As the machinery stiffness is not the same in the 3 directions, that’s why the machine vibration response to the forcing frequency will not be the same.
Noting that, different faults can occur (show high vibration) in different planes, and measuring only one axis will easily miss the fault.
Examples:
- Imbalance fault could be seen in the "Vertical Axis" depending on fan geometry.
- Misalignment fault could be seen in the "Axial Axis".