motor is extensively used in sewage treatment plants and at other locations to ensure safe operation.

The operation and operator maintenance of electric motors has four main aspects-( 1) lubrication of moving or rotating parts, (2) proper alignment of drives, (3) safety, and (4) cleanliness of windings and rotors. Pay careful attention to each of these factors to ensure motor efficiency and, in many cases, to prevent motor breakdown.

LUBRICATION. - Electric motors are fitted with bearings which reduce friction. The types of bearings most often used are sleeve bearings, roller bearings, or ball bearings. For these bearings to remove the heat generated by friction, they must be properly lubricated. The lubricant used is usually either grease or oil.

Some motors are equipped with ball bearings permanently lubricated or packed with grease when the motor is assembled at the factory. These bearings are usually covered with a nameplate that reads-Do Not Lubricate. Most electric motor bearings, however, must be lubricated at frequent intervals. In such cases, the lubricant is fed to the bearings through a pressure fitting or grease nipple from a hand-operated grease gun. Or, the lubricant may be metered to the bearings from a grease or oil cup, which must be periodically turned or screwed down by hand to keep the bearings supplied with lubricant (fig. 6-2).

Some rotating shafts are fitted with sleeve bearings that usually are soft brass cylinders that fit around the machine-shaft journal like a sleeve. In some installations, the lubricating oil is circulated through the sleeve bearings under pressure. Some sleeve bearings, however, may be lubricated by means of an oil ring, or rings, as shown in figure 6-3. The weight of the ring hanging on the journal is enough to cause it to revolve, as the shaft revolves. As the oil ring rotates, it dips into an oil reservoir directly beneath the shaft journal. The oil picked up by the ring is then diffused along the shaft, between the shaft journal and sleeve bearing. Proper lubrication of ring-oiled sleeve bearings depends on maintaining a sufficient oil level in the reservoir. For this reason, most sleeve bearings have oil filler gauges or overflow fittings installed to aid the operator in maintaining the oil at a proper level.

When the electric motor is in operation, the operator is required to make frequent checks and inspections for proper lubrication of bearings and for overheated bearings. Check for heat radiated to your hand or check with a thermometer. Note that one of the most frequent conditions that cause bearings to overheat is excessive lubrication. This is a very

Figure 6-2. - Grease-lubricated ball bearings.

Figure 6-3. - Ring-oiled sleeve bearing.