Page 124 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

them. Often, equipment has been aligned using any material
available, such as soft drink cans, galvanized sheet metal, tin cans,
and other scrap metal. Obviously, this is undesirable and must be
discarded, and the proper shim material used. Regardless of the
type of shim material to be used, all shims must be miked to
verify their thickness. All cut or stamped out shims should be
carefully inspected for burrs that can distort their thickness. The
use of brass or some of the newer plastic shims is highly recom-
mended.
One final caution about shims: The use of excessive numbers
of shims can cause uneven crush each time the machine hold-
down bolts are tightened. This will result in different indicator
readings and create many problems in the alignment process. It is
therefore recommended that no more than three shims are placed
under any machine foot. This may require machining a plate of a
given thickness for each foot, but the dividends will be proper
alignment and greatly extended equipment life.
Once the shims have been cleaned (or replaced) and the
machine feet and base plate have been cleaned, re-torque the hold
down bolts on the stationary machine to the proper tension. The
use of a torque wrench is highly recommended to assure not only
the proper preload, but to assure all bolts are tightened the same,
and that any subsequent re-tightening will place the same amount
of crush on the shims.

SOFT FEET

The first step is to assure the stationary machine is sitting
squarely on its foundation or base plate, and is supported equally
on all feet. A condition known as soft feet can cause internal mis-
alignment of the machine and lead to premature failures. Check-
ing for soft feet is a relatively simple task. Once the shims have
been removed, the machine feet have been cleaned and inspected,
and all hold-down bolts are properly torqued, loosen one hold
down bolt at a time. Use either a feeler gauge or a properly
mounted dial indicator to check the amount of movement or the