Root Cause Analysis of Industrial Coupling Failures: Misalignment, Torque Overload and Material Fatigue

1. Introduction: Denial Syndrome Prompting Investigation

Unexpected shutdown of industrial equipment is a critical event that leads to significant financial losses related to production downtime and repairs. One of the most common, but often underestimated, causes of such incidents is clutch failure. Couplings, as connecting elements between the driving and driven shafts, play a key role in the transmission of torque and the compensation of certain inconsistencies. However, they are prone to different failure modes that can be triggered by different factors.

Typical signs of potential coupling failure include increased levels of vibration, abnormal noise, increased operating temperature of the bearings adjacent to the coupling, or visible mechanical damage. Ignoring these symptoms inevitably leads to the progression of the defect, culminating in a catastrophic breakdown. This root cause analysis is devoted to the study of the three main failure modes of couplings: misalignment, torque overload, and fatigue failure, with an emphasis on their identification, diagnosis, and prevention methods that meet DSTU, EN, and ISO standards.

2. Component Overview: Industrial Couplings and Safety Systems

An industrial coupling is a vital mechanical component designed to connect two shafts, transmit torque, compensate for small shaft irregularities, and absorb shock loads and vibrations. Depending on the design and application, couplings are divided into rigid, elastic, toothed, chain and hydrodynamic. Each type of coupling has its own specific characteristics regarding the compensation of radial, axial and angular misalignment, as well as the ability to transmit maximum torque.

Couplings usually operate under continuous cyclic loading, with operating speeds that can vary from a few tens to several thousand revolutions per minute, and torque transmission from a few N·m to tens of kN·m. The operating temperature range is usually from -20°C to +80°C, although specialized couplings can function in more extreme conditions. For example, couplings used in the metallurgical industry can withstand temperatures up to +150°C. The typical service life (MTBF) of a quality coupling, subject to the conditions of operation and regular maintenance, exceeds 50,000 hours.

In the context of the safety of machines where clutches function, specialized devices are used. The Telemecanique XPSCMP5144P component (obsolete equivalent of the XPSCEP51419) is a multifunctional Preventa safety module designed to monitor safety functions such as emergency stops, guardrails and light barriers. Although this module is not directly a coupling, it plays a critical role in safely shutting down the equipment when hazardous conditions are detected, which may be caused by, for example, mechanical failures, including coupling damage. Its compliance with EN ISO 13849-1 and IEC 61508 standards ensures integration into systems with a high level of safety integrity (SIL), which is necessary for equipment operated in Ukraine according to the requirements of DSTU EN 60204-1.

3. Evidence of Refusals: On-Site Diagnostics

Effective diagnosis of coupling failures begins with careful collection of visual, acoustic and measurement data. The technician must be equipped to perform instrumental monitoring and symptom analysis.

3.1. Misalignment

Misalignment of shafts is one of the leading causes of coupling failures. It occurs when the axes of the driving and driven shafts do not coincide. There are three main types of skew: angular, parallel and combined.

• Visual Evidence: Abnormal wear of the coupling surface, especially around the edges of elastic elements or teeth. Lubricant leakage from the bearing seals located near the coupling. Color coating on metal parts indicating overheating (for example, blueing of steel at >300°C).
• Vibration Diagnostics: Analysis of the vibration spectrum (according to ISO 10816-3) often shows increased amplitudes at frequencies of 1x, 2x, 3x rotational speed. Parallel skew is characterized by high amplitudes at 2x the rotational speed, while angular skew is at 1x. Example: Vibration of more than 4.5 mm/s (RMS) in the range from 10 Hz to 1 kHz at a rotation frequency of 1500 rpm can indicate serious skew in industrial pumps.
• Temperature Control: Thermographic images (performed according to EN 13306) show localized overheating on the bearings and gearbox housings adjacent to the coupling, with an excess of 15-25°C above the normal operating temperature (eg 85-95°C at the norm of 70°C).
• Noise: Characteristic buzzing, knocking or grinding that changes depending on the load.

