The value of vibration measurement.
Many processes are shut down on routine basis for visual inspection of the machinery, whether or not anything is amiss. Routine analysis of vibration allows maintenance engineers to extend running time between shutdowns.
Vibration and temperature are the most common parameters used for machinery protection. Of these, temperature has been used longer, probably because it was available first and was better understood than vibration analysis.
However, vibration may be the better measurement over temperature since it becomes apparent earlier and the multitude of frequencies and amplitudes that are present in the signal allow for a more detailed analysis.
Sources of vibration
Unbalanced rotating equipment, misalignment of machines, bent shafts, oil whip in journal or sleeve bearings, gearboxes and vehicle movement – amongst others – all have the potential to generate destructive vibrations.
How is vibration defined?
To accurately analyse a vibration problem, it is necessary to describe the vibration in a consistent and reliable manner. The two most important numerical descriptors of machine vibration are amplitude and frequency. Amplitude describes the severity of vibration and frequency describes the rate of oscillation. Together, amplitude and frequency provide a basis for identifying the root cause of vibration.
How is vibration measured?
A vibration sensor or a transducer is a device used to convert the energy of vibration into a voltage or current. It is normally used to measure machinery or structural vibrations. The most common vibration sensors include velocity, accelerometers and proximity probes. Each type has its own distinct advantages.
Velocity transducers
These units monitor vibrations in rotating machinery, they are economical, easy to install and have frequency range from 0,5 Hz to 10 kHz.
Accelerometers
Most commonly used in the industry, light, compact and rugged while also possessing a wide frequency range from 2 Hz to 60 kHz. They utilise the piezoelectric principle to convert mechanical motion into a voltage proportional to the acceleration caused by vibration.
Proximity probes (Eddy current transducers)
These are preferred for monitoring vibrations on rotating machinery equipped with journal bearings. Eddy current transducers provide measurements of shaft displacement or measurement of shaft relative to the bearing.
Why measure vibration?
Changes in the ambient levels of machine vibration provide early warning of costly equipment failure. Analyses of vibration patterns enable machines to be shut down before catastrophic failure occurs and assist in the diagnosis of the root cause; spare parts and maintenance activities can thus be scheduled in advance and costly downtime kept to a minimum.
For more information contact Steve Edwards, R&C Instrumentation, 0861 11 4217, [email protected], www.randci.co.za
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