AE06.pdf

Mechanical inspection ultrasonic noise body noise

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Mechanical Inspection Ultrasonic noise (body noise) detected by the contact probe is generated by two rubbing or touching metal surfaces, especially loud when lubricant has been lost by leakage. This fact allows checks of bearings and other mechanical moving parts for tolerance and lubrication. A well running bearing whose lubrication is adequate may produce an ultrasonic noise that is transposed by the detector to a soft, whizzing tone. However, a bad bearing whose lubricant has leaked out, permitting metal-to-metal contact, produces a significantly louder and often scratchy tone. Applications of contact probes also include checking the sharpness of tools and application of cutting fluids on fast running, cutting machine tools, cam plates and eccentric plates, gears and other mechanically moved parts rubbing or striking on a metal surface because of wear or poor lubrication. Other examples are the checking and detection of dry running ball bearings and friction bearings, noisy transmissions that have lost fluid, leaking valves and slide valves with internal leakage through the closed valve. Bearing Analysis Bearings can be analyzed with a structure borne contact probe. Based on research conducted by the National Aeronautics and Space Administration and many years of experience, the first indicator of a bearing going into a failure mode (microscopic degradation of the bearing wear surface because of lack of lubricant) is the rise in ultrasound. This rise in amplitude is heard through the ultrasound detector as a rough or raspy sound. The experienced operator can detect this anomaly immediately. The amplitude can be trended for further investigation. This will allow the bearing to be changed long before it fails completely. If the ultrasound detector is used during inspections before plant shutdowns, problems can be identified and corrected during the planned shutdown rather than causing an unplanned shutdown later. Also, if the ultrasound detector is used during the lubrication procedure, it can help the operator to determine precisely when enough lubricant has reached the wear surfaces. 214 Acoustic Emission Testing M OVIE . Amplitude rise heard through ultrasound detector as rough and raspy.
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Ultrasound Detection of Electrical Leakage and Arcing Lightning is a good example of a large scale electrical leakage current conducted by arcing breakdown of the atmosphere. Current, often of hundreds of kiloamperes, rapidly heats the air in its conduction path. This rapid expansion of air acts like an explosion to generate intense, steep front sound waves that can be heard from a distance. The human ear can often detect these acoustic signals, which, if echoes do not interfere excessively, permit quick estimation of the direction and distance from the human observer to the sound source. Airborne sound at normal atmospheric pressures travels at speeds near 330 m·s –1 (1100 ft·s –1 ). Thus, a signal delay of 3 s corresponds to about 1 km (0.6 mi) of travel distance.
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  • Fall '19
  • Acoustic Emission

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