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Machinery and Equipment MRO
September 2020
TROUBLESHOOTING ROTATING MECHANICAL EQUIPMENT
USING VIBRATION ANALYSIS
V
ibration is technically defined as the oscillation of an object about its position of rest. These oscillations are responses to mechanical forces symptomatic of a problem. Before we discuss the technology as a troubleshooting tool, a review of terminology is important.
which is a measure of energy dissipated and consequent fatigue of machinery components. Overall velocity is also best for detecting a wide variety of different machinery defects occurring at the mid-frequency range.
Displacement refers to amplitude and “how much” the object is vibrating measured in Mils (1/1000 inch) peak to peak. Displacement (distance or movement) is generally the best parameter to use for very low frequency measurements (i.e., less than 600 cpm), where velocity and acceleration amplitudes are extremely low. Displacement is also traditionally used for machine balancing at speeds up to 10,000 or 20,000 rpm and where clearances are important criteria.
Acceleration of the object that is vibrating is related to the forces that are causing the vibration measured in “gs” (1g = 32 ¨/sec2 or 9.8 m/sec2) and is reported or shown as root mean squared (RMS). Acceleration (force) is best measured when it is known that all the troublesome vibrations occur at high frequencies, that is, above 60,000 cpm. For example, in detecting high frequency turbine blade vibration in the presence of many low frequency vibrations, acceleration will assist in emphasizing the high frequencies. It is important to remember that when using various transducers to monitor vibration, velocity leads displacement by 90 degrees and acceleration leads velocity by 90 degrees and displacement by 180 degrees. Vibration analysis should be part of any equipment reliability management program, but what must first be determined are which machines should be monitored and how o¨en monitoring should take place using the formula:
Velocity indicates “how fast” the object is vibrating measured in
MACHINE PRIORITY = CRITICALITY X RELIABILITY.
Frequency refers to “how many” of these oscillations in a given length of time (e.g., one minute), measured in cycles per minute (CPM), or cycles per second (Hertz) Hz, related to 1X sha¨ turning speed.
inches/second or mm/second peak. Velocity is frequently used for machinery vibration analysis where important frequencies lie in the 600 to 60,000 cpm range. For most machines, mechanical condition is most closely associated with vibration velocity,
Where criticality is the importance of the machine to production goals; and where reliability is a result of a review of the “probability of failure” based on history of machine repair.
Photo: photosoup / iStock / Getty Images Plus
BY L. (TEX) LEUGNER
