1. Introduction
An emergency stop of the main drive of a technological line was recorded at one of the heavy machinery plants in Ukraine. The failure symptoms included a sharp metallic knocking in the power transmission zone, a rapid increase in the temperature of the coupling node housing to 85 °C, and tripping of the frequency converter protection due to motor overload. The planned mean time between failures (MTBF) for this node is 45 000 hours, whereas the actual operating time until the failure amounted to only 6 200 hours. This required an immediate engineering investigation to identify the root cause and prevent recurrence within the framework of industrial reliability standards.
2. Component Overview and Operating Conditions
Coupling connections and related control elements for energy transmission operate within critical kinematic chains. In close proximity to the drive, pneumatic positioning elements are used, including cylinders of the series Festo ADV-20-10-I-P, which provide accurate linear movement and fixation of the tooling with a working pressure up to 10 bar.
Operating conditions of the unit:
- Ambient temperature: from -10 °C to +45 °C.
- Maximum torque: 320 N·m (nominal — 210 N·m).
- Speed of rotation: 1450 rpm.
- Allowable axial displacement: ±1.5 mm; radial displacement: up to 0.2 mm; angular displacement: up to 1°.
According to the standards ISO 21940-11 and DSTU GOST ISO 10816-3, the vibration level of similar devices should not exceed 2.8 mm/s in the frequency range from 10 to 1000 Hz.
3. Failure Evidence
During disassembly of the node, the mobile engineering group recorded the following data:
- Visual inspection: Fretting corrosion traces were found on the surface of the elastic element and the half coupling, as well as deep fatigue cracks along the spline groove axis on the driven shaft.
- Geometry measurement: Laser alignment of the motor and reducer shafts showed radial offset of 0.65 mm and angular misalignment of 1.8°, which exceeds the manufacturer's tolerances three times.
- Vibration monitoring data: Anomalous increase of peaks at the rotational frequency (1x) and the second harmonic (2x), as well as high-frequency components in the range of 2.5–5 kHz, indicating impact loads from coupling link breakage.
4. Systematic Root Cause Investigation
To determine the chain of events that led to the failure, the Ishikawa diagram method and the 5 Whys analysis were applied.
- Why did the coupling fail? Due to the destruction of the torque transmission elements as a result of cyclic fatigue crack.
- Why did the stresses exceed the strength limit? Due to significant shaft deflection during operation.
- Why has there been a misalignment? The reducer foundation bolts have loosened due to vibration.
- Чому болти ослабли? Під час попереднього монтажу не було дотримано моменти затяжки за стандартом ISO 898-1.
- Чому момент затяжки не контролювався? На підприємстві був відсутній діючий графік перевірки зусилля затягування різьбових з'єднань із використанням каліброваного динамометричного інструменту.
5. Defined Root Causes
| Primary Cause | Probability (%) | Evidence base |
|---|---|---|
| Incorrect Shaft Alignment (Angular/Radial) | 45% | Laser alignment data (offset 0.65 mm instead of 0.2 mm max) |
| Torque overload cycle | 30% | Plastic deformation traces on keys, motor current consumption graphs |
| Mounting errors and absence of tension control | 25% | Loosened reducer mounting bolts, absence of torque control markings |
6. Corrective and Preventive Actions
A two-step action plan has been developed to address the identified defects:
- Immediate Repair (Emergency Maintenance): Removal of the damaged coupling. Performing precise laser alignment of the units according to ISO 5406 with a radius tolerance less than 0.05 mm. Replacement of fastening elements with strength class 10.9 using a torque wrench. Replacement of worn pneumatic control components, including cylinders Festo ADV-20-10-I-P, operating in a single technological cycle.
- Long-term prevention: Implementation of regular vibration control every 720 hours of operation. Training of personnel on the rules of working with alignment equipment according to the requirements EN 13306.
7. Express Diagnostic Map for Engineers on the Production Floor
This table should be used during weekly rounds with a tablet directly in the workshop:
- [ ] Temperature check: Measure the temperature of the coupling hub with a pyrometer (normal: up to +50 °C; warning: > +70 °C).
- [ ] Visual inspection of the elastomer: Look for cracks, rubber peeling or rubber dust formation under the protective cover.
- [ ] Bolt Tightness Check: Check the torque indicators on the reducer and motor base frame.
- [ ] Vibration Analysis: Remove vibration spectrum in axial and radial directions on bearing housings.
- [ ] Pneumatic network: Check the pressure stability (9-10 bar) at the input of the control pneumatic cylinders Festo ADV-20-10-I-P.
- [ ] Integrity protection: Ensure the absence of play in the splined connections during drive reversal.
8. Failure Prevention Strategy
Reliability of mechanical transmissions is achieved through a comprehensive approach to technical maintenance (RCM). It is recommended to establish the following maintenance intervals:
- Щомісяця: Перевірка центрування лазерними приладами при перших ознаках шуму.
- Щокварталу: Повна ревізія та змащування рухомих з'єднань, інспекція ущільнень.
- Annually: Non-destructive testing (magnetic particle inspection) of shafts for fatigue cracks near coupling fits.
9. Conclusion
Analysis showed that premature failure of the coupling was caused by the combination of initial angular misalignment of the shafts and loosening of the foundation bolts. Compliance with installation standards and diagnostic schedule allows to avoid unplanned production stoppages. For selection of original replacement parts and quick restoration of units, visit UNITEC-D E-Catalog.
10. Regulatory Basis and References
- ISO 21940-11: Mechanical vibration — Rotor balancing.
- DSTU GOST ISO 10816-3: Vibration. Evaluation of machine vibration by measurements on non-rotating parts.
- ISO 898-1: Mechanical properties of fasteners made of carbon steel and alloy steel.
- EN 13306: Maintenance — Maintenance terminology.
- ISO 5406: Rotors — Guidelines for balancing heavy industrial drives.
