Vibration Measurement & Analysis Techniques Flashcards

An amplitude increase of 20 dB corresponds to what linear amplitude ratio?

Answer: 10 times

For amplitude, dB = 20 log10(ratio), so 20 dB is a 10× increase.

On a Bode plot taken during a coast-down, what happens to phase as the rotor passes through a lightly damped critical speed?

Answer: Phase shifts about 90° at resonance and about 180° in total through it

For a single-degree-of-freedom response, phase lags by 90° at resonance and the total change is about 180° across the resonance.

Comparing horizontal and vertical phase on the same bearing gives about a 90° difference, and 1× radial vibration is dominant. This most strongly suggests:

Answer: Unbalance

Unbalance creates a rotating force, so horizontal and vertical phase differ by about 90° (±30°).

Axial phase readings taken across a flexible coupling differ by about 180°. This most likely points to:

Answer: Angular misalignment

Angular misalignment drives the two shafts axially out of phase, so axial phase readings across the coupling are about 180° apart.

Which analysis tool is designed to identify families of evenly spaced harmonics or sidebands in a complex spectrum?

Answer: Cepstrum

The cepstrum (inverse transform of the log spectrum) turns a repeating spacing into one peak at the matching quefrency.

Why use order tracking (constant-order sampling) on a machine whose speed changes during data collection?

Answer: To stop speed-related peaks from smearing across several frequency bins

Sampling at a fixed number of points per revolution keeps each order in a single bin even as RPM changes.

A 25-tooth pinion turns at 1,800 RPM. What is the gear mesh frequency?

Answer: 750 Hz

GMF = teeth × shaft speed = 25 × 30 Hz = 750 Hz.