SLEEVE BEARINGS (Electric Motors)

3.12
There are two common types of sleeve bearings. The rigid steel-backed babbit bearing and the self-aligning sintered bearing are shown in Figs. 3.71a and 3.71b, respectively. Babbit is a soft alloy of tin and lead that has a low coefficient of friction. It is backed by a thin steel material to give the bearing rigidity. A wick notch and oil groove are normally formed in the bearing when it is manufactured.
Rigid bearings are pressed into the end frame and burnished or machined to size. An oil reservoir surrounds the bearing. If the bearing is a sintered bronze or sintered iron type, the oil wicks through the pores in the bearing by capillary action and provides lubricant to the shaft. In the case of steel-backed babbitt bearings, a wick of some type must be placed in a hole in the bearing so that oil from the reservoir can reach the shaft.
Self-aligning sintered bearings are held against the end frame by a retaining spring. Oil wicks from the oil reservoir through the pores in the material as in the rigid sintered bearings. Ball bearings are used where it is necessary to limit radial shaft play or where high side loads are expected in the application. These are the most costly bearings. Rigid bearings are used where moderate side loads are expected. Self-aligning bearings are used where light side loads are expected and starting friction has to be minimized.
The self-aligning bearing is the lowest-cost device, but it also carries the lowest load. The rigid sleeve bearing costs somewhat more than the self-aligning bearing, but it can carry a heavier load. Ball bearings can carry a much heavier load, but they also cost 5 to 20 times more than a sleeve bearing.
The most common shaft and bearing system consists of a steel shaft in a sintered bronze bearing. As a general rule, the harder the shaft, the longer the life of the system. Generally, Rockwell C scale 35 to 55 is a good range. Stainless steels are acceptable, except that the 300 series should be avoided with bronze bearings because excessive wear will result. When using bronze bearings, the load speed and shaft-to-bearing clearance must be considered. The pressure velocity factor PV must be calculated to ensure that it is within the rating of the material selected. Bronze is rated
at PV = 50,000.

FIGURE 3.71 Sleeve bearings: (a) rigid and (b) self-aligning.
where P = load, lb/in2
V = surface velocity of the shaft, ft/min w = total bearing load, lb L = bearing length, in N = shaft speed, rpm D = bearing ID, in
Additional lubrication is advised in order to reduce the coefficient of friction. These bearings may close down when installed and may need to be resized to maintain a shaft-to-bearing clearance between 0.0005 and 0.0015 in. The actual tolerance on bearing clearance will depend on the shaft diameter and the type of lubrication used. Sintered bearings should be resized with a hard, polished sizing pin or burnished roller to avoid closing down the pores that provide the lubrication to the bearing-shaft interface. There is a tendency to want to size the bearings with a reamer. This practice should be avoided, because the reamer cuts away the material and smears it across the pores. This results in closed pores and starves the shaft of lubrication. Shaft finish should be 16 |iin or better to reduce wear and avoid pumping oil out of the bearing.