Static and Couple Components of Dynamic Balancing
The information provided is for help with Commtest Instruments products:
- vbSeries® instruments
QUESTION
What is the recommended technique for performing a dual plane balancing job when both static and couple imbalance are present?
ANSWER
Separate the balancing job into two tasks and correct the static imbalance first.
The dual plane influence coefficient method of balancing is well known and works well for most balancing problems. Any dual plane dynamic balance solution can be separated into a static and couple component.
Narrow and overhung rotors are typically difficult to balance using the dual plane approach because they are dominated by static imbalance. Therefore, the best approach to balancing these types of rotors is to eliminate the static imbalance first, before trying to correct the couple imbalance.
This technique is also useful in balancing long flexible rotors, as the static correction can be applied in the center of the rotor (near the center of gravity), which reduces the rotor's flex near its first critical speed. This combined weight by vector addition is the static weight correction. This weight should be placed in the plane nearest the center of gravity, or distributed axially about the center of gravity plane.
Example:
The first run of our dynamic balancing procedure results in a Plane 1 correction callout of 6.47 grams at 85° and a Plane 2 correction callout of 4.31 grams at 195°.
- Use the Combine Weights feature. From the Balancing Menu press [5] to display the Balance Weight Calculations Menu then press [3] Combine Weights.
Sum = Combined Weight (from above) = 6.42 grams at 124° = Static Correction.
- We recommend that you do not proceed with the couple corrections at this time. Continue in trim mode and calculate the trim weights for Planes 1 and 2.
- After adding the static correction weight, run the rotor up to balancing speed and take further readings.
- Instead of adding the trim corrections in Planes 1 and 2, compute the static trim correction as above using the Combine Weights feature
- When the static imbalance is deemed acceptable, you can apply the couple correction.
The couple correction is the dynamic solution for Planes 1 and 2. In other words, when the static imbalance has been reduced to an acceptable level, the only imbalance left in the rotor is the couple imbalance i.e. two equal weights that are 180° apart in Planes 1 and 2. The couple corrections are half the vector difference of the Plane 1 and 2 corrections. [Plane 1 + (–Plane 2)] / 2 .
Example:
Plane 1 correction callout is now 1.47 grams at 66° and Plane 2 correction callout is at 1.21 grams at 135°.
- Use the Combine Weights feature in the Balance Weight Calculations Menu, but be sure to add 180° to the angle of Plane 2 to have the negative vector component. Thus, Plane 2 callout in the combined weight equation will be 1.21 grams at 315°.
Sum = Combined Weight (from above) = 1.54 grams at 18°
So, our resulting Couple Correction = 0.77 grams at 18° in Plane 1 and 0.77 grams at 198° in Plane 2.
The key to the Static/Couple approach is to correct the static imbalance first. You should apply this static correction on or about the rotor's center of gravity. For a multistage rotor you can divide the static corrections between the center two or three stages. For a narrow impeller you can split the static corrections in half and apply them on either side of the impeller. For overhung rotors, attach static weight in Plane (Plane 1) nearest the inboard bearing.
Use the Commtest vb instrument's Combine Weight feature to derive the static component. The vector sum of the Plane 1 and 2 corrections is the static correction. After removing the static imbalance the remaining dynamic imbalance is simply the couple imbalance. Calculate the couple component using the Combine Weight feature, remembering that the couple corrections are half the vector difference of the Plane 1 and 2 corrections. [Plane 1 + (–Plane 2)] / 2.
FAQ ID: 17984 Last Reviewed: 28 September 2006