Understanding the Rigid Rotor

1. Definition: What is a Rigid Rotor?

A Rigid Rotor is a rotor that does not significantly bend, flex, or change its shape under the influence of its own unbalance forces at its service operating speed. For balancing purposes, a rotor is considered to be rigid if it operates at a speed that is less than 70-75% of its first critical speed.

The key principle of rigid rotor balancing is that the unbalance distribution along the length of the rotor does not change when the rotor’s speed changes. This means that a state of balance achieved at a low speed on a balancing machine will still be valid and effective when the rotor is operated at its much higher service speed.

The vast majority of common industrial rotors, such as electric motor armatures, fans, pumps, and pulleys, are treated as rigid rotors.

2. Rigid vs. Flexible Rotor

The distinction between rigid and flexible rotors is one of the most important concepts in rotor balancing:

Rigid Rotor

• Operating Speed: Well below its first critical speed (typically < 75%).
• Behavior: Does not bend or flex due to centrifugal forces. Its unbalance characteristics are independent of speed.
• Balancing Procedure: Can be balanced at a single, convenient low speed. A standard two-plane balance is sufficient to correct any dynamic unbalance. The international standard for rigid rotor balancing is ISO 21940-11.

Flexible Rotor

• Operating Speed: Approaches, passes through, or operates well above one or more of its critical speeds.
• Behavior: Bends and flexes as it passes through critical speeds. The unbalance forces cause the rotor to change shape (deflect), and the location of the “heavy spot” can appear to shift.
• Balancing Procedure: Much more complex. Requires multi-plane balancing (often more than two planes) and must be performed at or near the service speed to account for the rotor’s flexing. Specialized techniques are required.

3. The Importance of the “Rigid” Assumption

The assumption that a rotor behaves rigidly is what allows for practical, economical, and safe balancing on industrial balancing machines. These machines typically spin rotors at relatively low speeds (a few hundred RPM) for safety and mechanical simplicity.

If a rotor is truly rigid, the unbalance measured at 400 RPM on the balancing machine will be the same unbalance that causes vibration at 3600 RPM in the field. By correcting it at the low speed, the problem is solved for the high speed. If the rotor were actually flexible, a low-speed balance would be ineffective, as the rotor would bend and exhibit a completely different unbalance state at its higher service speed.

4. When is a Rotor Considered Rigid?

The decision to treat a rotor as rigid is based on its geometry and operating speed:

• Short, Stubby Rotors: Rotors that have a large diameter relative to their length (like a grinding wheel or a disc brake) are almost always rigid.
• Long, Slender Rotors: Rotors that are long and thin (like a drive shaft or a multi-stage compressor rotor) are more likely to be flexible, especially if they operate at high speeds.

Ultimately, the definitive test is the ratio of the operating speed to the first critical speed. If this ratio is low, a rigid rotor balancing approach is appropriate and will be successful.

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