Mixed Deck — All CVA Topics Flashcards

What is the typical shape of a shaft orbit for a machine running with pure rotor unbalance?

Answer: A circular or slightly elliptical loop rotating at 1× shaft speed

Pure unbalance generates a nearly circular or elliptical orbit because the centrifugal force is constant in magnitude and rotates at shaft speed with a consistent phase relationship in both measurement directions.

Which standard focuses on vibration severity in rotating machinery?

Answer: ISO 10816

ISO 10816 is a key international standard specifically designed for the measurement and evaluation of mechanical vibration of non-reciprocating machines. It provides comprehensive guidelines for assessing the severity of vibration in various types of rotating machinery. This helps determine their operational health and the potential need for maintenance or repair.

Which bearing defect frequency is most affected by shaft speed variation?

Answer: All bearing defect frequencies scale linearly with shaft speed

All four bearing defect frequencies (BPFO, BPFI, BSF, FTF) are directly proportional to shaft rotational speed, so any change in operating speed shifts all defect frequencies proportionally.

What is a natural frequency (also called resonant frequency)?

Answer: The frequency at which a system vibrates freely after an initial disturbance, determined by its mass and stiffness

A natural frequency is a property of the system itself (fn = (1/2π)√(k/m)) that determines the rate at which it oscillates freely; it depends only on mass and stiffness, not on the excitation.

What is aliasing in digital signal processing?

Answer: False frequencies appearing when the sampling rate is too low

Aliasing occurs when the sampling rate is insufficient (below twice the highest frequency of interest), causing high-frequency signals to appear as false low-frequency components.

What type of maintenance uses condition monitoring data?

Answer: Predictive maintenance

Condition monitoring data is the cornerstone of predictive maintenance. By analyzing trends in vibration, temperature, oil quality, and other parameters, maintenance teams can predict when a machine component is likely to fail. This allows for maintenance to be scheduled precisely when needed, optimizing resource allocation and preventing unexpected breakdowns, unlike reactive or corrective approaches.

Which of the following best describes acoustic resonance as it relates to machinery vibration?

Answer: Resonance of fluid columns in pipes or cavities excited by machinery vibration or pulsation

Acoustic resonance occurs when pressure pulsations from pumps, compressors, or fans excite natural frequencies of piping systems or cavities, causing excessive vibration, noise, and potential fatigue failures in the piping.

What is operational deflection shape (ODS) analysis, and how does it differ from modal analysis?

Answer: ODS shows actual deformation under real operating forces at specific frequencies; modal analysis shows theoretical mode shapes from controlled excitation tests

ODS shows how a structure actually moves under its real operating loads at specific frequencies or time instants, while modal analysis isolates the intrinsic dynamic properties (natural frequencies, mode shapes, damping) from controlled excitation tests.

A 4-pole, 60 Hz induction motor runs at 1,780 RPM. What is the pole pass frequency?

Answer: About 1.33 Hz

Slip frequency = (1,800 − 1,780)/60 = 0.333 Hz, and pole pass frequency = slip × poles = 1.33 Hz.

Across a flexible coupling, axial 1X phase readings differ by about 180 degrees and axial amplitude is high. What does this indicate?

Answer: Angular misalignment

Angular misalignment produces strong axial 1X with roughly 180-degree axial phase shift across the coupling.

During a coastdown, phase shifts by about 180° while amplitude peaks then falls. What does this indicate?

Answer: The machine passed through a resonance (natural frequency)

Passing through a natural frequency produces an amplitude peak with a 90° shift at resonance and about 180° total.

Which of the following best describes the function of a tachometer in vibration analysis?

Answer: It provides a once-per-revolution signal used to determine shaft speed and as a phase reference for synchronous measurements

A tachometer (optical, magnetic, or proximity type) generates a pulse once per shaft revolution, providing precise speed information and a phase reference essential for balancing, order tracking, and synchronous averaging.

In motor current signature analysis, broken rotor bars are indicated by which feature?

Answer: Sidebands around line frequency spaced at pole pass frequency

Broken bars modulate current, producing sidebands around line frequency at the pole pass frequency (slip × number of poles).

An overhung fan shows high 1X in both radial and axial directions, with axial phase readings in phase across both bearings. What is the most likely cause?

Answer: Overhung rotor unbalance

Overhung unbalance typically produces high 1X radial and axial vibration with in-phase axial readings.

What is the ISO 10816 standard's primary concern regarding bearing vibration?

Answer: Setting velocity-based vibration severity limits for overall machine vibration assessment

ISO 10816 (now ISO 20816) establishes overall vibration velocity severity limits for evaluating machine condition, providing zone classifications that guide maintenance decisions based on RMS velocity.

A pump base shows many running-speed harmonics plus 0.5x subharmonics, and the time waveform is erratic and non-repeating. What is the most likely cause?

Answer: Rotating looseness (Type C) such as an oversized bearing bore fit

Type C rotating looseness produces truncated, non-linear response with many harmonics and half-order components.

What is the purpose of a velocity transducer (seismic pickup) in vibration measurement?

Answer: To measure the absolute velocity of a machine casing using an internal mass-spring system with velocity coil output

A velocity transducer (geophone or seismic pickup) uses an internal mass-spring-damper system and electromagnetic coil to produce an electrical signal proportional to absolute casing velocity, without requiring external power.

In the early stages of rolling element bearing failure, defect-related energy typically appears first in which frequency range?

Answer: Ultrasonic frequencies (20 kHz and above), then high-frequency resonances

Early bearing defects generate very low-energy, high-frequency stress waves that first excite ultrasonic frequencies and bearing/structural natural frequencies; bearing defect frequencies in the low-frequency spectrum appear in later fault stages.

ISO 1940-1 has been superseded by which standard for rigid-rotor balance quality requirements?

Answer: ISO 21940-11

ISO 21940-11 replaced ISO 1940-1 for balance quality requirements of rotors with rigid behavior.

What is the API 670 standard, and what type of machinery protection does it address?

Answer: The American Petroleum Institute standard for machinery protection systems, specifying online vibration monitoring requirements for critical rotating equipment

API 670 defines requirements for continuous online machinery protection systems (proximity probe systems, vibration monitors, speed detectors) for critical rotating equipment in oil and gas plants, including alarm and shutdown logic.