Page 152 - Know and Understand Centrifugal Pumps

Shaft Deflection

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LJ LJ
Fiqure 9-11

Fiqure 9-12
tolerances leaves a scratch or gouged circle around the rotary element,
and a gouged or damaged spot on the stationary elements. It is
absolutely necessary to distinguish and recognize these significant
differences.

If the pump is put into service with a bent or unbalanced shaft
assembly, its premature failure can be traced to inadequate maintenance
practices. The evidence does not lie. However, if the premature failure
leaves evidence of a deflected shaft, this would be an operations or
design failure. All too often, the mechanic is blamed. The two pictures
above show how a deflected shaft appears when rotated 180 degrees
(Figure 9-11 and Figure 9-12).
Shaft deflection is the result of an external radial load. The external
radial loading originates with the pump operator or process when the
pump runs away from its best efficiency point on the curve. The
resistance to deflection is a function of the shaft’s overhang length and
its diameter. The deflection resistance, also called the flexibility factor, is
known as the L/D factor.
The L/D indicates length/diameter. Because pumps are manufactured
with certain dimensional standards (ANSI, API, DIN, and ISO), the
L/D factor can and should be specified at the moment of specifying the
pump. The design engineer could request that the pump manufacturer
quote a pump based on its flow, head, metallurgy, and L/D factor,
awarding bonus points for a low L/D, indicating a high deflection
resistance. The high deflection resistance is an index of how far the
pump can be run away from its BEP on the curve without damaging
the mechanical seal and bearings.

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