What are Rigid Rotors?

Rigid rotors are rotating parts that do not exhibit flexibility at operating speeds, meaning they do not change shape. These rotors operate stably because their operating speeds remain below their natural resonant frequencies. Unlike flexible rotors, the unbalance occurring in a rigid rotor remains constant regardless of rotational speed. This allows balancing to be performed at a single speed, eliminating unbalance across the rotor's entire operating range. This feature makes them ideal for many industrial applications, such as pumps, motors, and fans operating at low and medium speeds. The stable structure of rigid rotors simplifies the dynamic balancing process and ensures more reliable results.

Rigid rotors are excellent candidates for single-plane or two-plane balancing operations. At MBS Balance, we offer balancing machines that can correct unbalances in these types of rotors with high precision. Balancing a rigid rotor is typically accomplished by measuring the mass distribution imbalance at the mounting position and adding or removing appropriate correction weights. This process minimizes vibration and noise while increasing the machine's overall efficiency, bearing life, and operational reliability. This stable structure of the rigid rotors reduces maintenance costs and the risk of failure.

Whether a rotor can be considered rigid is determined by the ratio of its operating speed to the first bending critical speed. In general engineering practice, if the operating speed remains below approximately 70 percent of the first critical speed, the rotor is considered rigid and low-speed balancing in two planes per ISO 21940-11 is sufficient. Above this ratio, the rotor's bending modes begin to come into play and the rigid-rotor assumption loses its validity.

In practice, the vast majority of parts such as pump impellers, electric motor rotors, pulleys, and small to medium fans fall into the rigid-rotor class. For these parts, a two-plane correction made in a single setup remains valid throughout the entire operating range. At MBS Balance, we guarantee reaching the target G grade in a single pass on rigid rotors with hard-bearing machines that provide low cross-effect and a high unbalance reduction ratio.