1. Introduction
An unexpected shutdown of a critical production line in a well-known DACH production company led to the immediate initiation of a forensic investigation. The failure manifested itself as loud, intermittent noises, followed by massive vibrations and the sudden stop of the system. The primary source of failure was quickly narrowed down to the connection between an electric motor and a hydraulic pump within a central hydraulic unit (HPU). This HPU supplies, among other things, a HYDAC SB50LT/112/FOR pressure accumulator, which is responsible for pulsation damping and energy storage in the hydraulic circuit.
The investigation focused on the clutch, an essential but often underestimated component whose integrity is critical to the smooth operation of rotating machinery. The aim of this analysis is to examine in detail the specific causes of failure - misalignment, torque overload and material fatigue - and to derive preventative strategies that prevent future failures.
2. Component overview
The affected coupling served as a connecting element between a 55 kW electric motor (nominal speed 1480 rpm) and an axial piston pump designed for a system pressure of up to 250 bar. This arrangement is typical for hydraulic drive systems in heavy industry, where high pressures and continuous load changes occur. The HYDAC SB50LT/112/FOR bladder accumulator, with a nominal volume of 50 liters and a maximum operating pressure of 250 bar, is integrated into this system to stabilize the hydraulic pressure and cover peak loads.
Couplings in such applications are subject to constant stress from torque transmission, axial and radial forces and vibrations. Their main function is to reliably transmit torque while compensating for minor axial, radial and angular shaft displacements. The coupling was selected in accordance with the recommendations of the VDI 2206 “Construction Methodology for Mechatronic Systems” and DIN 28180 “Couplings for Use in Hazardous Areas”. Although the specific application was not subject to ATEX, increased safety standards were implemented.
The coupling installed was a highly elastic claw coupling with a polyurethane elastomer as a transmission element. This type of coupling is known for its ability to dampen shock and vibration while tolerating a certain degree of misalignment.
3. Damage patterns and evidence
The first inspection after the standstill revealed catastrophic damage to the clutch:
- Visual inspection: Both coupling halves showed severe deformation and cracking. The polyurethane elastomer was completely destroyed, with fragments scattered throughout the clutch housing and surrounding area. Clear traces of fretting corrosion were visible on the shaft ends and the coupling hubs, particularly in the area of the keyways. The keys themselves were sheared off and severely deformed.
- Vibration Analysis: Recorded condition monitoring data for the last few weeks prior to the failure revealed increased vibration levels. One week before the total failure, the RMS value of the vibration speed on the motor mount rose from 2.8 mm/s to over 7.2 mm/s, which significantly exceeded the limits of Zone B according to DIN EN ISO 10816-3 for machines with operating speeds between 120 rpm and 15,000 rpm. Frequency spectral analysis showed dominant peaks at 1x and 2x the rotation frequency (approximately 24.7 Hz and 49.4 Hz), a clear indication of misalignment.
- Thermographic investigation: Retrospective analysis of the infrared images showed local overheating of the coupling and the adjacent bearings as early as 72 hours before the failure. Temperatures in excess of 105°C have been measured on the coupling surface, while the operating temperature should not exceed 60°C under normal conditions. This indicates increased friction and energy loss.
- Shaft Position Measurements: Although laser alignment was not performed prior to failure, measurements at the shaft ends after coupling removal showed significant axial and radial misalignments. The radial offset was 0.8 mm and the angular offset was 0.05 mm/100 mm, well outside the allowable tolerances of typically 0.05 mm radial and 0.01 mm/100 mm angular for precision industrial applications.
- Operating data analysis: The evaluation of the process control system data showed several load peaks shortly before the failure that exceeded the coupling's nominal torque of 350 Nm by up to 150%. These spikes occurred in connection with pressure surges in the hydraulic system, possibly caused by a pressure relief malfunction or unusual load requirements.
4. Root cause analysis
To systematically determine the root causes, a combination of the 5 Why method and a fault tree analysis was used. The investigation focused on the three primary suspected cases: misalignment, torque overload and material fatigue.
4.1 Misalignment
Why 1: The clutch has failed. Why 2: The coupling exhibited excessive wear and breakage of the elastomer. Why 3: Excessive radial and angular loads resulted in material fatigue and overheating. Why 4: The motor and pump shafts were not aligned correctly. Why 5: The last alignment was five years ago and has not been regularly reviewed; In addition, no laser alignment was used, only a straightedge method. The installation was carried out without sufficient consideration of thermal expansion and foundation settlement.
Evidence: Dominant vibration frequencies at 1x and 2x speed, fretting corrosion, uneven wear, high temperatures.
