The pump shaft is the central component of the rotating element and is responsible for transmitting energy to the rotating components. The shaft carries the impellers, sleeves and couplings for the rotating element assembly. The stiffness of the combined assembly is responsible for ensuring that the pump stationary components do not come into contact with the rotating components.

Unfortunately, this is not always achieved and many pump rotors cannot be described as classically stiff. This can have dire consequences for machine reliability, particularly if, during the rotor build process, the technicians are not aware of the subtleties of the machine design.

Source: https://www.pumpsandsystems.com/hidden-dangers-shaft-stiffness