Vibration analysis can be used to determine if a soft foot condition exists on a machine that is running. If you cannot shut the machine down to check for soft foot, using the simple procedure outlined in this article will help determine if soft foot is your problem:

  1. Mount your accelerometer on the suspected soft foot.
     
  2. Use the live monitoring mode of your vibration analyzer to monitor the 1x rotation speed vibration. Use enough resolution to distinguish the desired peak, but not so much that it requires long collection time between averages.
     
  3. While monitoring the 1x vibration, loosen the mounting bolt to approximately hand-tight.
     
  4. If the vibration at 1x rotational speed reduces significantly in amplitude, it is very likely that you have a soft foot condition.
     
  5. Retighten the mounting bolt and schedule the appropriate corrections.

When corrections are not made to eliminate a soft foot condition, the foot will deflect when tightened. This will affect the alignment and the motor air gap, and can cause significant vibration at rotational speed. By loosening the foot while running, the force deflecting the foot is removed.