Page 52 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

1. First, the external force that is exciting the component or
machine can be eliminated or its frequency changed. This
may not be practical in many cases since most equipment
must operate at some given speed.

2. Second, by adding or removing mass. Usually, mass changes
must be significant to alter the natural frequency sufficiently
to eliminate the vibration.

3. This leaves altering the stiffness of the component. Care must
be exercised when altering the stiffness of a component or
machine, so that its strength is not also effected. Altering the
location of support members will alter the effective length
and alter the resonant frequency. This is usually a good
choice for piping systems or other long, slender members.
Use caution so as not to place a support at a location which
will create other harmonics—i.e., 1/2, 1/3, etc.

It is always best to remove or at least decrease the source or
driving force of one or more of the vibrations.
The easiest way to measure the natural frequency of an object
is to first remove all sources of excitation. Next, the object is struck
and allowed to vibrate freely. Using a vibration instrument, the
frequency is recorded. The largest amplitude measured with a fil-
ter in reading will be the natural frequency.

BEAT FREQUENCIES

Beat frequencies are vibration frequencies caused by two or
more vibrating sources that are vibrating at slightly different fre-
quencies. The resultant beat frequency is the difference in the two
sources.

Example 3.5
A machine is found to produce a vibration at a frequency of
1,275 Hz. A second machine nearby produces a vibration at 1,280
Hz. What is the resulting beat frequency?