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Unstable RPM Measurement When Balancing


The RPM shown on the vb instrument's screen will not lock in and is unstable when balancing.


  1. This condition is common with irregular, duplicate or sporadic signals being fed back into the tachometer head. In a perfect setup, the reflective tape works to send back a signal to the tach receiver that corresponds to one revolution of the shaft. As extra reflected signals per revolution are received at the tach head, the instrument calculates for erroneous RPM readings. Also, if the shaft surface is too shiny, multiple reflections will be received by the sensitive tach receiver head.
  2. If the tach reading is far too high, and the tach cable is routed near a variable speed (VFD) motor, then the cause may be electrical interference.


  1. Using a slight overlap action, locate where the reflective tape will be placed and wrap the shaft with a black electrical tape so the surface is essentially dulled. The total width of the taped area can be 2-4 inches (5-10 cm). Then place a small piece of the supplied reflective tape on the new dulled surface.
    • Install the tachometer magnetic base on a stationary area of the machine being balanced. The magnetic base of the tach assembly must not be located on any surface that moves with rotation and imbalance, otherwise all phase angle measurements will be inaccurate and the balance session will not be valid.
    • Position the tach head a nominal 4-12 inches (10 to 30 cm) from the reflective tape on the shaft.
    • Aim the tach head so it has a slight angle to the tape and is not located perpendicular to the shaft surface. A 10-20 degree offset will work well.
    • Start the instrument running and adjust the angle of the tach head to get a stable, accurate reading.
  2. Move the cable away from any variable speed motors or their switching electronics.