Effect of damping on resonance graph
- The amplitude of the resonance peak decreases and the peak occurs at a lower frequency.
Phase and Resonance
- The phase relationship between the driving oscillation and the oscillation of the object being driven is different at different frequencies.
- Below resonance,e they are in phase with each other.
- At resonance, the phase relationship is 90o or π/2 rad.
- Above resonance, the phase relationship is 180o or π rad.
- 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.
- Resonance problems occur in two primary forms. They are:
- It occurs when a component rotates at its own natural frequency.
- 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.
- 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.
- The structure can be the machine housing itself or some nearby structure such as a hand rail or I-beam.
- The structure 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 shape".
- Testing for the structure's natural frequency is crucial (required) to confirming a resonance problem.
- Resonance, once diagnosed, can be simple to correct. It can also be extremely complex and difficult to correct.
- One method 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
- 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.
To correct 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.
- Change exciting 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 being generated.
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