Understanding Couple Unbalance

1. Definition: What is Couple Unbalance?

Couple Unbalance is a specific condition of dynamic unbalance where a rotor has two equal unbalance masses located at 180 degrees opposite each other, in two separate correction planes.

When this rotor spins, the two opposing unbalance forces create a turning force, or a “couple,” that tries to twist the rotor. This causes the rotor to wobble or rock end-to-end about its center of gravity. A key characteristic of pure couple unbalance is that the center of gravity lies on the axis of rotation, so it is statically balanced. You could place it on knife-edges and it would not roll to a heavy spot.

Couple unbalance can only be detected and measured when the rotor is spinning and can only be corrected by placing weights in two different planes.

2. Visualizing Couple Unbalance

Imagine a long, thin rotor. Place a 10-gram weight at the top (0 degrees) of the left end. Then, place another 10-gram weight at the bottom (180 degrees) of the right end.

• If you were to check this rotor for static balance, it would appear perfectly balanced because the two weights cancel each other out.
• However, as soon as the rotor spins, the weight on the left tries to pull the left end up, while the weight on the right tries to pull the right end down. This creates a powerful rocking motion.

This condition produces high vibration at 1X the running speed, and the vibration readings will be 180 degrees out of phase when comparing the left and right bearings of the machine.

3. Couple vs. Static vs. Dynamic Unbalance

It is important to understand the relationship between the types of unbalance:

• Static Unbalance: A single “heavy spot” that causes the rotor’s center of gravity to be offset from the axis of rotation. It can be corrected with a single weight in a single plane. The vibration at both bearings is in-phase.
• Couple Unbalance: Two equal and opposite heavy spots in two different planes. The center of gravity is on the axis of rotation. It requires two correction weights and can only be detected dynamically. Vibration at the bearings is 180 degrees out-of-phase.
• Dynamic Unbalance: This is the most common type of unbalance found in rotors. It is a combination of both static and couple unbalance. Correcting dynamic unbalance requires measurements and weight placements in at least two separate correction planes.

4. Correction of Couple Unbalance

To correct couple unbalance, the balancing machine or analyzer will calculate the size and location of two correction weights.

• One weight will be placed in the first correction plane to counteract the unbalance force in that plane.
• A second weight will be placed in the second correction plane to counteract the opposing force in that plane.

By correcting the unbalance in two planes, the rocking motion is eliminated, and the rotor will spin smoothly without the twisting force.

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