The Difference Between Static and Dynamic Unbalance

Published on | Category: Balancing Fundamentals

Static Unbalance

Diagram showing a rotor with static unbalance, where the center of gravity is offset from the axis of rotation.

In static unbalance, the center of gravity of the rotor is offset from the axis of rotation. This creates a one-sided force that tries to rotate the rotor so that its heaviest part points downwards. If you rotate such a rotor by 90 degrees, the "heavy spot" will always move downwards.

  • Occurs when the rotor is stationary.
  • Used for narrow, disc-shaped rotors.
  • Corrects the uneven mass distribution in one plane.

Dynamic Unbalance

Diagram showing a rotor with dynamic unbalance, with two offset masses in different planes.

In dynamic unbalance, there are at least two different mass displacements in different planes. This causes not only a one-sided force like in static unbalance but also moments that generate additional vibrations during rotation. Such a rotor does not turn by itself when rotated 90 degrees because the forces cancel each other out. This type of unbalance can only be corrected dynamically.

  • Occurs only when the rotor is spinning.
  • Arises from two unbalanced masses in different planes along the rotor's length.
  • Suitable for long rotors with two axes; requires correction in two planes.

For correcting dynamic unbalance, a device like the Balanset-1A with a two-plane balancing function is essential.