Before adding grease, however, be sure to check which grease is already in use and select either the same or a compatible type. Lubricant manufacturers can provide compatibility information. Most large electric motors contain a grease fitting and drain plug. A common practice is to pump new grease into the bearing through the appropriate fitting and let the old grease exit the drain plug. Don't add more grease than what is needed for proper lubrication. Excessive amounts raise friction and running temperature, shortening both grease and bearing life.
Unfortunately, there is no general rule governing grease relubri-cation intervals. Intervals are instead based on bearing size, type, operating speed and environment, and motor design. Vertically mounted motors, for example, need relubrication twice as often as those run in a horizontal orientation.
Consider oil lubrication when extremely high rotational speeds or operating temperatures make it impractical or impossible to use grease because of too-frequent relubrication intervals. In general, only large electric motors are oil lubricated, necessitating more sophisticated seals. Newer technologies, including circulating oil and mist, have advantages over conventional oil-sump systems in certain applications.