VIBRATION ANALYSIS TECHNOLOGIES:

VIBRATION ANALYSIS TECHNOLOGIES:

Amplitude Monitoring: We all know that all machines vibrate. But the question is what the acceptable limit of this vibration is? Basically it all depends on the machine type & its design. OEM gives a different vibration limit of alarm & trip for different machines. The different vibration technologies that can be employed for root cause analysis depends on the vibration transducer involved in picking up the data.

With contact type transducers following analysis can be performed-

1.    Amplitude monitoring & analysis of asset deterioration over a period.

2.    Frequency (FFT technology) - Raw time domain signal is broken into frequency domain signal by FFT technique (Fast Fourier Transform). As different frequencies are associated with a peculiar machine characteristic, by comparing the amplitudes of a good machine spectra & test machine spectra, it is possible to pinpoint troubles very accurately. For example 1X (one times the running frequency) frequency is responsible for unbalance, 2X for misalignment etc.

With proximity (non-contact) type transducers following analysis can be performed-

1.    Amplitude monitoring & analysis.

2.    Frequency (FFT technology)

3.    Phase: Phase is the position of a rotating part at any instant with respect to a fixed point. Phase tells about the vibration direction. Phase becomes very useful when the source of the vibration is not clear. Various useful insights regarding problems like Machine Soft Foot, Cocked Bearings and Bent Shafts, Imbalance confirmation, Looseness, Bending or Twisting, Shaft misalignment etc. can be identified by phase analysis.

4.    Form Analysis (Orbit Analysis): X-Y plane time domain signal plot: For performing orbit analysis, the input signals from the two proximity sensor placed at 90⁰ with each other on the bearing & a key phase (tacho) probe is viewed by the software & the resulting lissajous pattern is analysed. Orbit plot gives visual graph of actual shaft central line movement in bearing housing. With accelerometers & velometers also orbits can be achieved and as the sensors are mounted outside the casing of the machine, these orbits are called case orbits and provide absolute shaft motion with respect to space. But the orbits taken from proximity sensors are more common & useful. The shape of the orbit tells about the nature of the machine fault.

5.    Position: Position monitoring is done with the help of non-contact type of probes & various parameters like shaft eccentricity, Axial shift, housing & rotor (shaft) expansion & valve position can be monitored (A part of TG TSI system). With the help of X-Y probes the average shaft centreline can be plotted & rotor lift can be seen. These parameters prove valuable information regarding present condition of the machine.

6.    Bode Plot: Bode plot contains two graphs- Amplitude vs machine speed & Phase vs machine speed. It gives valuable information regarding amount of run out associated with a proximity probe, balance condition & system damping.

7.    Polar Plots: In polar plots also same variables are used as in bode plot.  They also serve the same purpose, only the methid of representation is different.