared to rolling bearings.
Fig. 11.29 shows the behaviour of a shaft (journal) in a plain bearing with the clearance space filled with oil and the load acting as shown. In plain bearings, when the journal is at rest position, there is metal to metal contact between shaft and bearing (Fig. 11.29 (a)).
As shaft starts to rotate, it rolls upto the left and a thin film of oil substitutes the metal to metal contact (Fig. 11.29 (b)). As the speed increases the oil drawn under pressure through friction caused by shaft rotation forces the journal to the right and balances the external load (Fig. 11.29 (c)).
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The value of minimum film thickness (h) depends on load W. Higher the load W, smaller the value of h. Fig. 11.30 shows relative magnitude of pressures normal to the surface of plain bearing spindle.
