In some cases the damage to the contacting surfaces

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In some cases, the damage to the contacting surfaces may be so advanced that they cannot be cushioned by an improved lubrication film and the acoustic emission signal may show little change. An increase in acoustic emission signal is usually an indication that preexisting wear debris has been pushed back into the rolling track. Influences on Measurement Sensitivity Transducer Positioning The positioning of acoustic emission transducers for machinery monitoring makes no exceptional demands. Transducers are placed where an adequate sound path exists between the expected source positions, usually where moving surfaces contact. In addition to minimizing the number of metal-to-metal joints in the sound path, geometry changes should be minimized. In the case of bearing monitoring, it is usual to mount the acoustic emission transducer as close as possible to the bearing housing. For gear teeth monitoring, the sound path through shafts and bearings is more tortuous and this attenuation may limit sensitivity to faults. Where acoustic emission measurements are made at multiple positions on a machine (for example, in the vicinity of shaft end bearings on a multistage reduction gearbox) then it is unlikely that attenuation will provide total isolation between the different measurements. However, sufficient attenuation usually exists to allow adequate identification of the area of concern. Background Noise In the context of machinery monitoring, background noise includes any detected signals that have a cause unrelated to the condition of the monitored item. Activity may be detected from nearby close coupled machinery — this is usually easy to recognize because signal levels will be higher on the interfering machine. The processes being carried out by the machine itself may generate background noise. The signals of interest for condition monitoring are usually conspicuous in rotating machinery — in motors, pumps, gearboxes, screw compressors, fans, roll supports and bearings. Principal exceptions are where the machine is carrying out some form of material reduction, where impacts occur (as from the movement of hard particles) or where friction is involved (as in braking systems). In addition, when monitoring pumps, the possibility of detecting cavitation or impacts from a second phase in the fluid must also be considered in addition to the signals expected from lubrication problems, bearing wear and rotor rub. Evaluation of Machine Condition Machinery condition monitoring enables servicing and maintenance actions to be carried out in a more timely way. Planning helps maximize machine life, minimize cost and avoid or reduce unplanned stoppages. These benefits can be gained only if a timely decision is made for each measurement regarding whether an action is required and, if so, what action. Such decisions benefit from previous experience and require knowledge of machine failure consequences, planned usage and availability of replacement parts.
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