Vibration Measurement & Analysis Techniques Flashcards

Two machines running at 29.5 Hz and 30.0 Hz produce a beating time waveform. What is the beat period?

Answer: 2 s

Beat frequency = 30.0 − 29.5 = 0.5 Hz, so the period is 1/0.5 = 2 s.

With Hanning windowing, what is the minimum resolution needed to clearly separate two peaks 1 Hz apart?

Answer: About 0.33 Hz or finer

Hanning's main lobe spans several bins, so separating two peaks usually takes about 3 bins between them, or ≤0.33 Hz resolution for 1 Hz spacing.

What is the main purpose of the analog anti-aliasing filter in a data collector?

Answer: To remove frequencies above the Nyquist limit before digitizing so they do not fold back as false peaks

Content above half the sample rate would otherwise appear as false lower-frequency peaks, and once the signal is digitized they cannot be removed.

A proximity probe system has a typical sensitivity of 200 mV/mil. What does it measure?

Answer: Shaft displacement relative to the probe mounting

Eddy-current probes measure the gap between the probe tip and the shaft, giving relative shaft displacement.

In an orbit plot with a once-per-revolution keyphasor mark, what is the most likely cause of a strongly flattened, elliptical 1× orbit?

Answer: Asymmetric support stiffness, preload, or misalignment

Unequal stiffness in different directions or a radial preload such as misalignment stretches the orbit into a narrow ellipse.

What is the main benefit of 50% to 67% overlap processing when averaging FFTs?

Answer: It shortens total data collection time for a given number of averages and makes better use of windowed data

Overlap reuses part of each time block, which cuts acquisition time and makes up for data the window tapers away.

In an impact (bump) test with an instrumented hammer, which window combination is normally used?

Answer: Force window on the hammer and exponential window on the response

The force window removes noise after the hammer pulse, and the exponential window makes the ringing response decay inside the record to limit leakage.