4.2 Torque overload
Why 1: The clutch has failed. Why 2: The elastomer was sheared and the keys were sheared off. Why 3: The transmitted torque exceeded the rated capacity and shock capacity of the clutch. Why 4: Unforeseen load peaks occurred in the hydraulic system. Why 5: A combination of incorrect setting of the pressure relief valve and a temporary blockage of the consumer led to extreme pressure peaks that overloaded the system. The coupling may have been undersized for the maximum possible load of the system under adverse conditions.
Evidence: Sheared keys, plastic deformation of the coupling hubs, operating data with high load peaks.
4.3 Material fatigue
Why 1: The clutch has failed. Why 2: There were visible cracks in the metal hubs of the clutch. Why 3: The cracks were caused by cyclic loading far below the yield point of the material. Why 4: The cyclic stress was increased by a combination of misalignment and recurring torque overloads, resulting in premature fatigue. Why 5: The initial misalignment resulted in permanent bending stresses, which resulted in rapid crack initiation and propagation due to the periodic torque overloads. The service life of the clutch has been massively reduced.
Evidence: Macroscopic and microscopic examination of the fracture surfaces (beach marks, crack propagation) that indicate fatigue fracture.
5. Identified root causes
Based on the evidence collected and root cause analysis, the following root causes were identified and assessed for their likelihood and impact on the failure:
- Misalignment (High probability, primary cause): The severe misalignment of the motor and pump shafts was the primary cause. Inadequate alignment during installation and lack of regular inspections resulted in increased radial and axial forces on the coupling and adjacent bearings. This manifested itself in increased vibration (above 7.2 mm/s RMS), local overheating (>105 °C) and accelerated fatigue of the coupling material. Fretting corrosion at shaft ends confirms relative movement under load caused by misalignment.
- Torque overload (average probability, secondary cause/trigger): The documented load peaks, which reached up to 150% of the coupling's nominal torque, resulted in the elastic limit of the coupling element and the keys being exceeded. Although the coupling was intended to tolerate short-term overloads, the combination with the existing misalignment resulted in cumulative damage. The sheared keys and plastic deformation are direct indicators of torque overload.
- Material fatigue (high probability, consequence of primary causes): The macroscopically visible cracks and the microscopic analysis of the fracture surfaces confirmed fatigue cracks. These were initiated and propagated by the cyclic stresses resulting from the combination of misalignment (alternating bending moments) and the recurring torque overloads. Clutch life has been drastically shortened, well below the expected MTBF of 20,000 to 50,000 hours of operation.
6. Corrective Actions
To prevent future failures and ensure operational reliability, the following corrective actions have been implemented:
6.1 Immediate measures
- Coupling replacement: The defective coupling was replaced by a certified spare part that meets the requirements of DIN EN ISO 14640 “Elastic Couplings” and has a higher service factor design.
- Precise shaft alignment: Immediate laser alignment of the motor and pump, taking thermal expansion coefficients into account. The residual deviation was set to less than 0.02 mm radially and 0.01 mm/100 mm angularly, which corresponds to the recommendations of VDI 3834 “Shaft Couplings – Selection, Calculation, Application”.
- Hydraulic system inspection: Comprehensive inspection and calibration of all pressure relief valves in the hydraulic system, especially those of the HYDAC SB50LT/112/FOR and the pump, to eliminate uncontrolled pressure peaks.
6.2 Long-term prevention
- Regular condition monitoring: Introduction of continuous vibration monitoring (according to DIN EN ISO 10816-3) with automated alarms when defined limit values are exceeded. In addition, thermographic inspections at quarterly intervals.
- Standardized alignment procedures: Implementation of a strict procedure for coupling alignment during every installation and after every major maintenance, using only laser alignment systems (e.g. Pruftechnik ROTALIGN or SKF TKSA series).
- Coupling sizing review: Recalculation of the required coupling size taking into account the actual load profile and an increased service factor (at least 1.8). Potential process disruptions and start-up moments must also be taken into account.
- Staff training: Conduct regular training for maintenance technicians in the areas of shaft alignment, vibration analysis and fault detection on couplings.
- Documentation and traceability: Introduction of a digital maintenance management system for complete documentation of installations, maintenance work, alignment protocols and replaced components.
7. Quick Diagnostic Checklist for Technicians
This checklist serves as a practical guide for on-site maintenance technicians to recognize early signs of clutch failure and take preventive action:
- Visual inspection: Check for cracks, discoloration, deformation or material abrasion on the coupling and shaft ends. (Red Flag: Visible cracks or loss of material)
- Hearing test: Listen for unusual noises (grinding, knocking, squeaking, howling). (Red Flag: Metallic rubbing or intermittent knocking)
- Temperature measurement: Measure the surface temperature of the coupling and adjacent bearings with an IR thermometer (setpoint: < 70 °C). (Red Flag: temperature rise above 80 °C)
- Hand check for play: Machine at standstill, turn clutch by hand and check for excessive play or looseness.
