Temperature Extremes.

(3) A bearing may become heated as a result of the temperature of the surrounding atmosphere or objects, or it may become hot because the frictional heat generated in the bearing has no adequate means of dissipation. On the other hand cold weather conditions or refrigeration may greatly reduce the temperature of the bearing. When these high or low temperatures are considered in selecting the oil, bearing troubles from these causes can be avoided. Unforeseen changes in these conditions, however, may cause trouble.

Hot surroundings tend to raise the temperature of a bearing to a fixed limit. The reduction of oil body by increased temperature is allowed for by the use of heavy-bodied oil.

Insufficient heat radiation from the bearing, due to a confined or non-ventilated location, may cause the accumulation of heat to such an extent that the high temperature of the bearing reduces the body of the oil to a dangerous degree. Proper ventilation should be provided for bearings, especially those of high-speed machines. When the circulation system is employed for lubrication, a large quantity of oil circulated through the bearing prevents heat accumulation where, with another system of lubrication, heat radiation might be insufficient to keep the bearing temperature moderate. In some cases bearing-housings or brasses are cored out and connected to a water-circulation system for cooling.

Cold surroundings sometimes reduce the temperature of the bearing below that at which the oil in use will flow, causing difficulty in starting a cold machine. Frictional heat generally raises the temperature to a point where the page 51 oil will flow and produce lubrication, but it is quite possible that injury may occur to the bearing before oil flow is established.

Where the bearings are expected to operate under very low temperature conditions, it is advantageous to design them in such a way that a supply of lubricant is held adjacent to the journal by waste packing. The first frictional heat generated on starting will then act upon the supply of oil in the packing adjacent to the journal. The transmission of heat throughout the bearing then gradually liquefies the entire mass of oil, so that the supply is maintained.

In some machines hot water or steam is circulated through water jackets while starting, being shut off later when frictional heat becomes sufficient to maintain the desired temperature. Depending on the type of machine and the method of oil application, it is often necessary to use an oil having a sufficiently low cold test to avoid congealing, even under the lowest temperature at which it has to operate.