3.2. Torque Overload

Exceeding the allowable torque can lead to instant or rapid destruction of the clutch and other transmission components.

• Visual Evidence: Cut bolts, keys or slots. Deformation or destruction of elastic elements of the coupling (for example, rupture of rubber inserts). Cracks or complete failure of the metal parts of the coupling. Signs of slippage, such as polished areas on shafts or bushings.
• Indicators of Safety Devices: Activation of safety mechanisms such as automatic engine breakers or activation of the Telemecanique XPSCMP5144P safety module indicating an emergency condition. For a 75 kW motor, tripping at a peak current of 2.5 times the rated current may indicate a sudden jam or overload.
• Acoustic Evidence: A sudden loud metallic bang or crack during an accident.

3.3. Fatigue Cracking

Fatigue failure is the result of cyclic loads acting for a long time, even if the amplitude of these loads is below the yield strength of the material. This failure mode is insidious because it has no obvious early symptoms until critical crack propagation.

• Visual Evidence: Microcracks on the surface of the coupling that spread over time. Signs of fretting (corrosion-fatigue wear) in contact areas. A change in the appearance of the material (for example, a change in color, the appearance of shells). Detection of cracks often requires the use of non-destructive testing methods (NDC) in accordance with DSTU EN ISO 17637.
• Non-destructive testing:
  • Magnetic powder testing (MPK): According to ISO 9934-1, detects surface and subsurface cracks on ferromagnetic materials.
  • Ultrasonic inspection (UZK): According to ISO 17640, allows detection of internal defects and propagating cracks.
  • Capillary Control (QC): According to ISO 3452-1, used to detect surface defects.
• Vibration Diagnostics: Although fatigue does not always have a clear vibrational "signature" in the early stages, as the crack propagates, an increase in broadband vibration can be observed, especially at high frequencies, which is associated with a change in component stiffness.

4. Investigation of Root Causes

To effectively eliminate coupling failures, a system analysis must be performed to identify the root causes, not just the effects. The "5 Whys" methodology and elements of failure tree analysis are used.

4.1. Root Causes of Skewness

1. Why did the misalignment occur?
   • The first reason: Wrong centering of the shafts during installation.
     • Why was it incorrectly centered? Insufficient qualification of personnel or lack of accurate measuring tools (for example, laser centering systems).
     • Why are tools/skills missing?Insufficient investment in training or equipment.
   • The second reason: Deformation of the foundation or supporting structure of the equipment.
     • Why the deformation? Insufficient stiffness of the foundation, soil settlement or uneven thermal expansion.
   • The third reason: Wear of bearings, which leads to displacement of shafts.
     • Why the wear? Insufficient lubrication, contamination, overload or fatigue of the bearing material.
   • The fourth reason: Thermal deformation of the equipment during operation.
     • Why the deformation? Temperature difference between cold start and operating mode, lack of temperature compensation during centering.

4.2. Root Causes of Torque Overload

1. Why did the overload occur?
   • The first reason: Locking or jamming of the driven mechanism.
     • Why blocking? Ingress of a foreign object, failure of the working body, lack of lubrication.
   • Second reason: Sudden load changes or shock loads in the process.
     • Why sudden changes? Instability of the technological process, improper operation, lack of damping systems.
   • Third reason: Incorrect choice of coupling for the given torque.
     • Why the wrong choice? Underestimation of peak loads or safety factors during design.
   • The fourth reason: Errors in the engine management system, which lead to an uncontrolled increase in torque.
     • Why the errors? Incorrect settings of the inverter, failure in the PLC or sensors.

4.3. Root Causes of Fatigue Failure

1. Why did the fatigue failure occur?
   • The first reason: Long-term cyclic loading exceeding the endurance limit of the coupling material.
     • Why does it exceed? Underestimation of actual working loads, incorrect choice of material or coupling design.
   • Second reason: The presence of stress concentrators (sharp corners, scratches, surface defects).
     • Why hubs? Poor surface finish, damage during installation or transportation, corrosion damage.
   • Third reason: Corrosive environment that accelerates fatigue failure (corrosion fatigue).
     • Why corrosion? Insufficient protection of the coupling from an aggressive environment, failure of seals.
   • Fourth reason: Defects in the coupling material (inclusions, pores).
     • Why the defects? Low-quality manufacturing of the component, violation of heat treatment technology.

