Page 148 - Know and Understand Centrifugal Pumps

Shaft Deflection

With pressures equal and more area in the E-F-G-H arc of the volute
circle, a tremendous radial force is created that will distort and deflect
the shaft toward a point approximately 60" around the volute from the
cutwater. This radial force can destroy the mechanical seal or packing
rings, bearings, and deform and even break the shaft. The evidence
would be rub or scratch marks around the circumferences of close
tolerance rotary elements, such as the outer diameter on open or semi
open impellers (see Point A in the next illustration, Figure 9-5), the
wear rings on closed impellers (see Point B, next illustration), the shaft
or sleeve at the restriction bushing in the bottom of the seal chamber or
stuffing box (see Point C), or on the posterior of the mechanical seal
(Point D).
The scratch marks on the circumference of these close tolerance rotary
parts will correspond to scratch marks on close tolerance stationary
parts at approximately 60" around the volute from the cutwater. These
marks will be visible on the back plate with open impellers, or on the
wear rings of pumps with enclosed impellers, or the ID bore of the
restriction bushing at the bottom of the seal chamber where the shaft
passes through, or the ID of the seal chamber bore at the back end of
the mechanical seal (Figure 9-6 and Figure 9-7, next page).

A - Impeller OD
A B -Wear Band
C - Restriction Bushing
D - Seal Posterior

h

B-

/

STRICT TOLERANCE
Figure 9-5
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