simply the natural frequency of a component or combination of components
(assembly). All structures have a resonant frequency. If you impact the
structure with enough force to make it move, it will vibrate briefly at
its natural frequency. A structure will have a resonant frequency in each
of its 3 directional planes (x, y and z, or as we call them, horizontal,
vertical and axial). Resonance serves to amplify the vibration
due to whatever vibration force is present at (or near) that resonant frequency.
It is important to note that resonance does not cause vibration
- it amplifies it.
problems occur in two primary forms. They are:
- occurs when a component rotates at its own natural frequency.
- This is far more common than a critical speed problem. It becomes a problem
when some forcing frequency comes close (+/- 10%) to the resonant (natural)
frequency of a structure.
A "critical speed"
is simply when the rotational speed (rpm) coincides with the natural frequency
of the rotor (cpm).
The tiniest amount
of residual unbalance (something that is always present) is enough
to cause huge amounts of vibration when rotating at a critical.
Rotors that are
sped up or slowed down slowly are susceptible to this (i.e. turbines).
In these cases, the critical speed is usually well known.
The most common
problem related to
unknown critical speeds is probably belts. Belts
rotating at their resonant frequency (or having a nearby source of excitation
of that resonant frequency) can vibrate excessively and cause other problems.
For example, if the natural frequency of the belts coincides with the rpm
of the fan, the belts will vibrate at their natural frequency.
2nd and 3rd criticals
also may occur if the rotor speed gets high enough.
diagnosed, can be simple to correct. It can also be extremely complex and
can be the machine housing itself or some nearby structure such as a hand
rail or I-beam.
A common example
of this is a vertical pump. Due to the lack of a support at the top of
the unit, these typically have very low resonant frequencies (~ 300 cpm).
While running, this is not a problem but during start-up or coast-down,
the unit experiences a "shudder" as it passes through the structural resonance
(this is not a critical speed - it is a structural resonant frequency).
itself will vibrate excessively - do not confuse with a critical speed.
The "shape" of
the structure's vibration is an important clue and is known as a "mode
Testing for the
structure's natural frequency is crucial (required) to confirming
a resonance problem.
The trick is in the diagnosis. But how do you diagnose it
for determining a critical speed is a "Coast Down/Start Up Plot". This
plot consists of the 1x vibration amplitude being collected simultaneously
with a 1x rpm phase reading as the machine coasts to a stop or goes from
stopped to full running speed. This test requires a 1x rpm reference (from
a photoeye or some other speed tracking signal) in order to track the amplitude
and phase at that frequency. Two things are observed as the rotor passes
through a critical:
The 1x rpm amplitude
will increase until the rotor reaches it's critical and then decrease to
the normal level as the speed continues to change.
Phase will shift
180° as the rotor passes through the critical. This is due to the rotor
changing from a rigid rotor (while operating below it's critical) to a
flexible rotor (while operating above it's critical). It practical terms,
on a rigid rotor, the heavy spot pulls the rotor around as it rotates.
On a flexible rotor, the heavy spot pushes the rotor around as it rotates.
resonances can be first suspected by several characteristics:
high amplitude at a single frequency (the resonant frequency) in the direction
in which the resonant frequency is being excited.
A "mode shape"
analysis shows the structure vibrating in a way that models resonance.
Those models are covered on the next page.
those characteristics confirms resonance as a problem. A
test must be performed that actually determines the natural frequency of
the structure in question - a "bump test". Although there are high-tech
methods available for this test (and some work very well), this test can
be as simple as bumping the structure (causing it to vibrate) while it
is not running and measuring the response (i.e. the frequency it vibrates
at). A simple method for doing this involves collecting a 2 second sample
(time domain plot) while bumping the structure, measuring the period of
one cycle and converting it to a frequency. The time sample may have to
be adjusted depending on the resonant frequency being measured (longer
sample for very low resonant frequencies, shorter sample for high frequencies).
|If the measured
response of the structure (i.e. it's resonant frequency) is within about
10% of the forcing frequency (i.e. the rpm of the machine although it can
be at any frequency), resonance should be considered a problem.
The closer the two frequencies are, the more of a problem it is.
a resonance problem, there are 4 methods:
Stiffen the structure
- This method raises the resonant frequency of the structure.
Add mass to the
structure - This method lowers the resonant frequency.
frequency - Change the speed of the machine.
Add a dynamic
absorber to the structure - This method attaches the equivalent of a tuning
fork to the structure. This attachment is tuned to have the same resonant
frequency as the structure and sets up an out-of-phase signal that has
the effect of cancelling out (reducing) the signal being generated by the
structure. The dynamic absorber must be properly sized to handle the forces