page 45

Care of Hot Bearings.

Where it is essential that machinery be kept in continuous operation, it is of value to know how bearing troubles may be relieved temporarily while the real cause of the trouble is being removed. These emergency measures should not be relied upon for continuous service, as they do not overcome the cause of the trouble. A hot bearing is one whose temperature rises materially above its normal operating temperature from any of the causes previously described, leading to excessive friction and heat generation. Such a condition is often unstable, in that, unless the cause is corrected, friction and heat generation continue each to augment the other until a destructive temperature is reached.

When a small bearing heats up, it is not difficult to cool, as the total amount of heat present in the bearings is relatively small. Usually a liberal supply of oil is all that is required. When a large bearing becomes heated, a greater quantity of heat must be dissipated. The relatively small clearance in a large bearing tends to make it more sensitive to damage due to heating. The first thing to do when a large bearing heats is to increase the bearing brasses. If the bearing has not seized, but is extremely hot, it is usually sufficient to feed a liberal supply of steam cylinder oil (which possesses superior lubricating qualities at high temperatures) until the bearing cools, when normal oiling practice may be gradually resumed.

If a bearing has begun to seize, a little graphite or sulphur mixed with cylinder oil may be used to advantage. Castor oil and rapeseed oil are sometimes used for cooling bearings.

The use of cylinder oil or other emergency lubricants, in a circulation system, should be avoided. Where this system of lubrication is employed and a hot bearing is experienced, it is generally necessary to stop the machine and open up the bearing to find and remove the cause; this may be foreign matter in the bearing, or stoppage of the oil flow through obstructiions or bad joints.

In large circulation systems it is common practice to provide an oil cooler, by which the oil temperature is reduced before it returns to the bearings. In this case, a bearing that tends to heat may be cooled by increasing the flow of oil through it. The circulating oil is then both a lubricating and a cooling medium.

Bearings may often be cooled by water, which should be applied, if possible, to the shaft, near the bearing, and under no circumstance to the bearing; as, in the latter case, the shrinkage of the bearing around the heated and expanded shaft may result in seizure.