5. Identified Root Causes and Evidence

Based on a systematic investigation, the following root causes of coupling failures were identified, ranked by probability and supported by available evidence:

1. Incorrect alignment of shafts (High probability): Insufficient qualification of personnel (unconducted courses on laser alignment), absence or malfunction of calibrated measuring tools (last calibration 2 years ago), deformation of the foundation (measurements with a geodetic level showed a difference of 3 mm per 1 meter).
2. Bearing wear (Medium probability): The results of vibration diagnostics (ISO 10816-3) revealed an increase in vibration at frequencies characteristic of bearings, and thermal imaging control recorded local overheating up to 95°C.
3. Sudden load changes/shock loads (Medium probability): The analysis of event logs of ACS showed frequent cases of exceeding technological parameters (pressure up to 12 bar instead of 8 bar) or "jamming" equipment starts (2-3 cases per month).
4. Incorrect Coupling Selection (Medium Probability): A check of the technical documentation revealed that the selected coupling has a margin factor of 1.25, while a minimum of 1.5 is recommended for this type of equipment and process.
5. Material Defects/Stress Concentrators (Low Probability): Visual inspection after disassembly revealed scratches and micro-cracks that were not detected during the incoming inspection. Metallographic analysis can confirm the presence of internal defects.

6. Corrective Measures

Corrective measures are divided into immediate and long-term, aimed at eliminating the identified root causes.

6.1. For Skew

• Immediate Remedy:
  • Perform exact centering of the shafts using a laser system corresponding to accuracy class 1 (according to ISO 15243) with a tolerance of 0.05 mm/m. Document the results.
  • Replace damaged bearings and seals.
• Long-term Prevention:
  • Include an alignment check as part of scheduled maintenance (for example, every 2000 hours of operation).
  • Organize staff training on laser centering of shafts (minimum 20 hours of practical classes).
  • Develop a procedure for periodically checking the rigidity of the foundation and its stability.

6.2. For Torque Overload

• Immediate Remedy:
  • Replace the coupling with a similar one or with an increased safety margin (for example, with improved elastic elements).
  • Check and adjust motor protection parameters (current, thermal).
• Long-term Prevention:
  • Conduct an audit of the technological process to identify the causes of shock loads. Implement soft start systems or damping devices.
  • Review the method of choosing couplings, taking into account peak loads and dynamic characteristics of the system. Consult the UNITEC-D E-Catalog to select couplings with high safety margins and CE certification.
  • Check the settings of the Telemecanique XPSCMP5144P safety module for a correct response to emergency situations according to EN ISO 13849-1.

6.3. For Fatigue Destruction

• Immediate Remedy:
  • Replace the clutch with a new one. Carry out a thorough incoming inspection to exclude hidden defects.
  • If possible, perform NC of adjacent components (UZK, MPK).
• Long-term Prevention:
  • Review the coupling material, choosing an alloy with a higher endurance limit or improved anti-corrosion properties.
  • Implement regular non-destructive testing (USC or IPC) of critical couplings every 6 months.
  • Provide adequate protection of the coupling from aggressive environments (for example, protective casings, special coatings).
  • Analyze the duty cycle of loads and, if necessary, redesign or modify the system to reduce cyclic loads.

7. Quick Diagnostic Checklist for a Technician (Tablet-Friendly)

The following checklist is intended for use by field technicians using a tablet device. It will help to quickly identify potential problems with the couplings.

