Ultrasonic diagnostics of couplings and shaft connections: early signs of failure

The coupling is the weak link between the motor and the load. Worn spider elements, cracks in the elastomer, loosening bolts — all of that develops over months. Vibration analysis often reacts late: the signal is masked by the mass of the gearbox or the foundation. Ultrasound captures micro-impacts and friction in the engagement zone much earlier.

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Why couplings are critical

On CHP plant pumps, conveyors and mining gearboxes the coupling compensates misalignment. As it degrades:

• the load on the bearings of both shafts grows;
• impact pulses appear in the gearbox teeth;
• an instant break with guard destruction becomes possible.

A shutdown to replace a coupling is planned; a break is an incident.

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What SDT hears

With a contact sensor on the coupling housing, the SDT340 registers:

Fault	Ultrasonic signature
Gear coupling tooth wear	pulses at the gear mesh frequency
Cracks in the elastomeric element	chaotic bursts, rising overall level
Loose bolts	periodic "knock" at 1× shaft speed
Misalignment	steadily elevated floor plus beating
Lack of lubricant (greased couplings)	dry friction, sharp rise in dBµV

Comparison against the baseline matters more than any absolute threshold.

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Route method

1. Measurement point — the coupling housing, as close as possible to the teeth / elastomeric block, not loose on the guard.
2. Mode — RMS plus a look at the TWF (time waveform) for impulses.
3. Load — measure under operating load; at no load the fault may not show up.
4. Trend — at least every 2–4 weeks on critical machines.

The SDT270 is fine for a quick screening; for archiving and LUBExpert use the SDT340.

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Combining with Artesis ESA

The coupling transmits impacts to the motor rotor. Artesis e-MCM sees the current modulation at running speed and its harmonics — an indirect indication of a coupling defect.

Recommended sequence:

1. Artesis — "amber" mechanical status with no clear motor bearing fault.
2. SDT — targeted measurement on the coupling and gearbox.
3. Localisation → planned replacement at the next window.

See SDT and Artesis integration.

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Typical assets in Kazakhstan

• Pump sets at water utilities and CHP plants — pin-and-bush elastomeric couplings.
• Mining and processing — gear couplings on crusher feeders.
• Metals — heavy couplings on rolling lines (add Bently vibration on the large machines).

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Related material

• Gearbox diagnostics
• Ultrasound at low speeds
• SDT340 for hydraulic systems
• Guide to ultrasonic inspection