Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Shaft Design and Axial Thrust 349
Figure 16-8. Axial thrust on single-suction open impellers without ribs on back
side.
It is recommended that the element maintain a small unbalance (minimum
100 Ib) in one direction to lightly load the bearing and prevent
floating of the rotor. Normal wear of the original ring or bushing clearance
will not affect balance. It is recommended that all impeller rings be
the same size and on even-stage pumps that the center sleeve diameter be
the same as the sleeve diameters in each stuffing box. To balance an odd
number of stages, the simplest solution is to change only the diameter of
the center sleeve.