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

530 Centrifugal Pumps: Design and Application
These requirements would probably be best satisfied by a different lubricant
for each specific application. For the relatively unsophisticated
rolling element bearing service found in the typical centrifugal pump, it
is often possible to economize by stocking only a few lubricant grades,
Maintaining a minimum base oil viscosity of 70 Saybolt Universal Seconds
(SUS) or 13.1 centistokes (cSt) has long been the standard recommendation
of many bearing manufacturers. It was applied to most types
of ball and some roller bearings in electric motors with the understanding
that bearings would operate near their published maximum rated speed,
that naphthenic oils would be used, and that the viscosity be no lower
than this value even at the maximum anticipated operating temperature of
the bearings [3].
Figure 20-2 shows how higher viscosity grade lubricants will permit higher
bearing operating temperatures. ISO viscosity grade 32 (147 SUS @ 100°F or
28.8-35.2 cSt @ 40°C) and grade 100 (557 SUS @ 100°F or 90-110 cSt @
40°C) are shown on this chart. It shows a safe allowable temperature of
146 °F (63 °C) for rolling element bearings with grade 32 lubrication. Switching
to grade 100 lubricant and requiring identical bearing life, the safe allowable
temperature would be extended to 199°F (93°Q. If a change from grade
32 to grade 100 lube oil should cause the bearing operating temperature to
reach some intermediate level, a higher oil viscosity would result and the bearing
life would actually be extended. Most ball and roller bearings can be operated
satisfactorily at temperatures as high as 250°F (121 °C) from the metallurgy
point of view. The only concern would be the decreased oxidation
resistance of common lubricants, which might require more frequent oil
changes. However, the "once-through" application of oil mist solves this
problem. Therefore, oil mist lubricated anti-friction bearings are often served
by ISO grade 100 naphthenic oils. Where these oils are unavailable or in extremely
low ambients, dibasic ester-based synthetic lubes have been very successfully
applied. These synthetics eliminate the risk of wax plugging that has
sometimes been experienced with mineral oils at low temperatures.
When applying greases to lubricate pump bearings, certain precautions
are in order. Soft, long-fibered type greases, or excessively heavy oils,
will result in increased churning friction at higher speeds, causing bearing
overheating due to the high shear rate of these lubricants. Excessive
amounts of lubricant will also create high temperatures.
Using oils of adequate film strength, but light viscosity, or using channeling
or semi-channeling greases has the benefit of substantially reducing
the heat-generating effects of lubricants. The advantages of these
greases rest in their ability to "channel" or be pushed aside by the rotating
ball or roller elements of a bearing and to lie essentially dormant in
the side cavities of the bearing or housing reservoir. Channeling greases
normally are "short-fibered" and have a buttery consistency that imparts