Couple or Dynamic Imbalance
Definition
Couple (dynamic) imbalance is a condition in which equal unbalanced masses are located in different axial planes of a rotating rotor, producing a rotating moment (couple) rather than a simple static unbalance force.
Physical Mechanism
Unlike static imbalance (single-plane), couple imbalance occurs when two equal but opposite unbalanced forces are separated along the shaft length.
This creates:
- A rotating couple moment
- Pure rocking motion of the rotor
- No single resultant force at the center of mass
The rotor tends to “wobble” around its center rather than translate in one direction.
Signal Signature
Couple imbalance typically appears in vibration data as:
- Strong 1× running speed vibration
- High vibration in both ends of the machine
- 180° phase difference between measurement points on opposite ends of the rotor
- Similar amplitude at inboard and outboard bearings
In time waveform:
- Smooth sinusoidal signal dominated by 1× RPM
- Consistent phase shift along shaft length
In phase analysis:
- Stable but opposite-phase response across bearing housings
Diagnostic Relevance
Couple imbalance is commonly associated with:
- Incorrect mass distribution in multi-plane rotors
- Assembly errors in coupled or multi-disc systems
- Thermal distortion causing differential expansion
- Manufacturing or repair imbalance in rotor components
It is significant because:
- It cannot be corrected with single-plane balancing
- It requires two-plane (dynamic) balancing for correction
Interpretation Notes
- Couple imbalance is often confused with misalignment due to phase differences, but key distinction is that couple imbalance remains consistent with speed and load.
- Unlike misalignment, axial vibration is not typically dominant.
- Presence of equal amplitude at both bearings with opposite phase is a key diagnostic indicator.
- Single-plane balancing will not resolve couple imbalance conditions.
Summary
Couple imbalance is a two-plane rotor imbalance condition that produces a rotating moment and characteristic 1× running speed vibration with opposite-phase behavior at each end of the rotor. It requires dynamic (two-plane) balancing for correction.