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Imbalance Will Not Reduce

 

SYMPTOMS

The imbalance will not reduce even though I keep adding weights.

CAUSES (match the number of the cause with its solution below)

1. The phase readings are unstable.
2. You are adding weights WR (with rotation) when the instrument is set to AR (against rotation) or vice-versa.
3. The trial weight used is too light.
4. You are not accurate enough when placing correction weights at specific angles.
5. The correction weights are inaccurate.
6. You are placing correction weights at a different radius than your trial weight was placed.
7. You are placing correction weights at a different plane relative to your trial weight.
8. You are not measuring from the same weight reference mark for the trial and correction weights.
9. You have the left channel sensor plugged into the vb instrument on the right side and vice versa. (This only applies if you happen to change the left/right sensor during the balancing process.
10. There is another vibration source nearby which is close to the 1X frequency (e.g. drive belt of balance stand).
11. The machine is running at slightly different speeds during each run (initial/trial/balance/trim).
12. You may be attempting a dual plane balance on a machine which has a severe static imbalance.
13. The machine is running near a natural resonance frequency (for its current mount or stand).
14. The machine is not 'rigid' i.e. it is flexing significantly at its normal running speed, or it may be running above its first critical speed.
15. The machine being balanced is an overhung rotor. These often have characteristics such as Disk Skew and Gyroscopic Effects that can make them hard or impossible to balance with standard single or dual plane balancing techniques.

SOLUTIONS

1. Allow phase angles to stabilize before making any corrections. A stabilized phase angle is considered good when there is no more than a three degree variance in three successive screen refreshes.
2. Make certain that the correction weights are placed in accordance with the way you have the vb instrument set up - either WR with rotation or AR against rotation.
3. When performing the trial weight tests use a heavy enough weight to offset the angle or amplitude by 30% or more.
4. The placement of the corrective weights is critical. You should have a tolerance of no more than 3-5 degrees. Corrections made outside of this parameter will result in the 'chasing syndrome'. The use of a magnetic protractor or degree wheel is essential.
5. Your scale is out of calibration and the weights called for are not being applied.
6. You are placing weights at a different radius than the original trial weight. Use the vb instrument's recommendation for weight and radius calculations.
7. You are placing your corrective weights at a different reference plane than the trial weights. If your trial weight was placed on a fan blade support that was 3 inches in from the bearing but your correction was made on the nose of an armature coil that was 5 inches in from the same bearing this will cause inaccuracies.
8. All weight angles should be measured from your chosen weight reference mark - see FAQ Determining Balance Weight Placement Angles for a full explanation.
9. To avoid accidentally switching the sensors during balancing use the green balancing extension cable for the left side trunion and the red one for the right. The green (left) cable will plug into CH1 on the instrument and the red (right) cable will plug into CH2.
10. Set the vb instrument to narrow filter bandwidth (+/- 15 RPM) to help cut out the nearby frequencies. To access this, from the Balancing Menu press [1] Set Balancing Paramerers then [7] Advanced Balance Options. Press [1] to select a bandwidth filter option.
11. Imbalance forces generally vary with the square of the running speed so variations can cause major errors in the balance calculations. Ensure that the RPM is stable and consistent between runs, certainly within 5% (usually for AC driven machines this is not an issue as they are locked to line frequency).
12. Perform a single plane balance on the worst end first then move on to dual plane balance - see FAQ Balancing Overhung Rotors for more information.
13. Does the machine appear to go through any resonances during run up or down? You can use the vb instrument to help diagnose this - see FAQ Run-up and Coast-down Recordings for more information. If possible run the machine at a different speed, preferably slower.
14. The instrument is designed to balance 'rigid' rotors only. If you can balance the rotor at a much lower speed, the imbalance forces may be reduced to a level where the rotor is effectively rigid at its operating speed. Try balancing at a slow speed first.
15. Read FAQ article Balancing Overhung Rotors, which describes two established and effective techniques for dealing with balancing overhung rotors.