The increasing costs associated with machinery repair and downtime has created a demand for higher-level maintenance techniques not formerly utilized and often not understood. Until recently, machinery maintenance programs were structured around Reactive Maintenance (RM), which is essentially on-demand type machinery repair; and Preventive Maintenance (PM), which utilizes calendar-based planned maintenance events. Unfortunately, these traditional machinery maintenance methods did not alert operators to existing or future problems based on the machine's actual operating condition.
Predictive Maintenance (PDM), the diagnostic technology of determining a machine's condition and the need for maintenance without internal inspection or disruption of normal operation, has become a proven part of a today's advanced maintenance methods. PDM utilizes the following condition monitoring technologies: vibration analysis, bearing high frequency fault detection, ultrasonic bearing and coupling analysis, lubricating oil analysis, and infrared thermal imaging, to evaluate machinery reliability based on its actual operating condition during normal operation. Condition Monitoring measures specific machine characteristics, defining an envelope in which the machine normally operates. Corrective Maintenance is then directed when a machine moves out of acceptable limits in order to stop the progression toward a catastrophic and costly failure. Additionally, mandatory Classification "open & inspect" requirements are no longer required for machinery operating within acceptable condition limits.
Machine vibration is one of the best indicators of overall machine condition for most types of rotating machinery. The filtered vibration, interpreted as spectral components, is an excellent indicator of vibration severity and overall machine condition. A vibration spectrum, graph of vibration amplitude vs. frequency, will indicate specific machine defects such as mis-alignment, imbalance, and bearing wear. The fundamental principle of PDM consists of periodic measurements of bearing housing vibrations with permanent or portable instruments, trending vibration levels to detect any developing problems, and interpreting the vibration spectra to diagnose specific machine defects.
In addition to vibration, ultrasonic detection is an essential tool for Condition Monitoring. Ultrasonic detection utilizes a hand-held contact probe and calibrated LED display to listen via headphones to high frequency (40 kHz - above human audible range) sound generated by friction in moving parts. Good bearings produce a smooth whistling sound with an amplitude typically of 0 - 25 dB (decibels). A bearing that is about to fail will exhibit sounds like crushing glass with a level of 30 - 50 dB or even higher. This principle is also used to diagnose the condition of shaft couplings via a non-contact microphone. The quality of the ultrasonic sound is extremely important in distinguishing a problem or defect from a lack of lubrication or a lubrication problem (i.e., oxidation). It must also be noted that vibration analysis will not detect inadequate bearing lubrication or allow for coupling fault detection by a non-contact method. To date, no other condition monitoring programs utilize ultrasonic detection to add to the vibration data fault detection with over ten (10) years of experience. We have found that the ultrasonic analysis information is an integral part of accurate machinery fault diagnosis that cannot be ignored.
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