nal from a machine is derived from many components and vibration monitoring requires a thorough understanding of the frequency response of all the components. This requires a skilled operator to be able to determine what ‘normal’ vibrations are, what indicates a change in condition and what level the abnormal vibrations need to achieve to signal an alert. Some systems can be programmed to do this ‘filtering’ work, but still requires a trained operator to set the system up. The other factor to keep in mind is that it normally requires more than one sensor on each piece of equipment. Although each application will vary, on a typical motor/gearbox assembly, for example, could typically require four sensors – two on the motor at the drive and non-drive ends and on the gearbox input and output shafts. They may also be different measurement types such as radial and axial. Although vibration sensors can be low cost, the potential need for multiple devices needs to be considered when evaluating the cost/benefit ratio compared to other monitoring technologies. Oil condition monitoring is also a popular technique although until recently, it too, required the use of skilled personnel. An oil sample is drawn off from the machine and sent to a laboratory where a number of parameters would be measured and a report sent back to the machinery operator. More recently, analysis kits have been introduced that allow the oil to be analysed on-site, but they can have reduced functionality or less sophisticated measurement equipment. Although a full laboratory analysis will provide an extensive view of the oil condition and machinery health, it does also have some drawbacks. It is relatively expensive, the oil needs to be drawn off the machine in a repeatable condition and not get contaminated outside the machine. There is also the delay between sampling and the time taken to receive results. It can be 2-3 days which, if you suspect a fault, may be too late before failure occurs. As with vibration monitoring, the results require analysis by trained staff to interpret and understand them correctly. The cost of analysis increases with the number of parameters measured plus it does require access to the machine and a location where the oil can be drawn off before it has passed through filtration. For health & safety reasons it may require the equipment to be halted whilst the oil sample is obtained. To retain the benefits of oil condition monitoring but to mitigate the drawbacks, in-line sensors have now been developed. Generally they measure a single parameter but provide continual, real-time analysis and range from particle counters to sensors which measure the viscosity or acidity of the oil, an indicator of the state of the oil itself. They can report their data locally or transmit into an asset management system. The perfect match? Oil condition and vibration have reached the levels of usage they have because they can provide a lot of information about machinery health. However, independently they are unable to provide coverage of the ‘Big Five’. If an electric motor starts to have a problem with a shaft bearing, it is only vibration monitoring that is going to provide you with the necessary indication of a problem. However, if a bearing is starting to fail in a gearbox, it will deposit debris in the oil before it produces a measureable increase in vibration levels. The principal of preventative maintenance is to provide the earliest possible indication of a problem developing allowing maintenance to be carried out in the most timely, cost effective way. In some studies it has been estimated that oil condition monitoring can provide up to 10 times earlier warning than vibra- 5There are primary root causes of machine failure: balance, alignment, lubricant quality, looseness and contamination. www.engineeringnews.co.nz 25
EN-June2017-eMag2
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