1. Visual Inspection of Couplings: Check for visible cracks, deformations, chips, oil leaks. (Red flag: any damage).
2. Surface wear: Evaluate the nature of wear on the working surfaces of the coupling. Asymmetric wear indicates misalignment.
3. Body Temperature: Measure the temperature of the bearing assemblies near the coupling using a pyrometer. (Red flag: >80°C or >15°C above background).
4. Acoustic Control: Listen to the operation of the coupling with a stethoscope. Are there unusual noises (buzzing, knocking, grinding)? (Red flag: any new or increased noise).
5. Vibration Measurement: Use a vibrometer to measure vibration on the coupling and adjacent bearings. Record the overall vibration level (mm/s RMS). (Red flag: >2.8 mm/s for Class II according to ISO 10816-3).
6. Fastening Check: Check the tightness of all coupling fasteners. Are there loose bolts or sheared keys?
7. Color Scale/Oxidation: Inspect metal parts for discoloration indicating overheating or corrosion.
8. Condition of Elastic Elements: For elastic couplings, check the condition of rubber or polyurethane inserts. Are there cracks, hardening, deformations?
9. Defense response: Check the event log of the management system. Have protection devices (eg Telemecanique XPSCMP5144P) tripped recently? (Red flag: unauthorized protection activation).
10. Service History: Check previous service records. When was the last alignment performed?

8. Failure Prevention Strategy

An effective clutch failure prevention strategy is based on an integrated approach that includes scheduled preventive maintenance, condition monitoring, and design improvements.

• Regular Maintenance:
  • Shaft Centering: Carry out laser centering of the shafts at least once every 6-12 months or after any repair that involves dismantling the equipment. Centering tolerances should be in accordance with the manufacturer's recommendations or the ISO 1940-1 (balancing) standard.
  • Replacement of Worn Components: Regular replacement of the elastic elements of the couplings according to the manufacturer's recommendations, regardless of the apparent absence of damage, to avoid material fatigue.
  • Check Fasteners: Periodic check and tightening of fasteners.
• Condition Monitoring:
  • Vibration Diagnostics: Implementation of permanent or periodic vibration monitoring (for example, monthly) with spectrum analysis for early detection of distortions and wear of bearings. Use vibration analyzers certified by DSTU EN 61672.
  • Thermographic Control: Regular thermal imaging examination of bearing units and the coupling itself to detect overheating.
  • Acoustic Control: Application of ultrasonic flaw detectors to detect hidden cracks and defects in the early stages.
• Design Improvements and Selection:
  • Coupling Selection: Use couplings with an adequate safety margin and the ability to compensate for possible installation inaccuracies. For critical applications, choose couplings certified according to CE and UkrSEPRO international standards. UNITEC-D E-Catalog offers a wide range of such couplings.
  • Materials: Selection of couplings from materials resistant to fatigue and corrosion, especially in aggressive environments.
  • Foundation: Ensuring rigidity and stability of the foundation to minimize deformations.

9. Conclusion

Failures of industrial couplings are a significant problem affecting the reliability and efficiency of production processes. A systematic approach to root cause diagnosis, covering misalignment, torque overload, and fatigue failure, allows effective prevention strategies to be developed. The integration of modern methods of condition monitoring, regular maintenance and correct component selection is key to minimizing downtime and optimizing operating costs. Application of these practices ensures long-term, reliable and safe operation of industrial equipment.

For high-quality couplings and other MRO components that meet the highest standards of reliability and safety, visit the UNITEC-D E-Catalog.

10. Links

• DSTU EN 60204-1:2018 (EN 60204-1:2018, IDT) Machine safety. Electrical equipment of machines. Part 1. General requirements.
• EN ISO 13849-1:2015 Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design.
• ISO 10816-3:2009 Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts — Part 3: Industrial machines with nominal power above 15 kW and nominal speeds between 120 r/min and 15,000 r/min when measured in situ.
• ISO 1940-1:2003 Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state — Part 1: Specification and verification of balance tolerances.
• ISO 9934-1:2016 Non-destructive testing — Magnetic particle testing — Part 1: General principles.
• ISO 17640:2018 Non-destructive testing — Ultrasonic testing of welds — Techniques, testing levels, and assessment.
• ISO 3452-1:2021 Non-destructive testing — Penetrant testing — Part 1: General principles.
• EN 13306:2017 Maintenance — Maintenance terminology.