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The permissible bearing operating temperature in the application limits the speed at which rolling bearings can be operated. Bearing types with low friction and corre- spondingly low heat generation inside the bearing are therefore the most suitable for high-speed operation. The highest speeds can be achieved with deep groove ball bear- ings when loads are purely radial and with angular contact ball bearings for combined loads. This is particularly true of angular contact ball bearings or deep groove ball bear- ings with ceramic rolling elements.

Locating and nonlocating bearings generally support shafts or other rotating machine components. Locating bearings provide axial location for the machine component in both directions. The most suitable bearings for this are those that can accommodate combined loads or can pro- vide axial guidance in combination with a second bearing. Nonlocating bearings must permit shaft movement in the axial direction so that the bearings are not overloaded when, for example, thermal expansion of the shaft and rotor assembly occurs. The most suitable bearings for the nonlocating position include cylindrical roller bearings. In applications where the required axial displacement is rela- tively large and the shaft also may be misaligned, the

toroidal roller bearing is the ideal nonlocating bearing.

The selection of an integral seal can be of vital impor- tance to the proper performance of the bearing. A large number of types and sizes are available for

■ deep groove ball bearings

■ angular contact ball bearings

■ spherical roller bearings

■ toroidal roller bearings.

All ball bearings with integral seals on both sides are filled with a grease of an appropriate quality and quantity based on the anticipated operating conditions and required service life. Because of this, the bear- ings are not designed to be relubri- cated in operation.

Calculation of Bearing Load

The loads acting on the bearing can be calculated according to the laws of mechanics if the external forces (forces from power transmission, work forces, or inertia forces) are known or can be calculated. When calculating the load components for a single bearing, the shaft is assumed

to be a beam resting on rigid, moment-free supports for the sake of simplification.

Radial bearings are often subjected to simultaneous- ly acting radial and axial loads. If the resultant load is constant in magnitude and direction, the equivalent dynamic bearing load can be obtained from the general equation:

P = XFr + YFa,

where P = equivalent dynamic bearing load, Fr = actual radial bearing load, Fa = actual axial bearing load, X = radial load factor for the bearing, and Y = axial load fac- tor for the bearing.

X and Y load factors can be obtained in bearing manu- facturers’ catalogs. Except for vertical applications, bear- ings in electric motors are subjected to little if any axial loading, hence P = Fr.

Requisite Minimum Load

If a bearing is to operate satisfactorily, it must always be subjected to a given minimum load. This minimum load ensures proper rolling element rotation, i.e., no skidding, and enhances lubricant film formation in the rolling contact areas. A general “rule of thumb” indi- cates that loads corresponding to 0.02 times the dynamic radial load rating should be imposed on roller bearings and loads corresponding to 0.01 times the dynamic radial load rating on ball bearings. The importance of imposing this load increases where accelerations in the bearing are high and where speeds are in the region of 75% of the permissible speed ratings quoted in the bearing manufacturers’ catalogs.

Bearing Life

The bearing type and size to be used in a motor applica- tion can be initially selected on the basis of its load rat- ings in relation to the applied loads and the requirements regarding service life and reliability. The basic dynamic load rating C is used for calculations involving dynamically stressed bearings, for example, when selecting a bearing, which is to rotate under load. It expresses the bearing load, which will give an ISO