Unstable Phase/Amplitude Readings when Balancing

The information provided is for help with Commtest Instruments products:

• vb instrument

SYMPTOMS

Unstable phase and/or amplitude readings when performing a balance.

CAUSE

Usually caused by tach signal variations. Sometimes caused by improper mounting of accelerometer or interference from another vibration source.

SOLUTIONS

Unstable RPM

If the balancing measurements are jumping all over the place, the most likely cause is that the tach is set up to point directly at a shiny shaft (from a perpendicular trajectory), allowing it to see reflections off the shaft surface as well as the reflective tape. If you position the sensor slightly away from the centerline of the shaft e.g.10-20 degrees away, the readings may suddenly become much more stable (remember that reflective tape is designed to reflect back at the source like a cats-eye, as opposed to just reflecting like a mirror).

Unstable Amplitude

If the RPM is stable but the amplitude readings are changing then it is possible that one of the following things are happening:

• The accelerometer is mounted poorly. Try mounting somewhere else (horizontal vs vertical maybe).

• There is another vibration source at a frequency close to the RPM of the motor in question. Try the narrow filter bandwidth setting. To set this, from the Set Balancing Parameters Menu, press [7] Advanced Balance Options then press [1] to change the filter bandwidth.

FAQ ID: 11691 Last Reviewed: 08/01/2001