Page 79 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Field Balancing

ating speed, balancing is usually very difficult. When a ma-
chine is in resonance, it is usually very sensitive to very small
changes in the location or amount of any trial weight.

4. EXCESSIVE CLEARANCE IN BEARINGS - Excessive loose-
ness or clearance in bearings will sometimes cause the system
to respond similarly to that experienced when operating in
resonance.

EQUIVALENT WEIGHTS

There are times when the final correction weight must be
placed at a different radius than the trial weight. When this is the
case, a new correction weight mass must be calculated for the new
radius. This is a straightforward process, since the balance correc-
tion is a product of both the mass and the radius (inch-ounce). All
that is required is that the product of the radius times the weight
remain the same.

Wt. × R = Wt. × R (5.12)
1 1 2 2
Example 5-2
If a 2.4-ounce correction weight were added to a cooling
tower fan at a radius of 54 inches, what weight would be required
at a radius of 18 inches?

Step 1. By rearranging Equation (5.12) and substituting the sup-
plied data, Wt. = 2.4 × 54/18 = 7.2 ounces. Thus the 7.2-ounce
2
weight at 18 inches produces the same amount of unbalance as the
2.4-ounce weight at 54 inches.
There are times when several weights can be combined into
one, or times when a single weight must be split into equivalent
weights to accommodate a particular piece of equipment. Once
again, this task is very straightforward.
First let’s examine the case of combining the weights. Figure
5-6 shows three weights being combined into one equivalent
weight.