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vb Instrument Specifies Very Heavy Balance Weights on Both Ends

QUESTION

While performing a balance job, the instrument told me to put massive amounts of weight on both ends about 180 degrees away from each other. Why is this?

SYMPTOMS

When doing a two plane balance, the vb instrument recommends that large weights be placed on both ends of the rotor, 180 degrees apart from each other. This seems counter-intuitive (and perhaps dangerous).

For example, a 250 g trial weight at both ends makes a reasonable difference to the vibration levels (e.g. going from 0.27 mm/s to 0.34 mm/s and the phase stays the same). The instrument then instructs the user to add 8.355 kg of weight on the left plane at 34 degrees, and 7.736 kg of weight on the right plane at 206 degrees. The machine state looks tenuous with the addition of the trial weights alone.

CAUSE

The balancing calculations suggest massive weights 180 degrees out of phase to try and reduce the residual couple imbalance of the rotor. If these weights were added exactly as indicated, the machine would run smoother. The problem can be that adding such large amounts of weight accurately is impossible in some situations.

Background Information


The imbalance forces in a rotor can be broken down into two components - the static imbalance and the couple imbalance.

To understand static imbalance, imagine a rotor with a single heavy spot on it. If the rotor was placed on frictionless bearings, the rotor would turn until the heavy spot was at the bottom.

Couple imbalance can be visualized as two heavy spots on the rotor, at opposite ends and opposite angles. The combined effect of their weights cancels out, so there is no static imbalance to drag any heavy spot to the bottom of the rotor swing as in the example above. But when the rotor is spinning, the couple imbalance will present imbalance forces to the bearings at either end.

So, if you have a machine that is not sensitive to couple imbalance but a couple imbalance exists on the machine already, the vb will suggest large amounts of weight on opposite ends of the machine at opposite angles to cancel out the couple imbalance.

SOLUTION


You can separate the static and couple components of the balance weights and choose to only correct the static imbalance present in the rotor. The vb instrument can perform this calculation for you.

  • From the main menu in the instrument, select [8] Balancing Menu.
  • Select [5] Balance Weight Calculations.
  • Select [3] Combine Weights.
  • Now enter the left and right balance weights as first mass and second mass on the Combine Weights screen. When you have entered this information, the instrument will indicate what the combined weight and angle are on the bottom half of the screen


Continuing the example above, we will combine the two weights of 8.355 kg @ 34 deg and 7.736 kg @ 206 deg. To get maximum resolution, we will shift the decimal point one place so that our units are 100s of grams instead of kg. (There are 2 decimal places available in the Combine Weights screen.)

  • Entering 83.55 @ 34 deg and 77.36 @ 206 deg gives us a combined weight of 12.80 at 91 degrees. Returning to our units of kilograms, this is 1.280 kg at 91 degrees. This is the weight required to remedy the static imbalance in the rotor.
  • Put half of this weight on both ends of the rotor i.e.


Left:
640 g at 91 degrees

Right:
640 g at 91 degrees.