Theory & Practice of Dynamic Balancing: A Comprehensive Guide

Published on July 22, 2025 | Category: Expert Knowledge

1. What is a Rotor and Why Does Vibration Occur?

A rotor is a body that rotates around an axis and is supported by its bearing surfaces in supports. These bearing surfaces (often called journals) transfer the loads to the bearings.

In an ideally balanced rotor, its mass is symmetrically distributed around the axis of rotation. During rotation, a centrifugal force acts on each element, directed outwards. In a balanced rotor, the centrifugal forces of symmetrical elements cancel each other out, and the total force on the bearings is zero.

However, if this symmetry is disturbed (e.g., by an uneven mass), an unbalanced centrifugal force is created. This force changes its direction with each revolution and creates a dynamic load on the bearings, leading to their accelerated wear and to vibrations.

Balancing is the process of eliminating this unbalance by purposefully adding balancing masses to restore the rotor's symmetry.

2. Rigid vs. Flexible Rotors

Depending on the material strength and the magnitude of the centrifugal forces, two types of rotors are distinguished:

• Rigid Rotors: These deform only insignificantly under the influence of centrifugal forces at operating speed. Most rotors in general mechanical engineering fall into this category, and the methods described here apply to them.
• Flexible Rotors: Their deformation cannot be neglected and requires more complex balancing methods. A rotor can behave like a rigid rotor at low speeds and like a flexible rotor at high speeds.

3. Types of Unbalance: Static and Dynamic

We have already covered this important difference in a separate article, but here is the key takeaway again:

• Static unbalance exists even at a standstill. The "heavy spot" turns downwards due to gravity. It is typical for narrow, disc-shaped rotors.
• Dynamic (or moment) unbalance occurs only during rotation. It is caused by two unbalanced masses in different planes, creating a tilting moment. This type of unbalance is typical for long rotors and cannot be corrected statically.

Theoretically, it has been proven that to completely eliminate the unbalance of a rigid rotor, **two correction weights** in two different planes are sufficient. These compensate for both static and dynamic unbalance.

4. The True Causes of Vibration

It is important to understand that unbalance is not the only cause of vibrations. Balancing can only eliminate the vibrations caused by an asymmetrical mass distribution.

Therefore, it is crucial: A defective mechanism must first be repaired and only then balanced. Balancing cannot replace a repair!

5. The Resonance Problem

Every mechanical system has a natural frequency. When the operating speed of the rotor approaches this natural frequency, the vibration amplitude increases dramatically – this is called mechanical resonance. In this state, normal balancing is impossible because the slightest changes in speed lead to massive changes in vibration.

A correctly designed mechanism should be designed so that its operating speed is far from the resonance frequency. Balancing machines that operate below their resonance frequency are considered "hard-bearing", while those that operate above are considered "soft-bearing".