New portable and online-monitoring systems help extend the value of vibration monitoring into the heaviest of industrial operations. Here’s a look at how users avoided serious motor failure in mining and detected a critical bearing failure in paper-pulp production by using the right vibration products at the right moment.
Case Study #1:
A Phosphate mine is garnering big returns by monitoring numerous pieces of processing equipment with online solutions from LUDECA. The mills use several low-cost VIBNODE® online systems. The VIBNODE is a comprehensive entry-level online monitoring system that allows the end-user to access customized spectrums and time waveforms from a remote location. The system will notify the end-user via email or text message when the vibration level exceeds an alarm band.
The new monitoring system has helped the mine’s vibration group catch several problems with a newly rebuilt drive motor. The waveform showed a fuzzy amplitude modulation that increased and decreased with every RPM.
A look at the acceleration spectrum indicated a large amount of high-frequency noise well over 1g. Upon inspection, several internal retaining bolts were found to be backing out and contacting the frame of the motor rotor. The bolts were tightened to torque specifications, which was believed to have solved the problem. A week later, however, the problem reemerged as the bolts had once again backed out and began to rub. The bolts were again torqued to specification, but this time with an application of thread locker, which held the bolts in place.
Had this problem not been identified by the fuzzy waveform and a high-frequency acceleration band alarm from the VIBNODE system, the errant bolts would have quickly eaten into the motor rotor and caused a catastrophic motor failure. The motor rebuild or replacement is valued at well over $100,000. And losses to production would have been many more times this amount.
Case Study #2:
Low-speed equipment turning below 40 RPM is often difficult to analyze because of the low energy it produces. If there is not much energy, there is often not much to see. For this reason, the dynamic range of a vibration analyzer/data collector and its signal-processing quality become critical for low-speed machine analysis. Recently, a pulp mill using a VIBXPERT analyzer from Ludeca took a spectrum and time waveform on a slow-speed 35 RPM roll. The VIBXPERT is a portable, lightweight, 2-channel, FFT data collector/vibration analyzer for monitoring and diagnosis of machine conditions. As a data collector, VIBXPERT records all forms of machine vibrations, bearing conditions, process data, and visual-inspection information.
Because of the dynamic range of the VIBXPERT, the mill’s maintenance technicians were able to see a repeating pattern in the time waveform. The recurring pattern was present for every RPM and would increase slightly, then disappear for about three-quarters of the roll’s revolution. A delta cursor was placed on the repeating pattern and the source frequency was 420 CPM. This worked out to be the frequency of the inner race. A 25,000-line spectrum was also collected with a bandwidth of 7.5 CPM per line of resolution. The amplitude was very low below 0.0008 inches per second, yet a definite harmonic pattern appeared. The main harmonic pattern was at 420 CPM, with each peak having another underlying pattern of 35 CPM sidebands. The frequencies represented the inner race and roll RPM. Had this data been taken using a conventional spectrum with a resolution of 6400 lines or even 12,800 lines, this low-frequency/low-energy event would have looked like a spectrum ski slope and been ignored.
Upon removal of the bearing, a crack in the inner race was plainly visible. There was evidence the inner race had begun to walk around the shaft. If the bearing had been left in service it would have damaged the shaft and required that both the shaft and bearing be replaced. Instead, only a minimal two-hour shutdown was required to replace the bearing. Thanks to early detection, the cost of replacing a roll was averted, as well as saving the eight or more hours of lost production that a roll replacement would have required.