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

Alignment 509
meet results. It should be checked at electric motor drivers, which usually
have four feet. Checking can often be omitted at turbines and pumps,
which are frequently supported at two or three feet. To do the check, apply
a vertical indicator mounted from a stationary base to each foot in
turn, and measure the upspring when the hold-down bolt is loosened. If
this exceeds 0.002 in., try shimming the indicated amount beneath the
foot with the largest upspring. This should help, if the cause was improper
shimming. Other causes may not be helped by this remedy
though. These include heel-and-toe effect, tilting feet or support pads,
and distorted or dirty shims. Remedies for these problems include remachining
supports, using tapered or liquid epoxy shims, and replacement
of bad shims with clean, flat shims. The best shim pack is a "sandwich"
having thick shims top and bottom protecting a few thin shims in between.
Measure the thickness of each shim. Markings are not always accurate,
and thin shims often stick together, doubling their marked thickness.
Stainless steel is generally the best material, and pre-cut shims are
desirable if available in the outline size required. Pre-cut shims 0,050
inch and thicker should be checked with a straightedge for excessive edge
distortion.
Axial Gap and End Float Check. For ball bearing motors, end float is
zero and axial gap depends on coupling geometry. For elastic centering
couplings (for example, metal disk), the coupling should be at zero axial
deflection with the motor in the center of its float, or manufacturer's
cold-spring recommendations should be followed. These are the easy
ones.
For those who enjoy messy mathematics, Murray [6] has some examples
showing how to calculate dimensions for achieving desired axial
gaps and end floats with sleeve bearing motors having gear couplings,
For most situations, however, it is easier to get these by intelligent trial
and error. By knowing our objectives, the task becomes fairly easy. We
wish to accomplish three things:
1. The motor shaft, when coupled, must be capable of passing through
its float center.
2. It must be restrained by the connected coupling so that it cannot rub
the outboard bearing stop.
3. It must be restrained by shaft contact, coupling thrust plates, or
thrust buttons from rubbing the inboard bearing stop.
In effect, the last two transfer thrust forces to the pump thrust bearing
through the coupling. Unlike the relatively weak motor thrust surfaces, it
is capable of handling these forces.