- Check fastening elements: Visual inspection and, if necessary, torque check of the fastening screws and keys. (Red Flag: Loose or missing screws, visible deformation of the keys)
- Vibration analysis (if available): Review of current vibration data compared to baseline values. (Red Flag: Exceeding the alarm limit according to DIN EN ISO 10816-3, e.g. > 4.5 mm/s RMS)
- Oil/hydraulic system check: Check for leaks or unusual smells around the HPU, especially on the HYDAC SB50LT/112/FOR and the pump. (Red Flag: Significant oil losses or emulsion formation)
- Alignment check (coarse): Use a ruler or straightedge to check the shaft ends for gross misalignment. (Red Flag: Visible gap or offset)
- Check load profiles: Short-term check of the current load and pressure values in the process control system. (Red Flag: Frequent or extreme load peaks)
- Maintenance History: Check last alignment record and clutch service date.
8. Prevention strategy
A comprehensive prevention strategy is crucial to optimize the reliability and service life of machine components. It is based on three pillars:
8.1 Maintenance intervals and planning
- Periodic maintenance: Coupling inspections and maintenance must be integrated into the maintenance planning in accordance with the manufacturer's instructions and the recommendations of VDI 2206. For critical systems, this should be done at least annually.
- Laser Alignment: Precise laser alignment should be performed every 2-3 years or every time the coupled machines are dismantled, even if there are no obvious problems. Compliance with the tolerances according to DIN ISO 21940-11 “Mechanical vibrations – shaft alignment” is essential.
- Spare parts management: Provision of certified replacement couplings and elastomers that meet the quality standards according to DIN EN ISO 9001.
8.2 Condition monitoring
- Continuous vibration monitoring: Installation of permanent vibration sensors on motors and pumps in order to detect deviations from the standard values according to DIN EN ISO 10816-3 at an early stage.
- Thermography: Regular thermographic scans to detect overheating on couplings, bearings and hydraulic components (e.g. on the HYDAC SB50LT/112/FOR).
- Operating Data Analysis: Continuous analysis of torque, pressure and speed profiles to identify load peaks and abnormal operating conditions.
8.3 Design improvements and material selection
- Selection of the service factor: When selecting replacement couplings, a higher service factor must be selected, which takes into account not only the nominal torque, but also starting torques, load peaks and the operating environment (e.g. temperature, shock load).
- Material quality: Use of couplings made of high-quality, certified materials that have high fatigue strength. When it comes to elastomers, attention must be paid to their resistance to operating temperatures and media.
- Protection devices: Testing the integration of torque limiters or overload protection devices to protect the clutch from extreme torque overloads.
9. Conclusion
The clutch failure in the hydraulic drive system examined was the result of a complex interaction of misalignment, torque overload and resulting material fatigue. This research highlights the critical importance of precise installation, regular maintenance and effective condition monitoring.
Compliance with relevant technical standards such as DIN EN ISO 10816-3 for vibration monitoring, VDI 2206 for construction methodology and DIN ISO 21940-11 for shaft alignment is not just a recommendation, but a necessity for the safe and efficient operation of industrial systems. Proactive action and investment in qualified personnel and modern diagnostic technologies are essential to avoid unplanned downtime and reduce total cost of ownership.
For certified spare parts, preventative maintenance components and high-quality shaft alignment tools, consult the UNITEC-D E-Catalog.
10. References
- DIN EN ISO 10816-3: Mechanical vibrations - Assessment of machine vibrations through measurements on non-rotating parts - Part 3: Industrial machines with nominal powers over 15 kW and nominal speeds between 120 rpm and 15,000 rpm when mounted on rigid or flexible foundations.
- VDI 2206: Construction methodology for mechatronic systems.
- VDI 3834: Shaft couplings – selection, calculation, application.
- DIN ISO 21940-11: Mechanical vibrations - Balancing of rotors - Part 11: Tolerances for balancing rotors in a rigid state.
- DIN 28180: Couplings for use in potentially explosive areas.
- Niemann, G., Winter, H.: Machine elements - Volume 2: General machine elements, Berlin/Heidelberg: Springer-Verlag.
- TÜV guidelines for machine safety and risk assessment.
- Manufacturer documentation HYDAC SB50LT/112/FOR bladder accumulator.
