Resonance - The Machines Natural Frequency
Symptoms: Large Amplitudes at Machines Natural Frequency - radial (V & H)
Resonant frequencies are sometimes reached even when operating rpm is low, as in our example above. Due to the impulses created by the pumps worn impellor, the pumps natural frequncy was excited , which resulted in the pump tripping. When the machine was off, a Bump Test indicated a resonance at approximatley 5.2 x of the machines running speed. this was close to the pumps vane pass frequency and so excited the natural frequency. The cure was to change the impellor and also instert dampening matting under the structure to absorb and resonance.
Types of Resonance
There are various types of Resonance: Structural and Shaft.
Resonance is characterised by:
- Wide Band noise floor (Resonance is a range not a peak)
- Radial vibration @ the machines natural frequency.
- high axial as well as radial amplitudes
- often higher in the least support direction due to stiffness
- The waveform should be very sinusoidaland modulate
- Phase is the best indicator - Random phase (Angle changes all the time) A machine is nearly impossible to balance if it is resonance at the time of balance