Page 69 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Field Balancing

anced weight. A reference mark for phase will appear to stand still
at some location under a strobe light triggered by the vibration. To
further illustrate this point, if a 4 ounce weight were added to a
balanced rotor, and caused a 3.5 mils vibration at 60 degrees,
doubling the weight to 8 ounces would increase the amplitude of
the vibration to 7 mils, but the phase angle would remain at 60
degrees.
Further, if the 4-ounce weight had been moved 60 degrees
counterclockwise, the amplitude would have remained the same,
but the phase reference angle would have shifted 60 degrees
clockwise to 120 degrees.
The three fundamentals of balancing are:

1. The amount of vibration is proportional to the amount of
unbalance.

2. The reference mark (phase angle) shifts in a direction oppo-
site to a shift in the heavy spot.

3. The angle the phase mark shifts is equal in degrees to the
angle the heavy spot was shifted.

The unbalance in a rotating element at the start of a balancing
process is referred to as the original unbalance, and thus the asso-
ciated phase angle and amplitude readings are called the original
readings. Polar graph paper is very useful in laying out balancing
problems. However, by using a protractor and a convenient scale
the same results can be achieved.
As an example, a rotor has a vibration amplitude of 3.5 mils
at 60 degrees, and its original readings are shown in Figure 5-1.
Note that the scale chosen is 1/2 mil per division, but any conve-
nient scale can be used.
The radial lines that radiate from the center represent the
angular position, and the concentric circles are spaced 1/2 mil
apart. This makes it convenient to visualize the problem.
When a trial weight is added to the rotor, one of three things
must happen: