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Making Sense Of Vibrations

jj 17 30Making Sense Of Vibrations

Vibration monitoring is a key technique within condition-based monitoring, identifying problems quickly and boosting the efficiency of maintenance operations, says Phil Burge, Country Communications Manager UK at SKF.

The idea of waiting for a machine to break before fixing it – so-called ‘run to failure’ maintenance – is fast disappearing from the manufacturing industry. (Read More) It has largely been replaced by preventative maintenance, where machines are serviced at regular intervals in order to stave off failure. While this is a vast improvement, there are still downsides to this approach: perfectly good machines end up being serviced for no reason – which can actually introduce problems; and, machines are still liable to fail between these scheduled overhauls.

A more sophisticated approach is predictive maintenance (or ‘condition-based maintenance’), which makes constant assessments of the machinery. By predicting imminent failure, it can recommend maintenance only when it is needed. This has multiple advantages: it catches problems before they happen; it only replaces faulty components; and it cuts the chance of unplanned downtime to almost zero.

A number of analytical techniques lend themselves to condition-based maintenance. Analysis of machine oil, for instance, can predict if wear has begun to affect moving parts; measuring motor current identifies problems such as rotor bar failure or incorrect motor windings; and infrared thermography detects ‘hotspots’ that indicate possible defects.

Good vibrations

The most widely used technique in condition-based maintenance is vibration analysis. Put very simply, this ‘listens’ to the vibration from rotating machinery and compares it with a ‘standard’ signature.

A vibration sensor, when attached to a component such as a pump or fan, can pick up four main problems in rotating machinery: imbalance; misalignment (such as in the shaft); looseness; and bearings faults.

Bearings in good condition make a ‘purring’ noise; when they are in poor condition, they grind and squeak – and this can be picked up using vibration analysis. Many bearing problems are caused by excessive vibration, so analysing this aspect of their behaviour can help to pinpoint problems correctly, and early. For instance, many types of surface damage – such as dented or etched raceways – can be traced back to excessive vibration.

Not only can vibration monitoring pinpoint a damaged bearing – allowing it to be replaced – but it can indicate the underlying nature of the problem, whether it be under-lubrication, improper installation or material fatigue.

Every mechanical problem generates a unique vibration frequency, so analysing this can help identify the root cause. Low frequency vibrations are generally caused by structural resonance, misalignment or mechanical looseness, while high frequencies include those generated by bearing defects. So, measuring the amplitude in terms of acceleration can give an early indication of developing bearing problems.

Each bearing component has a unique defect frequency, which enables a specialist to pinpoint damage. A number of ‘defect frequencies’ can be calculated, such as the ball/roller pass frequency – for either inner or outer ring raceway – as well as ball/roller spin frequency, and cage (or ‘fundamental train’) frequency. Identifying these frequencies helps to pinpoint specific damage.

Growing demand

As more maintenance regimes switch from preventive to predictive, the use of vibration monitoring will continue to increase. A recent report from Markets & Markets estimates that global demand for vibration monitoring systems will rise by around 50%, to reach nearly $1.5 billion by 2020. This equates to a compound annual growth rate (CAGR) of more than 6.5%.

“Vibration monitoring can detect faults and machine deterioration before the occurrence of other symptoms like heat, sound, greater electrical consumption, and lubricant impurities,” says the report. “Therefore, it is an integral part of the machine condition monitoring programme and remains the most preferred condition monitoring tool.”

The relatively low cost of vibration sensors will drive this growth, as they can now be fitted to ‘lower value’ assets, rather than only high value assets. However, this generates much more information – so the challenge is to manage this extra data.

While signals can be recorded ‘offline’ – using hand-held devices – it is becoming increasingly common to gather data automatically (or ‘online’), then transfer it to a control centre with real time monitoring.

The most sophisticated systems will transmit the data wirelessly, so that it can be analysed remotely. An example of where this is beginning to happen is in offshore applications such as wind turbines: vibration sensors are installed on the machinery, and the data is collected and transmitted to remote service centres – where it is analysed, and can be used to pinpoint problems instantly.

However, the greatest benefit to the manufacturing sector is that smaller, cheaper sensors bring this type of monitoring to lower-value assets – while more sophisticated data analysis allows maintenance staff to make sense of this extra data, and act on it quickly in order to avoid unplanned outages.

SKF (U.K.) Limited
Tel + 44 (0)1582 490049
www.skf.co.uk