Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

534 Centrifugal Pumps: Design and Application
brication—are functions of what is termed the DN factor. To establish th
DN factor for a particular bearing, the bore of the bearing (in millime
ters) is multiplied by the revolutions per minute, that is:
?5mm x 1000 rpm = 75,000 DN value
Speed limits for conventional greases have been established to rang<
from 100,000 to 150,000 DN for most spherical roller type bearings an<
200,000 to 300,000 DN values for most conventional ball bearings
Higher DN limits can sometimes be achieved for both ball and roller typ
bearings, but require close consultation with the bearing manufacture!
When operating at DN values higher than those indicated above, use ei
ther special greases incorporating good channeling characteristics or cir
culating oil. Typical relubrication intervals are given in Figure 20-3.
Figure 20-3. Relubrication intervals recommended by SKF Bearing Company.
Life-Time Lubricated, "Sealed" Bearings
As shown in Table 20-2, "lubrication for life" ranks last in order of
preference [8J. Lubed-for-life bearings incorporate close-fitting seals in
place of, or in addition to shields. These bearings are customarily found
on low horsepower motors or on appliances that operate intermittently.