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

546 Centrifugal Pumps: Design and Application
Table 20-3
Fatigue Life Reduction of Roiling Element Bearings Due to Water
Contamination of Lubricant (Mineral Oil)
Base Oil
Description
Water Content
of Wet Oil, %
Fatigue-Life
Reduction, %
Test Equipment
and
Hertzian Stress
Mineral oil,
dried over
sodium
0.002
0.014
3.0
6.0
48
54
78
83
Rolling 4-BaIl
test, 8.60
GPa(1.25x
10 4 psi)
pumps prompted the search for "life-time lubricated" rolling element
bearings that we had alluded to earlier, but which nevertheless have their
own particular vulnerabilities.
Problem incidents caused by dirt and water have been substantially reduced
by oil mist lubrication. However, serious failure risk can also be
introduced by certain specification practices, including some contained
in API 610. Without going into the many possible factors that could influence
bearing life, several items must be considered in the mechanical
design of reliable centrifugal pumps. First among these would be that
deep-groove Conrad-type radial bearings with loose internal clearance*
are more tolerant of off-design pump operation than bearings with precision
tolerances. Also, it should be recognized that centrifugal pumps
which undergo signficant bearing temperature excursions are prone to
cage failures. Phenolic cages are best suited for operation of radial bearings
below 225°F (107°C). New cage materials, such as Polyamide 66,
provide a much higher temperature limit and excellent lubricity at these
temperatures. Metallic cages are, of course, equally suitable and are
sometimes preferred in thrust bearings.
The pump designer and pump user must also come to grips with questions
relating to preload values and contact angles. The API requirement
to utilize duplex 40° contact angle thrust bearings was prompted by the
desire to obtain maximum load capacity from a given size bearing [11].
Similarly, the requirement of installing these bearings back-to-back with
a light axial preload was aimed at reducing axial shuttling of rotors to
prevent brinelling of contact surfaces (raceways) and to prevent ball skidding.
Occasionally, an effective way to prevent this destructive skidding
is to install matched, preloaded sets of angular contact bearings with dif-
* The AFBMA (Anti-Friction Bearing Manufacturers Association) calls this recommended clearance a
"C3" clearance as opposed to looser or tighter clearances (C1 or C5, for instance).