Bearing Lubrication

Bearing Lubrication

The whole hub assembly is partially packed with grease. A radial-lip grease-seal is pressed inside the hub next to the larger bearing. The lip of the seal faces this bearing and fits over the cylindrically machined surface on the stub-axle.

Over packing bearing hubs with grease causes churning and develops very high running temperatures. Due to high temperatures and excessive mechanical working the grease breaks down and becomes soft. The centrifugal force acting on the grease in the hub pushes soft grease outwards, so that it flows towards the lip seal and seeps past the seal lips. Bearing greases are based on mineral oil and to thicken the oil soaps of calcium, sodium, or lithium compounds are added. The consistency of a grease depends on the viscosity of the base oil as well as on the structure and properties of the metallic soap added.

The three most important properties of a grease includes :

(i) the melting temperature, at which the grease loses its semi-solid state,

(ii) its resistance to contamination and dilution by water, and

(Hi) its ability to withstand mechanical working before the grease breaks.

 The relative operating properties of commonly used bearing greases are provided in Table 27.1.

Table 27.1. Operating properties of commonly used greases.  

	Upper temperature limits
Types of grease soap	Short duration K	Continuous K	Water resistance	Mechanical stability
Calcium	353	323	Good	Fair
Sodium	393	353	Poor	Good
Lithium	423	393	Good	Good

For checking the bearing and seal, the split-pin and the castellated nut on the end of the stub-axle are removed and the hub assembly is withdrawn. All parts are thoroughly washed in paraffin, and the bearing rollers and tracks are examined for wear and damage. Before replacing the hub, the inside of the hub and the bearings are applied with grease (Fig. 27.7).

Fig. 27.7. Axle-beam and stub-axle assembly