Page 151 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

H Problems induced by Operations.

Problems induced by Design.
H Problems induced by Maintenance.
Let's analyze the evidence that pump mechanics have seen so many
times. Consider the difference between a deflected shaft and a bent
shaft.
A bent shaft is physically bent and distorted. Placing the shaft into a
lathe or dynamic balancer and rotating it will reveal the distortion. If a
bent shaft is installed into a aupp and run, it will fail prematurely,
leaving evidence and specific signs on the circumference of close
tolerance stationary parts around the pump's volute circle. The shaft
will exhibit a wear spot on its surface where the close tolerance parts
were rubbing.
A deflected shaft is absolutely straight when rotated in a lathe or
dynamic balancer. The deflection is the result of a problem induced
either by operation or system design. The deflected shaft also will fail
prematurely in the pump, leaving similar, but different evidence on the
close tolerance rubbing parts in the pump. The next two pictures show
how a bent shaft appears when rotated 180 degrees (Figure 9-9 and
Figure 9-1 0).
The basic difference between a bent shaft and a deflected shaft is the
following. A bent shaft spinning inside close tolerances leaves a scratch
mark around the circumference of stationary elements corresponding to
a damaged spot on the shaft. A deflected shaft spinning within close

n

Figure 9-9
___~

U
Figure 9-10

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