Page 68 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

speed or to the class of tolerable vibrations, will require precision
shop balancing.
Balancing in-place is a very straightforward process, and in-
volves following only a few simple rules. Unbalance is defined as
an unequal distribution of mass about a rotating center point.
Unbalance is further defined by International Organization of
Standards (IOS) as, “That condition which exists in a rotor when
vibratory force or motion is imparted to its bearings as a result of
centrifugal forces.” Unbalance will result in the vibration of the
rotating part and its supporting bearings and associated support
members.
Balancing is the process of identifying the amount and loca-
tion of a heavy spot on a rotating part, and then removing or
adding the correct amount of weight at the correct location to
cancel the centrifugal forces caused by the unbalance. The amount
of unbalance that exists is determined by the amplitude of vibra-
tion of the rotating part. The location of the unbalance is deter-
mined with the use of a strobe light to identify a phase location
with respect to any existing reference point on the rotating ele-
ment.

SINGLE PLANE BALANCING

The strobe light is configured to trigger on the vibration sig-
nal, at its peak. This point corresponds to the location of the heavy
spot at some time in the rotation cycle. The exact location of the
phase reference point is of no consequence in the beginning.
Next, a trial weight is attached to the rotating member, and
its location, amount and distance from the center of rotation are
noted. By adding the trial weight, the location of the resultant
heavy spot will be altered. Thus, the amplitude of the vibration
and its phase location will have been altered.
When a weight is added to a perfectly balanced rotating el-
ement, it will vibrate at a frequency equal to the rotating speed of
the element and have an amplitude proportional to the unbal-