Focus on Instruments: Listening for Trouble

Portable acoustic emission instrumentation can detect bearing defects down to 0.25 rpm — that’s four minutes per revolution — well beyond the capabilities of conventional analysis systems used in machinery condition monitoring.

Agood definition to introduce acoustic emission and ultrasonics is that it is a monitoring technique which analyzes elastic wave frequencies naturally generated above the human hearing threshold — those in the order of 25 kHz to 1 MHz.

One of the earliest examples of the detection of high-frequency, structure-borne, elastic wave activity was in ancient metalworking. The audible ‘cry’ when tin is deformed has been heard for literally thousands of years. But it was not until the late 1940s that high-frequency sensors and electronic amplification were systematically applied to ‘listening’ to a range of materials under controlled conditions. This research was done by Josef Kaiser, who published his thesis in 1950.

This pioneering work eventually spawned a new non-destructive technology — acoustic emission (AE) and ultrasonics — which today encompasses testing techniques that are well established and highly sophisticated.

There are now a wide range of AE applications to detect defect growth, for example, in bridges, dams, pressure vessels, pipelines, aircraft/spacecraft, and more. AE has also been developed to successfully monitor industrial processes, including machining and fabrication. The monitoring of airborne and fluid-borne ultrasound is also widely used for measuring leaks and the flow of gases and liquids, as well as electrical arcing and discharge.

Some of the people in the conventional vibration field regard the application of AE as something new and still to be proven, yet nothing could be further from the truth. Consider, for example, the chronology of some early work in this field:

1968 — H.L. Balderston, while working at Boeing published a paper on the detection of the incipient failure of bearings.

1974 — D.R. Hartling and J.W. Taylor, working at Boeing in Seattle, patented the concept of “demodulated resonance analysis.”

1975 — D.B. Board at Boeing Vertol published results using demodulated resonance analysis to detect defects in bearings and gearboxes and concluded that this approach “indicates a clear superiority over classical low-frequency vibration analysis techniques that have been employed to date for helicopter transmission diagnostics and prognosis.”

1977 — H.P. Bloch at Exxon published details of an 800-channel, plant-wide system and reported that “High frequency acoustic techniques have shown to be more effective in detecting mechanical failure at a very early stages than the popularly used low-frequency vibration and sound techniques.”

1970s-1980s — Classical vibration monitoring and analysis in the 0-25 kHz range was very successful in the 1970s and 80s in reducing the main causes of machinery failure — i.e. unbalance and misalignment. During this time, classical vibration also embraced the demodulated resonance technique, under a variety of trade names, using accelerometers. However, in Europe, the U.K. aerospace industry produced a quite different technology using sensors developed with a higher signal-to-noise ratio.

1986 — T.J. Holroyd, et al, produced a study, “The Uses of AE at Rolls Royce PLC.”

This new technology has not received much attention in North America. However, this is changing due to the simplicity of use of portable instrumentation and the ability to measure bearing defects down to 0.25 rpm. That is four minutes per revolution and well beyond the capabilities of conventional systems.

Each condition monitoring technique has its own special advantages, and not surprisingly, its distinct disadvantages. These are summarized for the use of AE/ultrasonics in the context of machinery condition monitoring.

Advantages:

Unaffected by typical environmental noise

More sensitive to activity from faults than normal running

Applicable to all machines irrespective of operational speed

Can provide sensitive yet simple indications of fault presence

Provides good trending parameters.

Disadvantages

Requires highly specialized sensors and signal processing

Sensitive to ultrasonic sources, i.e. turbulence, crushing, etc.

Insensitive to minor imbalance, misalignment and bent shafts

Signals are weaker than vibration signals.

Experience over the past decade has shown that AE portable instruments are an excellent addition to any machinery condition monitoring program. They allow untrained personnel to quickly monitor a greater number of machines while allowing the skilled analyst to concentrate on problem machines. For companies with no in-house expertise, employees can quickly determine the condition of their equipment.

This article was provided by Ron Wilson of Machinery Condition Monitoring, Saint John, N.B., and is based on “The Acoustic Emission & Ultrasonics Monitoring Handbook” from Coxmoor Publishing Co. (www.coxmoor.com). Information on acoustic emission products can be found at www.mcmpm.com.