Red Wolf Reliability
Vibration analysts often look at a time waveform and assume it is a valid representation of the component being monitored. Analysts may forget the time waveform is a digital representation of the raw analog signal. Due to it being a digital representation, the time waveform is limited by the signal processing parameters the analyst defines. Red Wolf Reliability commonly finds that analysts have incorrectly defined measurement parameters for the type of event they are trying to capture. The two main parameters in setting up a time waveform are total sample period and sampling rate.
When setting measurement parameters we want to make sure that we sample at a high enough rate to capture high frequency events, such as impacts from bearing defects, and that we can capture multiple instances of the fault in question. Red Wolf Reliability recommends a total sample period capable of seeing ten (10) revolutions of the shaft. The maximum frequency you will be able to accurately resolve is defined by the Nyquist theorem, sample rate divided by 2.5. Digital instruments will use the sample rate divided by 2.56 in order to provide a result in a binary number capable of processing quickly.
Depending on your hardware and software, you may be prompted to define the maximum frequency of interest, or the sampling rate. Instruments and software limit collections to a total number of samples for file size considerations. You will want to ensure that your settings give you a long enough time waveform and a high enough sample rate to capture the fault in question. Changing one parameter will automatically adjust the other based on the hardware and/or software limitations.
Applications such as large diameter bearings may require both long time waveforms and high sample rates, which will result in a high total number of samples. Many times faults are missed due to an inadequate sample period or sample rate. Consider the linear speed of the rolling elements in the bearing for defining more accurate collections specifications. Also check your portable instrument for advanced analysis modules that allow you to record long time waveforms at high sample rates to help get the data you need for proper analysis.
Do you have time waveform analysis questions or other vibration troubleshooting questions? Email Red Wolf Reliability or call 970-266-9005.