Page 48 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

When a machine is in operation, there are normally many
external forces that act upon the machine components. These re-
petitive forces are responsible for the vibration to appear as
though it were free and not dampened. If one or more of these
forces occurs at or near the natural frequency of one of the com-
ponents, it will vibrate with an increased amplitude.
Resonant frequency in shafts or rotors is often referred to as
a critical speed. Criticals are often experienced as a machine comes
up to operating speed. This is often true for high-speed machines,
which may pass through one or more criticals before reaching
operating speed.
The first critical is simply the natural or resonant frequency
of the vibrating element. There are also multiples of this natural
frequency called the second harmonic, third harmonic, etc. These
are also referred to as the second critical and third critical, etc.
Figure 3-4 shows the wave shape of a component vibrating at
its natural frequency.

Figure 3-3. First Critical Vibration

Note the different shape of the wave for the simply sup-
ported component versus the cantilever or overhung component.
Even without a vibration analyzer, feeling along the length of a
component undergoing vibration at one of its critical frequencies,
resonance can be detected. For the simply supported component,
if the vibration is the highest at its midpoint, and decreases as
either end is approached, the component may be vibrating at its
natural frequency.
The cantilevered or overhung component will have its high-