A vibration signal is first measured by an accelerometer, then filtered by band pass filters. After passing through a peak-to-peak detector, overall and spectrum results are extracted. The spike energy spectrum shows the defect frequency and its harmonics.
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Nothing runs as smooth as a new machine. After time, as components wear, the hum of productivity becomes the whine of a tired collection of loose fitting parts beating out of unison. This condition is often monitored using vibration analysis of one form or other. Unfortunately, conventional techniques may not spot machine problems until it’s too late.

There is a way, however, to use filtered high-frequency analysis for catching problems that previously went unnoticed. Since its introduction, the method (referred to here as spike energy) has been used in a variety of rotating equipment to flag machine faults before they actually occur. The method relies on a signal filtering and detection process that captures the most minute influences of a defect, while greatly amplifying and exposing its fundamental frequency and multiples.

What’s measured?

As bearings, gear teeth, and other machine components wear, they develop microscopic cracks and spalls, which in turn cause bumpier operation. The mechanical knocking produces short pulses, or spikes, of vibratory energy that excite component natural frequencies. (Side note: pump cavitation, turbulence in liquids, and control-valve noise have a similar effect.) The impacts of microscopic cracks and spalls also excite the natural frequencies of spike energy accelerometers gathering vibration signatures from around the system; acting as carrier frequencies, they lead the machine defect frequencies that flutter with them. Impact energies (labeled in acceleration units gSE of spike energy) are registered by the accelerometers as functions of spike amplitude and repetition rate, and are sent on for further analysis.

It starts with the setup

Conventional vibration parameters (displacement, velocity, acceleration) typically fall within the linear frequency response range of most transducers, and are therefore fairly easy to measure. But spike energy detects frequencies beyond the linear range of most industrial transducers. Because mounting methods affect higher frequencies, spike energy results vary with different setups.

Impact-induced resonant frequencies of industrial accelerometers typically range from 10 to 50 kHz, varying greatly with construction and mounting. If two accelerometers had the same frequency response characteristics, it would be a coincidence; thus, spike energy readings made with different accelerometers shouldn’t be compared. Because of spike energy’s great sensitivity to setup, the most meaningful way to use spike energy for machinery condition monitoring applications is to observe trends in the returned signal. For consistency, the same accelerometer, mounting method, and measurement location should be used throughout any data collection.

Permanent mounting

Mounting methods change high frequency results, and some cut signals out entirely. If an accelerometer is mounted insecurely, a mounted resonant frequency is introduced. It is always lower than the accelerometer’s inherent resonance, editing frequencies above it. When it is much lower, the usable frequency range becomes much smaller.

The less securely attached a sensor is to machinery, the lower its maximum detected frequency.
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The best method for collecting spike energy data is with stud mounting because there is only one interface: accelerometer-tomachine. This allows greater transmission of high-frequency signals, and returns the most consistent results. Some tips:

• Any threaded holes should be perpendicular to mounting surfaces to prevent “working out.”

• Stud length should be shorter than hole length to allow direct contact between accelerometers and mounting surfaces.

• Cable connectors should be sufficiently tightened to the ac-dicelerometer to prevent rattling and erroneous readings.

• If the stud is mounted to a moving component, the extension cable should be as well; this minimizes cable wiggling during measurement.

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