Figures 8a and b: Various type of adverse torsional vibration which can occur during start-up (left) or process upsets (right).


The spectral data will provide the analyst with information on the frequencies present in the waveform. Process conditions and angular clearances such as backlash or wear can produce torsional oscillations within the equipment; however, it is more common to see the system natural frequencies in the in the spectral data. This is the frequency which the shaft(s) oscillate at after an impacting load or when the operational speed coincides with a torsional natural frequency. While there are an infinite number of torsional natural frequencies for a drivetrain, it is likely that only the first and sometimes the second TNF are observed. Higher-order natural frequencies are much less energetic and more difficult to excite. The spectral data can be utilized to verify theoretical calculations of torsional natural frequencies so that changes to operational speeds can be properly evaluated, ensuring that adverse conditions are avoided. Additionally, the empirical data will allow for the mathematical model of a drivetrain to be ‘normalized’, providing confidence in evaluating any changes (motor upgrades, coupling changes, etc.) proposed to the drivetrain to improve reliability without introducing unforeseen problems.


For applications that are susceptible to torsional vibration (either due to resonance at operating speeds or from impacting loads), the integration of torque measurements with radial and axial vibration from eddycurrent probes or case-mounted accelerometers can provide a comprehensive condition monitoring system. First, the torque measurements can be used to trigger predicate-based recording of vibration data. Because light and intermittent loads can pose difficulties in the collection of useful vibration data, torque measurements can help analysts distinguish between data which is representative of a fault and that which is just a characteristic of how the equipment is operated. To illustrate this, data from an online vibration monitoring system which simultaneously records torque and vibration data on a metal rolling application is shown in Figure 9. The figure shows spectral data recorded with a case-mounted accelerometer on the reduction gearbox, which is intermittently loaded. The vibration measurements for lightly-loaded (blue) and fully- loaded (purple) conditions were triggered by a signal from the torque measurement system installed on the output shaft of the gearbox. The difference in the spectral data is obvious, but without knowing the transmitted torque, it would be difficult to determine if the vibration increases are due to load or a fault in the machinery.


Figure 9: Loaded (purple) versus unloaded (blue) vibration spectrum data acquired from case mounted accelerometers. THE REPORT | SEP 2024 | ISSUE 109 | 73


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