Page 77 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Field Balancing

cially if it normally operates above the first critical speed. The
subject of critical speed will be discussed later.
As a general rule, a trial weight should be selected that will
cause a 30% change in amplitude or a 30-degree shift in the refer-
ence mark location. Changes of this magnitude will insure accu-
rate results are obtained from your calculations. A common
practice used in selecting the size of a trial weight, is selecting a
mass that will produce a force equal to 10% of the rotor weight on
the supporting bearings.

2
F = 1.774 × (rpm/1000) × inch-ounces (5.11)

Example 5-1
A rotor turns at 1750 rpm and weighs 175 pounds. The rotor
is simply supported by two bearings. Since each bearing supports
87.5 pounds, a force of 8.75 pounds will be sufficient to add the
10% force required for balancing. What trial weight is required?

Step 1. Using Formula 5.11, rearranging and substituting the sup-
plied data, the unbalance is found to be inch-ounces = 8.75/[1.774
2
× (1750/1000) ] or inch-ounces = 1.61.
Step 2. If the trial weight were to be added at a radius of six
inches, a weight of .268 ounces would be required. For practical
purposes, a .25 or quarter ounce weight would be selected.

Be sure to attach trial weights securely to assure they do not
fly off during the subsequent trial runs. There are numerous ways
to add trial weights to a rotor, and a close inspection of the exact
application will reveal the most appropriate method. If the rotor
has a recessed lip, modeling clay can be used for trial weights. Flat
washers, lead weights, hose clamps, reinforced tape, epoxy, and
custom-made clamps have all been used as trial weights.
Trial weights and especially permanent correction weights
can be added by soldering, brazing, and welding. Adding washers
to existing assembly bolts or drilling and tapping new bolt holes
may also be used. Occasionally there will be no location for trial