Problem: In a number of real-world applications the speed of a machine cannot be held to a fixed RPM while vibration data is collected.
Solution: The order tracking capability of the VIBXPERT® allows accurate data collection on machines that experience constant speed changes during data acquisition.Applications such as a winder that spools paper coming off the end of a paper machine, by its design continuously decrease in RPM as the diameter of the paper accumulated on the winder increases. Other applications such as a pump that feeds a production process may be required to continuously change speed as the process demand dynamically changes. A spectrum collected during an RPM change will result in smeared and skewed data. During the spectrum measurement process a digitized time waveform of the vibration is collected. The time waveform is collected for enough time to gather repeated cycles of vibration. The FFT process converts the waveform into a spectrum that displays vibration amplitude versus frequency. If the speed of the machine changes during the waveform collection process the peaks generated in the waveform will not be evenly spaced. The resulting spectrum will have vibration peaks widened or smeared and frequency information from low frequency to high frequency that is frequency shifted or skewed. The resulting spectrum data taken from a machine whose speed is changing is most often unusable. In this changing speed scenario if frequency peaks are displayed in the spectrum both the frequency and amplitude will be inaccurate at best. Order Tracking Spectrums allows one to successfully collect spectrum data from a machine that is continuously changing speed. The resulting spectrum amplitudes will be accurate and the frequencies will be displayed in orders of running speed. The collection process requires a tachometer to track the machine’s speed as the time waveform data is acquired. As the equipment speed changes during the measurement process, the start of the actual measurement has to be synchronized from average to average. The number of averages is user selectable. Otherwise, the 1× on one average will not be the 1× on the next average, etc., for the measurement. This results in the same peak showing up in multiple places or being merged with other peaks. This can create analysis havoc. Order tracking results in an accurate spectrum with no data skewing or smearing.
Setting Up Order Tracking Spectrums
There are 5 preset Order Tracking Spectrum setups in OMNITREND®. To view these setups in OMNITREND simply select the menu item <Tools> then <SetupManager> from the drop-down menu. Measurement setups 260 through 264 are examples of Order Tracking Spectrums. The preset Order Tracking Spectrums are available in 5 different units of measure. Like all preset measurements in OMNITREND, these are fixed and cannot be changed. If the preset measurements are not suitable for your application, they may be modified per the following instructions:1) From the <Setup Manager> window, highlight and right mouse click on the Order Tracking Spectrum you are interested in modifying.
2) Click on <Duplicate Setup>
3) An identical measurement will be created and show up at the bottom of the setup manager list.
4) The spectrum parameters of this newly created spectrum may now be customized to better fit your needs. Once you have finalized your Order Tracking Spectrum setups we simply add them to a Measurement Location on a Machine in the equipment database or in the Template Editor. The Machine can then be downloaded to your VIBXPERT data collector as part of a route or as a machine template. At the present time, Order Tracking Spectrums are not available via the Multi-Mode area of the VIBXPERT.
Collecting Order Tracking Spectrums
When collecting Order Tracking Spectrums the appropriate accelerometer and tachometer are required per the measurement setup in OMNITREND. The tachometer must be aimed at a speed reference such as reflective tape mounted on the turning shaft. When the machine is running, the RPM may change while you are taking the measurement.
To the left is a spectrum taken as the speed constantly changed between 1775 RPM and 910 RPM. Notice the waveform time units are recorded as the number of rotations. The spectrum frequency units are orders of running speed. The lower half of the display can be changed to list the maximum 10 amplitudes in the spectrum. Toggle to the lower screen by pressing the F key and then select the peak you would like to see. In the spectrum, the cursor will jump to the corresponding frequency. In OMNITREND the data is equally simple to view and analyze.
Vibration Analysis by Ana Maria Delgado, CRL