Dynamic Balancing (Two-Plane Balancing) Explained

Definition: What is Dynamic Balancing?

Dynamic balancing is a procedure for correcting unbalance in a rotor by making mass corrections in a minimum of two separate planes along its length. It is the most comprehensive form of balancing because it resolves both types of unbalance simultaneously: static (or force) unbalance and couple unbalance. A rotor that has been dynamically balanced will not have a tendency to vibrate or “wobble” from either a heavy spot or a rocking motion when it rotates.

Static vs. Dynamic Unbalance: The Key Difference

To understand dynamic balancing, it’s crucial to distinguish between the two forms of unbalance:

• Static Unbalance: This is a condition where the rotor’s center of mass is offset from its axis of rotation. It acts like a single heavy spot. This can be corrected with a single weight in a single plane and can even be detected with the rotor at rest (statically).
• Couple Unbalance: This occurs when a rotor has two equal heavy spots on opposite ends, positioned 180° apart. This condition is statically balanced (it won’t roll to a heavy spot when at rest), but when it rotates, the two heavy spots create a turning force, or “couple,” that causes the rotor to wobble end-over-end. Couple unbalance can *only* be detected when the rotor is spinning and can *only* be corrected by placing weights in two different planes to create an opposing couple.

Dynamic unbalance, the most common condition in real-world machinery, is a combination of both static and couple unbalance. Therefore, correcting it requires adjustments in at least two planes, which is the essence of dynamic balancing.

When is Dynamic Balancing Required?

While single-plane (static) balancing is sufficient for narrow, disc-shaped objects, dynamic balancing is essential for most industrial rotors, particularly when:

• The rotor’s length is significant compared to its diameter. A common rule of thumb is that if the length is more than half the diameter, dynamic balancing is necessary.
• The rotor operates at high speeds. The effects of couple unbalance become much more severe as rotational speed increases.
• The mass is distributed unevenly along the rotor’s length. Components like multi-stage pump impellers or long motor armatures require two-plane correction.
• High precision is required. To meet stringent balance quality grades (e.g., G2.5 or better), dynamic balancing is almost always needed.

Examples of rotors that always require dynamic balancing include motor armatures, industrial fans, turbines, compressors, long shafts, and crankshafts.

The Two-Plane Balancing Procedure

Dynamic balancing is performed on a balancing machine or in the field using a portable vibration analyzer. The process, typically using the influence coefficient method, involves:

1. Initial Run: Measure the initial vibration (amplitude and phase) at both bearing locations.
2. First Trial Run: Add a known trial weight to the first correction plane (Plane 1) and measure the new vibration response at both bearings.
3. Second Trial Run: Remove the first trial weight and add a new trial weight to the second correction plane (Plane 2). Measure the vibration response at both bearings again.
4. Calculation: From these three runs, the balancing instrument calculates four “influence coefficients.” These coefficients characterize how a weight in Plane 1 affects the vibration at both bearings, and how a weight in Plane 2 affects the vibration at both bearings. Using this information, the instrument solves a set of simultaneous equations to determine the precise size and location of the correction weights needed for both planes to eliminate the initial unbalance.
5. Correction and Verification: The trial weights are removed, the calculated permanent correction weights are installed in both planes, and a final run is performed to confirm that the vibration has been reduced to within the specified tolerance.

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