Ultrasound at low speeds: how to diagnose a gearbox running at 0.1 rpm

Why vibration cannot see slow machines

Low-speed equipment — decanters, conveyor drives, cement mill gearboxes, rolling mill sections — is notoriously difficult for vibration analysis. Signal energy is low, the shaft revolution period is long, and a confident FFT needs a skilled analyst and a long recording.

Ultrasonic monitoring does not measure shaft speed directly; it measures friction and impacts, which are present at any speed. That is why SDT recommends recording at least three full revolutions of the asset for accurate measurements.

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Case: wastewater decanter at a pulp and paper mill

At a pulp and paper mill the suspects were either the pinion (2.95 rpm) or the bull gear (0.1 rpm) in the wastewater decanter drive.

Drive kinematics

Assembly	Speed
Electric motor	1785 rpm
Primary gearbox (belt plus planetary, 52.1:1)	34.26 rpm
Secondary gearbox (11.6:1)	2.95 rpm (pinion)
Bull gear	0.1 rpm

Thanks to recordings of up to 10 minutes on the SDT340, at least one full pinion revolution was captured — ideally SDT recommends three, but here one was enough for the diagnosis.

What the data showed

The time waveform (TWF) shows a characteristic impulse once per revolution of the faulty tooth — at the moment it tries to engage with the bull gear. The interval between impacts matches 1/RPM of the pinion.

The typical broken-tooth pattern:

• a distinct peak in the TWF with a period equal to the rotation period of the faulty gear;
• in the FFT — an elevated ultrasonic level at 1×RPM of the affected shaft.

Conclusion: a broken tooth on the 2.95 rpm pinion. A planned shutdown instead of sudden gearbox destruction.

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Practical recommendations for maintenance teams

1. Do not drop vibration — but below 10 rpm add ultrasound to the route.
2. Increase recording time — 0.1 rpm needs minutes, not seconds.
3. Look at the TWF, not just the overall level — one impulse per revolution is more reliable than an average dBµV.
4. Trend it — comparison with a baseline beats a one-off measurement.

Where this matters in Kazakhstan

• Pulp, paper and packaging — decanters, drying drums
• Mining — conveyor and crusher drives
• Water utilities — pump sets and mixers
• Metals — slow sections of rolling mills

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Related material: SDT340 + LUBExpert · Gearbox diagnostics · SDT340 catalogue