CVA Rotor Dynamics & Balancing 1 — Questions and Answers
Question 1: What is a critical speed in rotor dynamics?
- The maximum allowable operating speed of a machine
- A rotational speed at which the shaft excitation frequency coincides with a rotor natural frequency (Correct answer)
- The speed at which bearing load capacity is exceeded
- The minimum speed required for hydrodynamic lubrication
Correct 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.
Question 2: What type of rotor unbalance causes a pure couple (moment) with no net centrifugal force?
- Static unbalance
- Dynamic unbalance
- Couple unbalance (Correct answer)
- Overhung unbalance
Correct 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.
Question 3: What is the vibration signature characteristic of rotor unbalance?
- Strong 2× running speed with high axial vibration
- Dominant 1× running speed component, typically with consistent phase (Correct answer)
- High-frequency random broadband noise
- Sub-harmonic components at 0.5× running speed
Correct 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.
Question 4: What is the influence coefficient method in rotor balancing?
- A method to calculate critical speed from bearing stiffness
- A mathematical approach that determines correction weights by measuring the rotor response to known trial weights (Correct answer)
- A technique for measuring shaft bow using proximity probes
- A procedure for balancing flexible rotors at multiple planes simultaneously
Correct 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.
Question 5: What is shaft bow, and how does it appear in a vibration spectrum?
- Axial bending of a shaft due to thermal gradients; appears as high axial 1× vibration
- Permanent or temporary curvature of a rotor shaft; produces 1× vibration that does not change with speed changes the way pure unbalance does (Correct answer)
- Excessive radial clearance in bearings; produces broadband noise
- Torsional oscillation of the shaft; appears at gear mesh frequency
Correct 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.
Question 6: In ISO 1940 balance quality grades, what does a higher G number indicate?
- Tighter balance tolerance requiring more precision
- Looser balance tolerance allowing more residual unbalance (Correct answer)
- Higher allowable vibration velocity
- Greater rotor mass requiring larger correction weights
Correct 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.
CVA Rotor Dynamics & Balancing 1
What is a critical speed in rotor dynamics?