Page 368 - Centrifugal Pumps 2E

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Shaft Design

and

Axial Thrust

Shaft Design
The pump rotor assembly consists of the shaft, impellers, sleeves,
bearing or bearing surfaces, and other components such as balancing
disks, shaft nuts, and seals that rotate as a unit. The primary component
of the rotor assembly is the shaft. The pump shaft transmits driver energy
to impellers and consequently to the pumped fluid. This section will be
concerned primarily with the sizing of the pump shaft.
The pump shaft is subject to the combined effects of tension, compres-
sion, bending, and torsion. As a result of the cyclic nature of the load,
when shaft failures occur they are almost exclusively fatigue-type fail-
ures. Therefore, the first consideration in sizing the shaft is to limit
stresses to a level that will result in a satisfactory fatigue life for the
pump. The degree of detail involved in the stress analysis will be depen-
dent upon the intended application of the pump. The analysis can be a
simple evaluation of torsional shear stress at the smallest diameter of the
shaft or a comprehensive fatigue evaluation taking into consideration the
combined loads, number of cycles and stress concentration factors.
Sizing the shaft based on stress is not the only consideration. Shaft de-
flection, key stresses, fits for mounted components, and rotor dynamics
must be evaluated by the designer. The analytic tools available range
from simple hand calculations to sophisticated finite element computer
programs. The following sections are intended to present the fundamen-
tal considerations with which the designer can begin the design of the
pump shaft. In some situations, satisfying these fundamental require-
ments can be considered adequate for a complete shaft design. In other,
more critical services, further analysis is required before finalizing the
design.

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