Application note | Rotating machinery
Rotating Axle Vibration Analysis with Single-Beam LDV
A single-beam Ommatidia Qmini measured a rotating drill axle without physical contact. The measurement reveals the main running-speed response near 29 Hz, a secondary component near 21 Hz, and harmonic content that helps engineers pinpoint rotating-system behaviour.
Why it matters
Measure shaft vibration without mounting a sensor on the rotating part
This is where laser Doppler vibrometry is useful. The Qmini captures velocity optically when contact sensors are impractical or would change the response. Use it on shafts, spindles, couplings, hot surfaces, energized equipment, and other targets that should remain untouched during the test.
No contact
Keep the test article unchanged
The laser reads velocity without sensor mass loading, adhesive preparation, or cabling on the rotating component.
Faster diagnosis
See more than a single running-speed peak
The measurement exposes the main response, its harmonics, and mixed-frequency terms that help separate simple rotation from nonlinear behavior.
Use case
Well suited to rotating machinery troubleshooting
Use it for targeted checks on shafts, bearings, couplings, small motors, and other components where access is limited or contact sensing is undesirable.
Measurement setup
A domestic drill axle measured with the Ommatidia Qmini
The Ommatidia Qmini observes one point on the rotating axle, capturing data ready for standard vibration-analysis workflows. It shows how a focused non-contact measurement can turn a difficult-to-reach target into a clear diagnostic signal.
Measured result
The time trace shows a Doppler offset, and the FFT shows the machine signature
Over a 10-second interval, the measured velocity stays close to a positive mean value instead of oscillating around zero. That offset comes from the measurement geometry: the beam captures the desired vibrational motion plus a projected component of the axle’s tangential velocity. Once that bias is understood, the remaining oscillatory content can be read as the shaft vibration itself. For readers less familiar with spectral analysis, this is the same type of frequency decomposition typically associated with an FFT.
Measurement readout
The plot shows a time-domain trace with a positive Doppler offset, a dominant peak near 29 Hz, a secondary component near 21 Hz, and a harmonic-plus-combination pattern that points to nonlinear rotating-system behavior.
Dominant peaks
Two main components appear at approximately 21 Hz and 29 Hz, with the 29 Hz peak showing the larger amplitude.
Harmonics
Integer multiples of the dominant frequency are visible, including 2f2, 3f2, 4f2, and 5f2, with weaker harmonics of f1 also present.
Combination terms
Mixed components such as f2 minus f1, f2 plus f1, and higher-order combinations indicate interaction inside the mechanical system rather than a single clean tone.
Geometry
Why the time trace has a positive offset
If the laser beam is not perfectly aligned with the local shaft radius, the line of sight includes both the vibration velocity and a projection of the tangential surface speed. In compact form, the measurement can be written as vLDV = vvib + vtan cos(theta). That tangential term stays nearly constant here and appears as a Doppler bias in the time trace.
Interpretation
The spectrum shows more than a simple running-speed tone
Harmonics and frequency sums and differences reveal interaction between periodic sources. Use them to focus the next inspection: imbalance, misalignment, clearance, or secondary rotating dynamics.
Repeat the measurement at different operating speeds to separate rotation-locked effects from structural resonances and drivetrain interactions.
Measurement considerations
Set the measurement geometry before interpreting the result
Projection matters
The measured velocity is the component of motion projected onto the laser beam, so alignment changes the reading.
Surface optics matter too
Speckle, reflectivity, and spot placement affect amplitude stability and spectral clarity on rotating cylindrical surfaces. See how SpeckleGuard minimizes noise.
One beam is one component
A single-beam setup cannot separate radial, tangential, and axial motion by itself. Multiple synchronized views are needed for full vector reconstruction.
Connected reading
Related applications and next steps
This drill-axle example sits at the focused, single-point end of the workflow. If you need product positioning, broader motor diagnostics, or a refresher on the measurement principle, these pages are the natural next click.
Product fit
Compare the Q range
See where Qmini fits relative to the multi-point and higher-channel platforms.
Industrial example
See the WEG motor work
For a broader industrial case, see how contactless vibrometry is applied to motor and transformer diagnostics in the field.
Measurement principle
Refresh the Doppler basics
Useful if you want a quick technical refresher on Doppler-based vibration measurement.
Larger motor studies
Go from point data to spatial maps
When the question moves from one point to full motor behavior, this case study is the next scale up.
Need a non-contact measurement plan for rotating machinery?
Bring us the target geometry, operating-speed range, access constraints, and failure mode. We’ll recommend the measurement approach and the right single-beam or multi-point workflow.
