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Good vibrations

02 April 2012

Vibration monitoring is a modern maintenance aid that has been adapted to meet a variety of industrial needs. Phil Burge reviews the techniques and describes some equipment innovations that are easing the burden of the maintenance engineer.

Vibration of rotating equipment is a common problem, sometimes resulting from misalignment due to poor installation, sometimes the consequence of natural wear and tear. But sources of wear can now be identified at an early stage and with considerable accuracy by the simple expedient of vibration monitoring, preventing what may at first appear to be a minor problem from developing into a big one!

One of the most challenging environments for vibration monitoring – more generally termed ‘condition monitoring’ - is the wind turbine, where the nature of the machinery, the location and exposure to adverse weather all present their own challenges. The key to wind turbine condition monitoring is to do it remotely. The latest systems provide interconnectivity with a variety of maintenance based equipment, including centralised lubrication and gearbox oil condition systems. Vibration sensors – essentially accelerometers - can be mounted strategically to monitor turbine main shaft bearings, drive train gearbox, and generator. The data available from these sensors is available in real time as well as being recorded for purposes such as root cause failure analysis.

Vibration monitoring for cranes, on the other hand, presents a whole different set of challenges. One, in particular, is posed by the fact that cranes move extremely slowly, the slewing bearings rarely even completing a full revolution.
Historically, accurate condition monitoring has been hard to achieve for cranes because it is so difficult to interpret data from slow-moving systems. While a single accelerometer would suffice to measure the frequency range of most fast rotating machinery, in slower-moving applications, both the frequency and amplitude of vibration is at a much lower level, making it difficult to distinguish between a fault-generated signal and background noise.

This problem has recently been addressed by a new approach, in which a series of sensors are placed around the bearing, each taking a separate reading for a series of repeating part-revolutions made by the crane operator during a test cycle. By looking at the peak differences and their repetitive nature, the so-called Enveloped Acceleration (gE) technique can discriminate fault signals from the background noise. With data such as this to hand, it may be possible to extend scheduled maintenance intervals - an intensive process for cranes that can render equipment out of action for weeks.

Field instruments
The easy-to-use, ergonomically designed ranges of portable data collectors and analysers now available to engineers have done much to simplify maintenance. Portable data collectors and analysers make bearing condition assessments based on a wide range of signal sources, including handheld and magnetically-mounted accelerometers and permanently mounted vibration sensors.

Compact portable devices are able to measure vibration signals and automatically compare them with pre-programmed ISO guidelines. If measurements exceed these guidelines, the unit displays an alert or danger alarm to indicate potential bearing damage.

When used as part of a planned predictive maintenance programme, these modern instruments can have a significant impact on maintenance planning, plant performance and, ultimately, plant profitability.

Phil Burge is with SKF

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