Shaft diameters are usually larger than what is actually needed to transmit the torque. A factor that assures this conservative design is a requirement for ease of rotor assembly.
The shaft diameter must be stepped up several times from the end of the coupling to its center to facilitate impeller mounting (see Figure 65). Starting with the maximum diameter at the impeller mounting, there is a step down for the shaft sleeve and another for the external shaft nut, followed by several more for the bearings and the coupling. Therefore, the shaft diameter at the impellers exceeds that required for torsional strength at the coupling by at least an amount sufficient to provide all intervening step downs.
One frequent exception to shaft oversizing at the impeller occurs in units consisting of two double-suction, single-stage pumps operating in a series, one of which is fitted with a double-extended shaft. As this pump must transmit the total horsepower for the entire series unit, the shaft diameter at its inboard bearing may have to be larger than normal.
The shaft design of end-suction, overhung impeller pumps presents a somewhat different problem. One method for reducing shaft deflection at the impeller and seal chamber, where the concentricity of running fits is extremely important, is to considerably increase the shaft diameter between the bearings.
Except in certain smaller sizes, centrifugal pump shafts are protected against wear, erosion, and corrosion by renewable shaft sleeves. In small pumps, however, shaft sleeves present a certain disadvantage. As the sleeve cannot appreciably contribute to shaft strength, the shaft itself must be designed for the full maximum stress. Shaft diameter is then materially increased by the addition of the sleeve, as the sleeve thickness cannot be decreased beyond a certain safe minimum. The impeller suction area may therefore become dangerously reduced, and if the eye diameter is increased to maintain a constant eye area, the liquid pickup speed must be increased unfavorably. Other disadvantages accrue from greater hydraulic and seal losses caused by increasing the effective shaft diameter out of proportion to the pump size.
To eliminate these shortcomings, very small pumps frequently use shafts of stainless steel or some other material that is sufficiently resistant to corrosion and wear that it does not need shaft sleeves. One such pump is illustrated in Figure 66. Manufacturing costs, of course, are much less for this type of design, and the cost of replacing the shaft is about the same as the cost of new sleeves (including installation).