TEST & MEASUREMENT FEATURE Pushing the envelope
ACHIEVING SUCCESSFUL RESULTS While acceleration enveloping in many ways is the ideal option for detecting bearing failure, it does have a number of potential imitations. The first 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. Where an application is deemed to
Bearing frequencies are often hidden within a range of
machine signals and harmonics, making it difficult to detect bearing faults at their early stages of development. Chris Hansford, managing director at Hansford Sensors, explains how acceleration enveloping can help to overcome this
A
variety of proven techniques can be used to detect wear, imbalance and
misalignment of rotating parts within machinery. As an example, vibration analysis uses accelerometers to detect potential problems with industrial equipment caused by incorrectly aligned, loose or unbalanced rotating parts. Such techniques tend to be most effective during the later stages of the wear cycle, when damage has already begun to occur. In the early stages of wear, however, when vibration signals are of low intensity, it can be
failure of, for example, rolling element bearings at the earliest possible stage. The rate of wear can then be monitored and maintenance work planned accordingly. In practice, what tends to occur is that
a defect in a rolling element causes repeated impact events that generate resonant frequencies in the surrounding machine surfaces, causing it to ring. Although the amplitude of the ringing signal decays between impacts, and becomes part of the overall vibration signal of the machine, it will nonetheless affect the natural
“Instead of waiting for wear rates to progress to a later stage,
maintenance and production engineers can take advantage of a signal processing technique called acceleration enveloping”
difficult to separate the wear signature from underlying and background machine frequencies. Instead of waiting for wear rates to progress to a later stage – when machinery performance is likely to be declining and the potential for unscheduled machine downtime will be increasing – maintenance and production engineers can take advantage of a signal processing technique called ‘acceleration enveloping’. This enables engineers to overcome
the limitations of conventional velocity spectrum measurements and detect the
resonance response of the machine at the impact frequencies. Using a high performance accelerometer, acceleration enveloping works by progressively filtering out unwanted parts of the vibration spectrum, until the signal of the bearing defect can be isolated from the noise around it and clearly be identified. This information can then be easily collected from the accelerometer using a data collector, ready for review by a specialist who is able to make an informed decision on whether or not maintenance work is required immediately.
be suitable, however, 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. Once installed and calibrated, data readings should then be taken at regular intervals to allow accurate trend analyses to be produced. However, while the potential benefits of
acceleration enveloping are clear to see, it would be unwise to rely on this technique alone. Implementing it as part of a wider monitoring and analysis programme can be a far more effective strategy.
Hansford Sensors
www.hansfordsensors.com
Vibration analysis uses accelerometers to detect potential problems with industrial equipment caused by incorrectly aligned, loose or unbalanced rotating parts
INSTRUMENTATION | MARCH 2016 27
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