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

548 Centrifugal Pumps: Design and Application
most insignificant preload of approximately 22 Ibs, whereas an interference
fit of 0.0007 in. would result in a mounted preload of 200 Ibs. A
much more significant preload would result from temperature differences
between inner and outer bearing rings. Such differences could exist
in pumps if heat migrated from high temperature pumpage along the
shaft or if the pump design incorporated cooling provisions that might
artificially cool the outer ring and would thus prevent it from expanding,
By far the worst scenario would be for a pump operator to apply a cooling
water stream from a firehose. It is sad to see this done today, in the age of
high tech, space travel, and information explosion.
Figure 20-15A is very important because it allows us to visualize how
the cage inclination of back-to-back mounted angular contact bearings
with steeper angles promotes a centrifugal outward-oriented flinging action
from side "A" to side "B" [8]. If conventionally lubricated angular
contact ball bearings are back-to-back mounted as shown in Figure
20-15B, lubricant flow may become marginal or insufficient. Subject to
proper selection and utilization of proper installation procedures, face-toface
mounting (Figure 20-15C) may be advantageous because it promotes
through-flow of lube oil. This presupposes that the temperature
difference between inner and outer race is minimal. If the temperature
difference were substantial, growth of the inner ring would force the
bearing into a condition of high axial preload.
The least vulnerable thrust bearing execution is illustrated in Figure
20-16, Here, the vendor has opted to guide the lube oil into spacer ring
"A." This ensures that lubricant flows through both bearings before exiting
at each end. A second spacer ring "B" facilitates making preload adjustments.
Flinger disc "C" tosses lube oil onto the surrounding surfaces
and from there it flows into trough "D" and on towards both inboard and
outboard bearing locations. The periphery of flinger "C" dips into the
lube oil level; however, the lube oil level is generally maintained well
below the center of the lowermost ball. This reduces oil churning and
friction-induced heat-up of lube oil and bearings. Needless to say, unless
lubricant application methods take into account all of the above, bearing
life and reliability may be severely impaired.
The pump designer and user should also realize that double row "filler
notch*' bearings are considerably more vulnerable in pump thrust applications
than other bearing types and should not be used. Similarly, ball
bearings are sensitive to misalignment and must be properly mounted to
eliminate this cause of failure. Misalignment must be no greater than
0.001 inch per inch of shaft length. Bearings operating in a misaligned
condition are subject to failure regardless of cage type, although riveted
cages seem particularly prone to rivet head fatigue in misaligned condition.