Page 258 - Industrial Power Engineering and Applications Handbook

Installation and maintenance of electric motors 10/237
Relubricating interval (RG)
in operating hours
C b a
1.5 - 2.5 3
104 - 2 2.5
- 2
- 1.5 1.5
104
2-
0 104
9
103 - 6 a
0- 4 7
6- 3 6
5 - 2.5 5
4- 2 4
3 - 1.5 3
2.5 - 2.5
2 - 103 2
1.5 - 7.5 1.5

10' - 5 103
9 - 4.5 9
a- 4 0
7 - 3.5 7
6- 3 6
5 - 2.5 5
4- 2 4
3 - 1.5 3
2 - 10' 2
1.5 - 7.5 1.5
Speed (rpm) -
10' - 5 1 0'

(Source: SKF,
a = Radial ball bearings
b = Cylindrical roller bearing
c = Spherical roller bearings, taper roller bearings, full complement cylindrical roller bearings, thrust ball
bearings, cylindrical roller thrust bearings

Figure 10.6 Curves to determine relubricating periods for different types and sizes of bearings at different speeds

may also cause shaft currents as a result of capacitive 2 Insulation of both the bearings will not prevent the
coupling. circulating currents, as illustrated in Figure 10.7.
Both these effects cause circulating currents to flow Therefore a sheet of insulating material (rubber lining
through the motor frame, forming a complete electric or Bakelite) may be provided between the motor's
circuit via the bearings (Figure 10.7). These currents are feet and the base frame, and insulated holding-down
detrimental to the life of bearings due to sparking and fasteners used for fixing the motor. This will prevent
consequent pitting and heating effects. Such currents, if the eddy current or leakage current paths forming a
not prevented, may cause indentation (pitting) on the closed circuit through the bearings, as indicated at
inner races of the bearings, as well as on their balls or location 1 in Figure 10.8. One of the bearings, generally
rollers. The pitting may not only cause noise but may the driving end, may also be grounded as illustrated,
also loosen the motor shaft within the bearing, causing to prevent a build-up of electrostatic currents in the
heating and chattering, so that a stage may arise when rotor.
the rotor may start to rub the stator laminates and cause 3 Sometimes a coupling insulation as indicated at location
damage. These shaft currents, therefore, must be prevented 2 in Figure 10.8 may also prevent the eddy currents
while installing such motors and the following are some forming a closed circuit and generate the bearing
preventive measures: currents.

1 One of the bearings should be insulated inside its The insulation, as discussed above, must be protected
housing by providing a layer of thin insulation as during normal operation to avoid any damage to it
indicated at location 3 in Figure 10.8 to prevent otherwise the circuit may defeat the purpose of shaft
circulating current through the motor bearings. insulation. It is advisable to check the insulation periodi-