Checking for Data Validity

After determining the shaft rotation rate and locating it on the spectrum (it will be the first order in a normalized spectrum), the vibration analyst must check the validity of the spectrum. Data validity can be corrupted by such things as incorrect labeling of accelerometer orientation or position, improper accelerometer attachment, rapid accelerometer temperature changes, and incorrect machine operating conditions.

When data are to be compared to previously collected data from the same point, similar test conditions must be maintained, especially machine speed, load, and operating temperature.

The integrity of the accelerometer cable is crucial to the collection of valid data. If the central conductor in the cable is intermittent or open, the measured signal will consist mostly of random noise, and if the cable shield is intermittent or broken, the data will be contaminated with 60 Hz noise and harmonics. (50 Hz in countries with 50 Hz power lines.) In electrically driven machines, the 60 Hz line frequency will produce a series of 120 Hz harmonics in the vibration spectrum, as explained in the section on electrically induced vibration.

If an accelerometer is exposed continuously to a higher temperature than that for which it is rated, it will become desensitized, and the data it senses from then on will be worthless. Some accelerometers will operate up to 400 degrees F, but most give up the ghost at about 200 degrees F.

Care must be taken that the accelerometer is not dropped onto a hard surface lest the piezo-electric element be damaged. If the element is cracked, the stiffness of the internal assembly will decrease, reducing the resonant frequency of the accelerometer, and this can greatly change its sensitivity at high frequencies.