Grease lubrication is generally used where ball and roller bearings operate at normal speeds, temperature and loading conditions. Grease has certain advantages by comparison with oil: it is more easily retained in the bearing housing and assists in sealing against the entry of dirt and moisture.

In general the free space in the bearing and housing should only be partly filled with grease (30 to 50%). Overfilling causes rapid temperature rise particularly if speeds are high. Manufacturers supply details regarding suitable weights of grease for particular bearings.

Bearings can be supplied which are sealed after prepacking with the correct type and quantity of grease. Where relubrication is more frequent, provision must be made by fitting grease nipples to the housing. Grease will then be applied by a grease gun and a lubrication duct should feed the grease adjacent to the outer ring raceway or between the rolling elements. Examples are shown in Fig. 28.24.

Oil lubrication

Oil lubrication is generally used where high speeds or operating temperatures prohibit the use of grease, when it is necessary to transfer frictional heat or other applied heat away from the bearing, or when the adjacent machine parts, e.g. gears, are oil lubricated.

Oil bath lubrication is only suitable for slow speeds. The oil is picked up by rotating bearing elements and after circulating through the bearing drains back to the oil bath. When the bearing is stationary the oil should be at a level slightly below the centre of the lowest ball or roller. An application is shown in Fig. 28.25. At high speeds it is important that sufficient oil reaches the bearing to dissipate the heat generated by friction and oil jets provide an effective method (Fig. 28.26).

Desperdicios Transporte

Fig. 28.26

The illustrations in this section (Figs 28.10 to 28.26) are reproduced by kind permission of SKF (U.K.) Limited Bradbourne Drive, Tilbrook, Milton Keynes, MK7 8BJ.

Trouble-Free Bearing Operation

When bearings fail, they can bring equipment to an unscheduled halt. Every hour of downtime due to premature bearings failure can result in costly lost production in a capital intensive industry. Substantial investment in research and development has resulted in the manufacture of bearings of the highest quality. Quality alone cannot guarantee trouble-free bearing operation since other factors may affect life span including the following:

1 Operating environment Machinery must be kept in peak operating condition. Bearings should be properly aligned and protected from extreme temperatures, moisture and contaminants.

2 Proper installation Knowledge of the proper installation techniques and tools is required to ensure that the bearings are not damaged.

3 Proper maintenance Following lubrication and maintenance schedules using recommended materials and time intervals is essential. A familiarity with operating procedures, basic trouble shooting, condition monitoring and vibration analysis is also desirable.

However, bearing manufacturers do have a full line of products and services to make installation and maintenance easy to perform and should be consulted. This will certainly contribute to long bearing life and ensure cost effective operation.

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