Article
Condition Monitoring by Stress Wave Analysis

by David B. Board

ABSTRACT

This is a well written paper on the theory behind and use of the Stress Wave Analysis technique for condition monitoring and defect condition analysis. Stress Wave Analysis is a high frequency acoustic sensing technology that filters out background vibration and audible noise and measures and quantifies shock and friction events in rotating machinery. This paper is intended for beginning and intermediate analysts that have not had very much experience with this technique. The author uses graphs, illustrations, and pictures to ensure an understanding of the subject and provide examples of the technique in use. This is definitely a ‘practical’ paper intended for the analyst that uses the tools of our trade. This paper is descriptive in nature showing the different stages of the processing that is performed for this technique and does not provide mathematical examples of the processing techniques. Several case histories are included to emphasize the capabilities of the tool on specific equipment and fault types. The primary focus of the examples are bearing and gear defect early detection, but additional examples also show this technique identifying cavitation and recirculation in a pump and show this technique identifying different classes of foreign object damage incurred in an aero-derivative combustion turbine. This technique is not as well known or understood as other technologies that are available, this is a very good introductory paper for this technology.

PREVIEW

“Stress Wave Analysis (SWAN):

Stress Wave Analysis (SWAN) is a state-of-the-art instrumentation technique for measuring friction, shock, and dynamic load transfer between moving parts in rotating machinery. SWAN provides an electronic means of detecting and analyzing sounds that travel through a machine structure at ultrasonic frequencies. This structure-borne ultrasound (or stress wave) is caused by friction and shock events between the moving parts of a machine. An externally mounted sensor on the machine’s housing detects stress waves transmitted through the machine’s structure. A piezoelectric crystal in the sensor converts the stress wave amplitude into an electrical signal, which is then amplified and filtered by a high frequency band pass filter in the analog signal conditioner, to remove unwanted low frequency sound and vibration energy (Figure 1).

The output of the signal conditioner is a Stress Wave Pulse Train (SWPT) that represents a time history of individual shock and friction events in the machine. The digital processor then analyzes the SWPT to determine the peak level and the total energy content generated by the friction/shock event. The computed Stress Wave Peak Amplitude (SWPA) and Stress Wave Energy (SWE) values are displayed and stored in a database for comparison with and historical trending with “normal” readings. SWAN measures even slight shock and friction events that occur between contact surfaces. The level and pattern of anomalous shock events becomes a diagnostic tool.

The digital analysis of stress waves consists of computing both the amplitude and the energy content of detected stress waves. The amplitude (or peak level) of a stress wave is a function of the intensity of a single friction or shock event. The Stress Wave Energy (SWE) is a computed value (the time domain integral) that considers the amplitude, shape, duration and rates of all friction and shock events that occur during a reference time interval. In a spalled bearing, for example, the peak level of the detected stress waves is primarily a function of the spall depth, while the SWE is a function of spall size, as shown in Figure 2.

The ability to separate stress waves from the much lower frequency range of operating machinery vibration and audible noise makes SWAN an indispensable tool when monitoring operational equipment for damaged gears and bearings. In most cases, during early component fatigue, the energy released between the contact surfaces is too small to excite gearbox or engine structures to levels significantly above background vibration levels, until catastrophic failure or extensive secondary damage occur.”

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