Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)
Shaft Design and Axial Thrust 335 Solving for D: For design round up to nearest Vs-in. increment: D = 2.375. Example A 2-stage pump has been designed with a 2 5 /8-in. shaft diameter at the coupling. The maximum horsepower at 3,560 RPM is 900. What is the maximum operating speed for a limiting stress of 7,000 psi? Solution Using the pump affinity laws described in Chapter 2:
336 Centrifugal Pumps: Design and Application Substituting and solving for N: Shaft Sizing Based on Fatigue Evaluation Pump shafts are subjected to reversing or fluctuating stresses and can fail even though the actual maximum stresses are much less than the yield strength of the material. A pump shaft is subject to alternating or varying stresses as a result of the static weight and radial load of impellers, pressure pulses as impeller vanes pass diffuser vanes or cutwater lips, driver start-stop cycles, flow anomalies due to pump/driver/system interaction, driver torque variations, and other factors. In order to perform a fatigue analysis, it is first necessary to quantify the various alternating and steady-state loads and establish the number of cycles for the design life. In most cases, the design life is for an infinite number of cycles; however, in the case of start-stop cycles, the design life might be 500 or 1,000 cycles depending on the application. Once the loads have been defined and the stresses have been calculated, it is necessary to establish what the acceptable stress values are. The use of the maximum-shear-stress theory of failure in conjunction with the Soderberg diagram provides one of the easier methods of determining the acceptable stress level for infinite life (Peterson; Shigley; Roark). Since the primary loads on pump shafts are generally torsion and bending loads, the equation for acceptable loading becomes:
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336 <strong>Centrifugal</strong> <strong>Pumps</strong>: <strong>Design</strong> <strong>and</strong> <strong>Application</strong><br />
Substituting <strong>and</strong> solving for N:<br />
Shaft Sizing Bas<strong>ed</strong> on Fatigue Evaluation<br />
Pump shafts are subject<strong>ed</strong> to reversing or fluctuating stresses <strong>and</strong> can<br />
fail even though the actual maximum stresses are much less than the yield<br />
strength of the material. A pump shaft is subject to alternating or varying<br />
stresses as a result of the static weight <strong>and</strong> radial load of impellers, pressure<br />
pulses as impeller vanes pass diffuser vanes or cutwater lips, driver<br />
start-stop cycles, flow anomalies due to pump/driver/system interaction,<br />
driver torque variations, <strong>and</strong> other factors. In order to perform a fatigue<br />
analysis, it is first necessary to quantify the various alternating <strong>and</strong><br />
steady-state loads <strong>and</strong> establish the number of cycles for the design life.<br />
In most cases, the design life is for an infinite number of cycles; however,<br />
in the case of start-stop cycles, the design life might be 500 or 1,000<br />
cycles depending on the application.<br />
Once the loads have been defin<strong>ed</strong> <strong>and</strong> the stresses have been calculat<strong>ed</strong>,<br />
it is necessary to establish what the acceptable stress values are.<br />
The use of the maximum-shear-stress theory of failure in conjunction<br />
with the Soderberg diagram provides one of the easier methods of determining<br />
the acceptable stress level for infinite life (Peterson; Shigley;<br />
Roark). Since the primary loads on pump shafts are generally torsion <strong>and</strong><br />
bending loads, the equation for acceptable loading becomes: