Experienced operators can often tell if a machine is not working correctly on the basis that is doesn’t sound right. The same principle can be applied – using modern electronics – to identify the exact cause of the problem.
Sensitive accelerometers can detect and analyze the vibrations from industrial equipment, highlighting problems such as misalignment or bearing imbalance. The technique is known as vibration analysis. It can identify bearing failure in the very early stages, when there is a microscopic defect in the raceway, for example. The problem is that the identifying signal is usually drowned out in other noise emanating from the machine.
Filtering signals with acceleration enveloping
It is vital to catch defects as early as possible, to stop them developing into more serious problems. One way of homing in on the signal of interest, and filtering out the noise, is to use a signal processing technique called acceleration enveloping. It works by progressively filtering out unwanted parts of the vibration spectrum until the exact signal of the bearing defect can clearly be identified, thereby separating low-level, repetitive vibrations from the noise around it.
The unfiltered waveform from a defective bearing is a mix of low and high frequencies, with no obvious pattern. The first step is to apply a band pass filter, which isolates only the frequencies in which the signal of interest is concealed. The filtered output will identify repeating, high frequency signals, though more steps are required to pinpoint the one specific to the bearing defect.
First, the waveform is rectified – inverting the negative part to positive – and this is then enveloped (or demodulated) by tracing a line over the general shape of the rectified signal. 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. Other causes of noise, such as shaft rub, are random – so will not produce evenly spaced peaks.
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 is 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 using a data collector, ready for review and interpretation by a specialist who can decide whether or not maintenance work is required immediately or can be planned as part of routine schedules.
While acceleration enveloping may seem to be the definitive answer to detecting bearing failure, 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.
Achieving Success
While acceleration enveloping in many ways is the ideal option for detecting bearing failure, it does have a number of potential imitations that must be taken into account before being implemented. The first consideration for plant engineers is the suitability of each machine because acceleration enveloping isn’t fit for use with any and all machines. The technique detects faults involving repetitive, metal-to-metal interactions, which means that anything that masks this, such as gaskets or dampers, may reduce its effectiveness.
However, where an application is deemed to be suitable, there are several factors that will help to ensure better results. First of all, accelerometers to measure the low level signal should be selected carefully – in the proper frequency range – to suit the needs of the particular machine or application.
Once specified and ready for use, accelerometers should be correctly mounted in close proximity to the component being monitored on a flat, clean surface to guarantee consistent results. Poor mounting reduces the reliability of results and can make collected data redundant, preventing the correct decisions and appropriate actions from being taken. After accelerometers are 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.
While the potential benefits of acceleration enveloping are clear to see, it would be unwise to rely on this technique alone. Instead, implementing it as part of a wider monitoring and analysis program can be a far more effective strategy, enabling plant engineers to better safeguard not only the health of assets but also their performance and productivity.
Case Study
According to the Global Wind Energy Council, there were 268,000 wind turbines in operation at the end of 2014, with an average of 8,000 separate components per turbine. Of these, a large number are associated with the drivetrain, which is considered as the major cause of extended downtime. Wear in gearboxes, and bearings in particular, are known to cause problems. Regular vibration monitoring can prevent these issues occurring, eliminating the need for expensive repairs.
The complexity of a typical wind turbine does, however, present a challenge for vibration monitoring. For example, the main turbine, gearbox and generator often have more than 15 rolling element bearings installed, while the gearbox incorporates a series of stages, each with multiple gears. These components generate unique vibration signatures, with different amplitudes and frequencies, which can be difficult to isolate from each other and that can be masked by noise from surrounding systems.
This is where a technique such as acceleration enveloping can play a crucial role, enabling vibration analysts and maintenance engineers to separate vibration signatures and identify the changes in signal conditions, which can indicate increasing wear. To be effective, acceleration enveloping requires the use of multiple accelerometers, fitted to all key rotating parts. These include the main bearings, planetary, intermediate and high speed gear stages, the generator (inboard and outboard bearings), and ideally the nacelle traverse and axial movements.
In each case, there are several critical factors that must be considered. In particular, each accelerometer must be mounted 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 consistently, to enable any change in operating conditions or trends over time to be accurately identified at the earliest possible stage.
About the Author
Chris Hansford is Managing Director of Hansford Sensors, a leading manufacturer of accelerometers and ancillary equipment. He is a qualified electro-mechanical engineer with over 30 years’ experience in the vibration monitoring industry. To learn more, visit http://www.hansfordsensors.com/about/meet-the-team.