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Uncovering those hidden vibrations

08 September 2015

Chris Hansford explains how a technique called 'acceleration enveloping' can be used for the early detection of faults in rotating equipment.

Vibration monitoring is a key tool for detecting wear, imbalance and misalignment in many different types of machines with rotating parts, from sleeves and shafts to bearings and power transmission.

In many instances, such as gearboxes with gear meshing problems, rolling bearings and other machines with repetitive metal-to-metal contact, the vibration signature caused by a small defect can be masked by harmonics emanating from the surrounding components and machine structure.

Even in a relatively simple system with a small number of shafts and bearings there can be a confused mixture of mesh frequencies, harmonics and bearing defect frequencies; for example, a single bearing can produce four defect frequencies from its outer and inner ball races, cage and elements.

One of the challenges of early fault detection is that minor defects often produce signals at high frequencies but low velocities, or at a much lower amplitude than that of other vibrating components.  Typically, this might be a bearing with a displaced element where, during each rotation, the change in speed is briefly and abruptly changed but where the overall velocity signal is unaffected.

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 surface, 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 resonance response of the machine at the impact frequencies.  

This ringing signal is, however, extremely localised and requires a high performance accelerometer, matched to the anticipated range of machine and fault resonance frequencies for it to be effectively detected.

Given that the high frequency nature of a defect signal will not travel far from its source and will quickly be attenuated by surrounding media or structures such as gaskets, oils and joints, it is important that the accelerometer is mounted in close proximity to the component being monitored – a solid mounting is also critical to eliminate the risk of errors being introduced.

The output from the accelerometer will generally contain three frequencies, including the low frequency, high amplitude shaft rotation, the structural resonance frequency and a combination of high frequency elements.  Using a technique known as enveloping acceleration, this complex signal can be processed by a data collector, connected to the accelerometer, to isolate the actual defect signal. 

The acceleration enveloping processes normally uses band-pass, or high-pass, filters to remove unwanted parts of the composite vibration signal, with the remaining signal components around the resonant frequency being conditioned and then fed through a Fast Fourier Transform algorithm to be presented as an enveloped acceleration spectrum.

This is a far more sensitive process than that used for the raw acceleration signal and allows the repetition frequencies of the defect to be clearly shown on the data collector display, together with associated harmonics.

It should be noted that a degree of experimentation can be required with different filter settings until the required results are achieved. Acceleration enveloping can provide an important early indication of defects, enabling the rate of wear to be monitored over time and for maintenance work to be planned in advance to eliminate the risk of sudden failure and unscheduled downtime.  

The system is not, however, without its potential pitfalls.  Correct accelerometer mounting is critical; so too is an understanding of the errors that can be introduced by, for example, inadequate machine lubrication or contaminated lubricants, both of which can adversely affect the nature of any defect signal.

A further consideration is consistent machine speeds at the time at which measurements are taken, as changes in shaft rotation speeds will cause different acceleration enveloping readings. Also, bear in mind that as a defect develops the nature of its frequency response may change, to the point where the amplitude of the acceleration enveloping reading may actually decrease as component failure becomes imminent.

Finally, in common with all condition monitoring techniques, a degree of expertise is essential if data is to be interpreted correctly and the appropriate action taken.

Chris Hansford is managing director of Hansford Sensors

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