SPECIALIZED UNITS OF MEASUREMENT

Specialized units of vibration have been developed to determine the force in transient energy waves and high frequency vibration. Most commonly these units are recorded in conjunction with a standard unit to provide an additional means of evaluating bearings or gears.

The majority of these units apply the principle that an accelerometer resonance frequency (Fn) is induced by the ringing caused by metal to metal impact. The two largest factors in calculating resonance are mass and stiffness (material composition affects both). It would seem unusual that all accelerometers and mounting situations have the same resonanct frequency. So while the resonance of a specific accelerometer may be at a certain frequency, there is certainly no guarantee that this will match the resonance of any structure or component (i.e. bearing) as some instrument manufacturers once claimed. The method these processing techniques use is the effect that occurs by tapping something to excite its natural frequency, much like a drum is excited. This does produce an erratic signal that can be used as a general measure of non-periodic events.

The signal processing techniques are often calculated and defined with a specific sensor and sensor mount. Using another sensor model or other deviations will distort the data from what the expected output should be. Additionally, in taking hand held readings, the resonant frequency of a persons hand may affect the measurement. For these reasons, and many more, these units should not be the sole method of determining bearing condition. They are simply an indication that further investigation is warranted.

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Last updated: November 24, 2002