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maintenance teams and engineers will first learn of damage and failure when overall vibration increases, lubricants are contaminated and temperatures rise. By the time this occurs, the remaining usable life of the failing machine elements could be short and the damage might be more extensive than if any faults had been detected earlier. In worst case scenarios, bearings may fail and the machine could break down before operators notice or resolve problems. The first step is to apply a band pass filter to the mix of low and high frequencies of a defective bearing’s unfiltered waveform. This isolates only the frequencies in which the signal of interest is hiding. The filtered output will identify repeating, high frequency signals. The second step in the process is to pass the filtered output through an enveloper, which rectifies (or demodulates) the waveform, by inverting the negative part to positive, and extracts the repetition rate of the energy bursts. This ‘envelope’ is now used as a true vibration signal – helping it to stand out from the noise. The envelope helps to contain regularly spaced signals, such as a single defect on a raceway, but other causes of noise, such as shaft rub, are random – so will not produce evenly spaced peaks. Some experience is required to ensure the steps in this process are completed correctly, especially when selecting the correct high- and low-pass frequencies in the first stage of the process.

ACCELERATION ENVELOPING IN ACTION Acceleration enveloping is most commonly used in roller bearing systems but can also be applied in areas such as electric motors and gearboxes. It has repeatedly been proven to be a key factor in the success of condition-based maintenance (CBM) programmes. Once the signal has been filtered, the information can be collected from the accelerometer ready for review and interpretation – for example, whether or not maintenance work is required immediately, if it can be planned as part of routine schedules, or if no action is required for the time being. Despite acceleration enveloping appearing to be the definitive answer to detecting bearing failure, it is important to make clear that there are certain times when it might not be the best option. This is because it cannot be universally applied to any machine. The technique detects faults involving repetitive, metal- to-metal interactions. Anything that masks this, such as gaskets or dampers, may put a machine outside its scope of use.

USE IN WIND TURBINES The average wind turbine has around 8,000 separate components. Of these, a large number are associated with the drivetrain, which has separately been recognised as the major cause of extended downtime. Wear in gearboxes and bearings in particular is known to cause problems and can lead to expensive repairs. Regular vibration

monitoring can prevent these issues occurring. The complexity of a typical wind turbine does, however, present a challenge for vibration monitoring. Components such as the main turbine, gearbox and generator produce unique vibration signatures, with different amplitudes and frequencies, which can be difficult to isolate from each other and can be masked by noise from surrounding systems. This is where acceleration enveloping plays a crucial role. To be effective, accelerometers should be fitted to all key rotating parts in a turbine. These include the main bearings, planetary, intermediate and high speed gear stages, the generator and ideally the nacelle traverse and axial movements. Accelerometers should be selected depending on the frequency of enveloping signals. For example, on wind turbine drivetrains, specifically the generator output shaft, rotational speeds can be relatively slow and may therefore require the use of special purpose low frequency AC accelerometers, with a sensitivity of between 100mV/g and 500mV/g.

Each accelerometer must be mounted White Paper

Acceleration enveloping to detect bearing damage

A guide to using envelope signal processing to pinpoint bearing failure at an early stage.

In any manufacturing or processing plant, machine breakdown has significant consequences. Productivity and profitability are usually affected

❱❱ Hansford Sensors has published a new white paper on the envelope signal processing technique 6 /// DAQ, Sensors & Instrumentation 2017

securely on a clean and solid base, and as close to the component being monitored as possible; normally, standard M8 mountings are used. It is also important to collect data regularly and consistently, to enable any change in operating conditions or trends over time to be accurately identified at the earliest possible stage. This can be done on-site using hand held data collectors, which feature software capable of automatically calculating acceleration enveloping, or are transmitted to a remote monitoring centre for subsequent analysis. Secondly, accelerometers should then be correctly mounted – close to the component being monitored, on a flat, clean surface to guarantee consistent results. Poor mounting reduces reliability and can make collected data redundant – which means a machine might break down without warning even though it seems healthy and in fine working order. Finally, once accelerometers have been installed and calibrated, data readings should then be taken at regular intervals over a period of time to allow accurate trend analyses to be produced. This allows a steadily deteriorating condition to be identified and for action to be taken before any bearing damage worsens. EE

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