Page 359 - Improving Machinery Reliability

Extending Motor Life in the Process Plant Environment 325

Motor Mounting Basics

A motor is part of a system that also includes the driven machinery, the power
source, the atmosphere surrounding the motor, and the surface or mounting base to
which the motor is attached. Improper mounting or attachment of a motor to the base
can lead to otherwise unexplained bearing failures, vibration, shaft breakage and,
ultimately, premature motor failure.
The complete motor support structure-the understructure as well as the motor
baseplate-must be adequate to resist forces resulting from the weight and operation
of the system comprised of motor and driven machinery. The four motor attachment
points must be fastened to a flat, rigid, machined surface. The motor should be dow-
elled as well as bolted; bolts alone are not always adequate to prevent lateral shift-
ing. If a motor base is not flat, the frame will twist when the bolts are tightened
down, causing misalignment with the driven machinery, vibration, and eventual
bearing failure.
The motor base or mounting should be stiff enough to withstand deflection from
belt pulls and other operating loads. Welded bracing can be added to improve
mounting stiffness, but the heat of welding may also cause distortion and impose
new stresses. Ideally, a base that has been modified should be stress-relieved and the
mounting surfaces remachined.
Grouting should be properly installed between the steel motor base and the con-
crete block or cement pad on which it rests. A large base resting on only a few sup-
port points will sag and cause vibration problems. Inspect grouting periodically for
cracking, powdering, or crumbling. The grout usually fails before the motor requires
replacement, and the remaining motor life will be reduced without adequate support.
Often, when vibration is detected, loosening one attachment bolt at a time and not-
ing the result will identify and locate a support problem. Adequate shimming should
then be added under the affected mounting point and all bolts should be properly
retightened.
When using shims to compensate for unavoidable irregularities in the motor base,
or when making vertical adjustments to align components, as few shims as possible
should be used. Shim packs of many thin shims are difficult to tighten down uni-
formly. Ideally, a single shim should be used at each point requiring shimming.
Tapered shims are a last resort when the motor feet and base plate cannot be made
parallel. This will avoid distortion of the feet when the bolts are tightened.
Consideration should always be given to alignment changes that occur while the
motor system is in operation. Thermal expansion of tightly coupled shafts, for exam-
ple, can transfer loads from one component to the next, affecting bearing life and
even causing shaft breakage.
Changes in process operating conditions after original installation of the motor
can cause new problems. Any time a motor is removed for servicing or shifted
because of load changes, all dowels and shims should be inspected and original
installation procedures followed.