Page 65 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Vibration Due to Unbalance
2
due to unbalance is expressed as F = 1.774 * (rpm/1000) * in-oz.
Here, the inch-ounces of unbalance is to be determined, so rear-
ranging the equation, in.-oz. = F/1.774 * (rpm/1000) 2 in.-oz. =
2
14.47/(1750/1000) = 4.72 inch-ounces of unbalance.
Step 3. The center of the missing key material would be at a ra-
dius of 1-3/4 inches, thus the missing material would weigh
16.54/1.75 or 2.7 ounces. Steel weighs approximately 4.49 ounces
per cubic inch, therefore the missing material would be 2.7/4.49 or
.6 cubic inches. Since the key is 1/2 × 1/2, the missing portion
would be 1/2 × 1/4 or 1/8 ounce per inch of length. The length
of the missing material is .6/.125 or 4.8 inches.

In the case of electric motors, measure the amplitude and
then turn off the power. If the vibration disappears immediately,
the problem is most likely electrical. However if the amplitude
decreases with the decrease in speed of the motor, check for bal-
ance and/or misalignment.
There are a number of published vibration severity charts
that give guidelines as to how much vibration is too much. One
major pitfall of using these guidelines is the relationship of the
weight of the rotating element to the total equipment mass and
stiffness.
A small rotating element in a massive machine that is well
anchored may not exhibit severe vibration when readings are
taken on the machine frame. Some vibration analyzer compa-
nies provide shaft riders which allow the vibration probe to be
placed against the rotating shaft to obtain a more accurate read-
ing.
Regardless of the method employed, it is obvious that the
smoother a part runs, the longer the life of the equipment. Many
conditions of unbalance are best left to shop balancing rather than
field corrections. However, there are many conditions of unbal-
ance that can be recognized and corrected during routine mainte-
nance. Cleanliness and attention to detail can avoid many of these
situations.