A pump shaft is a major part whose function is to transmit input power from the
driver to the impeller/s mounted on the shaft. It is subjected to several stresses -
flexural, shear, torsional, tensile, etc. Of these, the torsional stress is most
significant and is usually used as basis for sizing the shaft diameter. Commonly
used shaft materials are 4140 carbon steel, and stainless steel such as 310,
410, or 416.
In large, multistage, or high speed pump that requires high brake horsepower
(BHP) it may be necessary to reduce the shaft diameter by using higher strength
material, such as 13-4 or 17-4, to reduce the bearing size, mechanical seal size,
and stuffing box to reduce the over-all size and cost of the equipment.
On single stage, overhang pump where the BHP is small, sizing the shaft based
on torsional stress alone may result in a shaft diameter so small that the shaft
deflection may become excessive and unacceptable. In such situation the shaft
size is increased above that required for torsional stress consideration to reduce
the shaft deflection.
There are other situations where the shaft size is determined by neither torsional
stress, nor shaft deflection consideration. If the rotor natural frequency at a given
pump speed coincided with the natural frequency of other pump components,
such as the bearing housing, the frequency is said to be in resonance. The pump
speed at which the frequency is in resonance is called the critical speed of the
Operating the pump at its critical speed is almost certain to result in excessive
and destructive rotor vibration. One way of detuning the resonance is to change
the pump shaft size to change the rotor natural frequency.
The shaft position, horizontally or vertically, defines whether a pump is considered
a horizontal pump, or a vertical pump.
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