The shaft of reciprocating positive displacement pumps takes the shape of a crankshaft or a camshaft.
The pump shaft is the central component of a centrifugal pump rotor and carries the impellers, the shaft sleeves (e. g. spacer sleeves between the impellers or shaft protecting sleeves in the shaft seal area), the bearings (rolling element bearings or plain bearing sleeves and thrust collars), in some cases balance discs or drums (see Balancing device) and the coupling as well as other components that are part of the rotor (e. g. throwers, balancing discs, inducers, shaft nuts). Close-coupled pumps do not have a coupling (see Shaft coupling), and the hollow shafts of propeller pumps incorporate the adjusting rod which actuates the adjustment mechanism (see Impeller blade pitch adjustment).
As well as transmitting energy, the pump shaft is designed to centre the rotating components on the pump rotor within the bores of the pump casing in such a way that the former do not touch the latter when the pump is running, taking shaft deflection into account.
It is necessary to keep the radial distance between pump rotor and pump casing, i.e. the clearance gap, as small as possible in order to reduce the flow losses (see Clearance gap losses) and the leakage rate. This means that some contact may occur at these clearance gaps under certain operating conditions in multistage pumps, and therefore the materials used in these places must have certain minimum sliding properties.
When sizing a pump shaft, it is essential to take into account not only the maximum torques and permissible shaft deflections, but also any bending and torsional vibrations which may develop (see Vibration). Establishing the critical speeds requires a good deal of practical know-how as aspects such as the damping influence (which is hardly predictable theoretically) exercised by the throttling gaps and the special stiffening effects resulting from different modes of installation have to be taken into account. Finally, the effects of corrosion on the shaft material must be considered, with the exception of "dry" shafts (shafts which do no come into contact with the fluid handled), i.e. a corrosion-resistant material grade must be used (see Material selection). In all cases, however, the shaft material must be selected so as to ensure that it does not suffer permanent deformation over time as a result of temperature fluctuations.