tion monitoring. Oil condition also has advantages in low-speed machinery (< 5 rpm) where the amplitude of the vibration signal may not be strong enough to overcome the noise factor. Monitoring the oil will give an early warning of component breakdown, but gearboxes generally have multiple bearings so will not help pinpoint where the exact problem is, which additional monitoring by vibration will assist with. If you already have vibration monitoring, would adding oil condition monitoring improve the performance of your preventative maintenance programme and provide a sufficient return on investment? Two is better than one In an article from Practicing Oil Analysis magazine, the example of the Palo Verde Nuclear Generating Station is given. They combined vibration and oil condition monitoring into a common group and worked together. In an assessment of bearing defects detected by the technologies, they found that oil condition was responsible for 40% of the defects found, vibration was responsible for 33% and both techniques converged on the remaining 27% of the defects found. The loss of either technology would have reduced their detection performance and their ability to control the root causes of machine failure. Gearboxes, for example For an example of this complimentary approach, let’s look at gearboxes. Pretty much every plant in the world uses gearboxes which is tough on most lubricants because of heat, aeration and chemically active particles. Likewise, misalignment, imbalance and looseness are also a risk. Occasionally, gears go wrong, with gear tooth wear and gear tooth fracture the most common failure modes. Using vibration analysis to detect wear related gear faults can be challenging due to the number of competing vibration signals. Oil condition, on the other hand, can provide excellent results in detecting contact fatigue, abrasion and adhesive wear, making it an ideal choice for early detection of these modes of failure. It is unlikely breakage of a gear tooth will produce a detectable quantity of wear particles, especially if the failure is caused by a sudden impact or defective material. By monitoring the gear mesh frequency using vibration analysis will pinpoint a broken gear tooth virtually every time. Together, vibration and oil condition come together for monitoring the essential health of gearboxes. What have we learnt? Generally, we can come to the following conclusions about combining oil condition and vibration analysis in detecting and analysing machine faults; Both methodologies are required to understand and manage the root causes of machine failure. In many cases one technique will serve as the primary indicator with the other as the secondary confirmation. Oil condition is generally stronger in failure detection for gearboxes, hydraulic systems and reciprocating equipment. Vibration analysis is generally better in detecting high-speed bearing failures and localising the point of failure, depending on the application. For determining which wear mechanism is inducing failure, oil condition is often stronger. Correlation between oil condition and vibration analysis is very good, but there can be contradictory instances. By combining oil condition and vibration monitoring a number of benefits compared to conventional condition monitoring practices can be realised. These include accurate, efficient and early fault detection as well as the ability to perform root cause analysis. This combination of the two measurement techniques covers the ‘Big Five’ causes of machine failure. After all, there is little point in implementing a preventative maintenance strategy with only one technology which does not, on its own, prevent all of the most common causes of failure. An additional function to preventative maintenance, other than simply preventing a breakdown, is to understand the root cause of a failure. The combination of a primary and secondary means of detection provides more data that will enable a deeper understanding of a failure, thereby guiding changes in maintenance, operation or design that will permanently eliminate the cause of a breakdown. With the arrival of real-time measurement sensors for oil condition monitoring this combination has become more practical and affordable than the sampling and laboratory technique. The new generation of sensors compliment the vibration sensors in terms of continuous measurement with outputs that can be accessed locally or combined onto an asset management system dashboard. The commercial availability of more powerful magnets has also increased the capabilities of ferrous debris sensors. It is now possible to collect larger failure particles as well as wear debris. Capturing the debris allows inspection of the particles, aiding problem diagnosis. MOTORS, DRIVERS & TRANSMISSIONS 26 June 2017
EN-June2017-eMag2
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