Rolling bearings in electric motors and generators

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www.bergab.ru Берг АБ bergab@ya.ru Тел. (495)-228-06-21, факс (495) 223-3071 1 Rolling bearings in electric machines Design requirements Maintenance Electric motor maintenance typically includes lubricating the bearings, servicing the stator windings and monitoring the performance of the motor. For motors equipped with bearings that are sealed and lubricated for life, relubrication is not necessary and the motor is considered to be maintenance-free. Precision The required accuracy of any machine determines the required precision of the bearings. Bearings with higher precision are available for high accuracy/high speed machinery. However, for a machine to benefit from the running accuracy of its bearings, its roundness and surface finish of the bearing seating must be machined accordingly. Condition monitoring Bearings in operation generate vibrations. With the methods and equipment available today, bearing condition can be effectively monitored and diagnosed. Suitable procedures for condition monitoring of electric motors are: • Comparative measurements on a number of identical motors, running under the same operating conditions. • Trend measurements on a motor at given intervals, to observe the change in bearing condition. SKF has developed the tools and the knowledge base to effectively measure, trend, and diagnose bearing condition. Product availability During the design stage, SKF recommends checking product availability with your local SKF representative. Up-to-date bearing data can be found in the SKF Interactive Engineering Catalogue online at www.skf.com. Handling, tools and transport Rolling bearings are precision products, which must be handled carefully if they are to perform properly. When mounting or dismounting bearings it is important to use the correct methods and tools. Instructions can be found in chapter 5 “Mounting and dismounting”, starting on page 77. To prevent premature bearing failure, it is also important to prepare the motor properly for transport. 14 www.bergab.ru Берг АБ bergab@ya.ru Тел. (495)-228-06-21, факс (495) 223-3071
www.bergab.ru 1 Rolling bearings in Берг electric АБ machines bergab@ya.ru Тел. (495)-228-06-21, факс (495) 223-3071 Bearing selection Bearing selection Electric motors and generators use a wide variety of bearing types including deep groove ball bearings, angular contact ball bearings, cylindrical roller bearings, spherical roller bearings, CARB toroidal roller bearings and spherical roller thrust bearings. In small horizontal machines, the most common arrangement consists of two deep groove ball bearings. In larger or heavier loaded machines, roller bearings are typically used. In vertical machines deep groove ball bearings, angular contact ball bearings or spherical roller thrust bearings are typically used, depending on the loads, speeds, temperature and environment of the application. As mentioned earlier, the design requirements and operating conditions of the application will influence the bearing arrangement. The bearings selected for the arrangement should be verified by calculating bearing life. A number of examples of bearing arrangements for electric machines are shown in chapter 2 “Bearing arrangements”, starting on page 37. Bearing internal clearance Bearing internal clearance is defined as the total distance through which one bearing ring can be moved relative to the other ring in the radial direction (radial internal clearance) or in the axial direction (axial internal clearance). The internal clearance in ball bearings (not angular contact ball bearings), cylindrical, spherical and CARB toroidal roller bearings is always measured radially. A bearing initial clearance is chosen to accommodate: clearance can result in premature bearing failure. For deep groove ball bearings radial clearance in electric machines is typically one class greater than Normal (suffix C3). When bearing types other than deep groove ball bearings are used in high speed applications, (where speeds are 70 % or higher than the reference speed of the bearing) a C3 clearance should be selected. A C3 clearance should also be used when the temperature difference between the inner and outer rings exceeds 10 °C (15 °F). Increased clearances may also be necessary when an interference fit is needed for both bearing rings (usually cylindrical roller bearings). The noise level of the bearing increases as internal radial clearance increases. Therefore, clearances should be chosen carefully. If an application is designed to use a bearing with C3 clearance, do not use a bearing with Normal clearance. Bearings with Normal clearance have no clearance marking on the outer ring. Tables for bearing internal clearance can be found in the SKF General Catalogue or the SKF Interactive Engineering Catalogue, available on CD-ROM or online at www.skf.com. 1 • Expansion of the inner ring caused by its interference fit on the shaft. • If applicable, compression of the outer ring caused by its interference fit in the housing. • The reduction in radial clearance due to the temperature difference between the inner and outer ring during operation. • The needed internal clearance during operation. It is important to choose the right initial clearance, as insufficient operating www.bergab.ru Берг АБ bergab@ya.ru Тел. (495)-228-06-21, факс (495) 223-3071 15
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Rolling bearings in electric motors and generators

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