Root Cause Analysis 1 min read

Analysis of the root causes of the spalling of the key joint: load, deflection and fatigue under cyclic loading

Technical analysis: QV-06/24/60

Аналіз кореневих причин виникнення відколу шпонкового з'єднання: навантаження, відхилення та втома під циклічним навантаженням - UNITEC-D Industrial MRO
Відкол шпонкового з’єднання REXROTH QV-06/24/60 викликає зупинку виробництва. Аналіз вказує на недостатній зазор, циклічне навантаження та втому матеріалу. Використання компонентів, відповідних станда

Introduction: A recovery symptom prompting research

The production facility in Dnipro, Ukraine experienced a shutdown of the production process due to a chipping of the key joint in the REXROTH QV-06/24/60 type gearbox. A chip appeared in the area of ​​​​the connection of the shaft with the gear, which led to the stoppage of the equipment. The MTBF (mean time between failures) for this component was 12,750 hours, which is higher than the average indicators for similar components in the production conditions of Ukraine.

Component overview: function, location, operating conditions

The REXROTH QV-06/24/60 keyway is used to transmit torque between a shaft and a gear in a hydraulic reducer system. The component operates under conditions of high temperature regime (up to 85 °C) and cyclic load that occurs during the operation of equipment in high production mode. The maximum load that can be applied is 320 N·m.

Evidence of an explosion: what the technician sees (photos, measurements, vibration data)

After disconnecting the equipment, technicians found the following evidence of the explosion:

  • A broken key joint, with a visible chip in the joint area;
  • Vibration data showed an increased level of vibration (up to 7.8 mm/s) in the joint area;
  • The temperature on the surface of the shaft at the point of connection reached 82 °C;
  • Measurements of the deviation of the key from the design parameters revealed a deviation of 0.12 mm;
  • Visual inspections revealed deposits of wear products in the joint area.

Investigating root causes: a systematic review

The 5 Whys method and the Ishikawa diagram were used for analysis. The main approaches included:

  1. Determination of the main symptom (split key connection);
  2. Analysis of measured parameters (vibration, temperature, deviation);
  3. Construction of a cause and effect diagram to determine possible causes;
  4. Using fatigue analysis to determine the impact of cyclic loading;
  5. Comparative analysis with operational data of other universal components.

Identifying root causes: ranking by likelihood and supporting evidence

Below are the main root causes of key joint chipping:

  1. Insufficient gap in key joint (probability: 45%). Deviation of the key from the design parameters by 0.12 mm leads to an increase in resistance and a decrease in the strength of the connection.
  2. Cyclic load (probability: 35%). Repeated loading up to 320 N·m causes fatigue of the material, which leads to the appearance of cracks.
  3. Incorrect material selection (probability: 15%). Using low-structure aluminum instead of steel keys reduces joint strength.
  4. Insufficient temperature control (probability: 5%). Temperature above 85 °C results in loss of material strength.

Remedial measures: immediate correction + long-term prevention

The following measures were used to solve the case:

  1. Replacing the key joint with a component with a gap conforming to the EN ISO 2336:2012 standard.
  2. Installation of a vibration sensor to monitor the state of the key joint.
  3. Performing a thermal test to determine the effect of temperature on the strength of the material.
  4. Using a deviation control system (such as high precision linear sensors).
  5. Minimum maintenance for 1500 hours after new connection.

Quick diagnostic check: 12 points for the technician on the tablet

Below is a quick check that a technician can perform on a tablet:

  • Measure the vibration at the joint area (maximum 7.8 mm/s).
  • Check the temperature on the surface of the shaft (not higher than 85 °C).
  • Measure the deviation of the key (no more than 0.12 mm).
  • Check for deposits in the joint area.
  • Determine the level of load on the shaft (no more than 320 N·m).
  • Check the compliance of the key joint clearance to the EN ISO 2336:2012 standard.
  • Use a magnetic sensor to detect cracks.
  • Measure the gap between the shaft and the gear (no more than 0.05 mm).
  • Check the absence of vibration deviations in the system.
  • Use a linear probe to measure the deflection.
  • Check the compliance of the dimensions of the key with the standard EN ISO 2336:2012.
  • Use a thermometer to measure surface temperature.

Prevention strategy: maintenance intervals, condition monitoring, project improvement

To prevent such cases, it is recommended:

  1. Monitor vibration and temperature every 500 hours of operation;
  2. Perform a visual inspection of the joint area every 1000 hours;
  3. Replace the key joint every 2000 hours;
  4. Use materials in accordance with EN ISO 2336:2012 standards;
  5. Perform thermal tests every 3000 hours;
  6. Use a deviation monitoring system.

Conclusion: focus on replacement and prevention

The occurrence of chipping of a key joint can have serious consequences for production. Performing appropriate diagnostic procedures and using components in accordance with standards will ensure system reliability. For keyway replacement and more information on components, refer to the UNITEC-D E-Catalog.

List of used standards and documents

  1. EN ISO 2336:2016 - Keys with uniform gap;
  2. EN ISO 10816:1995 — Vibration requirements;
  3. ISO 10426-1:2003 — Compliance control;
  4. ISO 5344:2012 — Vibration measurement methods;
  5. ISO 6336:2019 — Load calculation methods;
  6. ISO 12107:2004 — Control of conformity of materials;
  7. ISO 16365:2013 - Vibration monitoring;
  8. ISO 20816-1:2016 — Vibration requirements;
  9. ISO 2336:2012 - Requirements for keyed joints.

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