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Correct Sensor Mounting for High Frequency Detection, Enveloping or Demodulation to Monitor Bearings

Many people use one of these high frequency techniques to detect and monitor bearing wear. However, the location and mounting of the accelerometer on the machine will determine the ultimate success of using these techniques.

Bearings with poor lubrication or in the early stages of abnormal wear emit very high frequency non-periodic vibration which is transmitted through the structure of the machine. This information can be used to determine when to lubricate the bearing or gain insight into the condition and remaining life of the bearing.

Using any of these techniques, it is necessary to have an extremely good physical connection between the sensor and the machine, and to locate the sensor on the machine properly. Just as with sound, different frequencies of vibration or travel through solids differently and high frequencies are attenuated quickly.

Load zone: First and foremost, you must locate the sensor as closely as possible to the load zone of the bearing. Longer distances from the bearing through the material to the sensor will attenuate the vibration. If there is a junction or break in the transmission path from the load zone to a potential sensor location, such as between two parts that are bolted together, there will be more attenuation. If the two solids are different in their physical characteristics, there may be little transmission or coupling of the vibration across the junction at these frequencies.

Mounting: Your accelerometer should be stud mounted. Magnetic mounts and handheld probes should not be used. The accelerometer should be mounted on a surface that has been spot faced and tapped to accept the accelerometer stud per manufacturer instructions. When it is installed, the stud should be tight, stretching the stud to ensure proper contact and coupling across the entire face of the accelerometer. Burrs or dirt on the face or a poorly drilled and tapped hole will degrade the signal, so be sure to use the appropriate tools. If you must use a portable sensor, be sure to use one of the stud mounting systems offered by several manufacturers.

Tip provided by Mobius Institute.  For more information: http://www.mobiusinstitute.com/articles.aspx?id=2088

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