Rotor Dynamics & Balancing Flashcards

What is a critical speed in rotor dynamics?

Answer: A rotational speed at which the shaft excitation frequency coincides with a rotor natural frequency

A critical speed occurs when the shaft rotational frequency matches a rotor natural frequency, causing resonance and potentially large amplitude vibrations.

What type of rotor unbalance causes a pure couple (moment) with no net centrifugal force?

Answer: Couple unbalance

Couple unbalance exists when two equal and opposite unbalance masses are located in different axial planes, creating a pure couple that can only be detected and corrected during rotation.

What is the vibration signature characteristic of rotor unbalance?

Answer: Dominant 1× running speed component, typically with consistent phase

Rotor unbalance produces a dominant 1× (one times running speed) vibration component with a phase angle that remains stable, as the heavy spot always lags the high spot by a consistent angle.

What is the influence coefficient method in rotor balancing?

Answer: A mathematical approach that determines correction weights by measuring the rotor response to known trial weights

The influence coefficient method uses trial weights placed at known locations and measures the resulting vibration change to calculate the correction weight magnitude and angle needed for balance.

What is shaft bow, and how does it appear in a vibration spectrum?

Answer: Permanent or temporary curvature of a rotor shaft; produces 1× vibration that does not change with speed changes the way pure unbalance does

Shaft bow is a bend or curvature in the rotor shaft that produces a 1× vibration symptom similar to unbalance but differs in that the phase and amplitude change differently with speed and may be present at very low speeds.

In ISO 1940 balance quality grades, what does a higher G number indicate?

Answer: Looser balance tolerance allowing more residual unbalance

ISO 1940 balance quality grades (e.g., G1, G2.5, G6.3, G16) represent the product of eccentricity and angular velocity; a higher G number permits greater residual unbalance and is assigned to less precision-sensitive machinery.