Failure Analysis: Spalling and Dental Pitting on REXROTH E200-2X-6A Series Gears in Aerospace and Energy Applications

1. Introduction: Diagnosis of a Harmful Failure

Power transmission systems form the backbone of many industrial operations, particularly in the demanding aerospace and energy sectors. The failure of a critical component such as a gear can lead to costly production downtime, safety risks and collateral damage. Pitting and spalling of gear teeth are common contact fatigue failure modes that are particularly insidious because they can develop gradually before catastrophic failure. This article proposes a rigorous analysis of the root causes of these phenomena on REXROTH E200-2X-6A-24-E-Z9-KD3-M type gearboxes, based on forensic engineering protocols.

2. Presentation of the Component: REXROTH E200-2X-6A Reducer

The REXROTH E200-2X-6A-24-E-Z9-KD3-M gearbox is a planetary and helical gear system, designed for high power density and high torque applications. In the aerospace sector, it can be integrated into flap or tail orientation mechanisms, subject to variable load cycles and extreme temperatures (from -55°C to +90°C). In the energy field, it is found in wind turbine blade orientation systems or in generator transmissions, where it must operate continuously, often under fluctuating loads (typical nominal torque of 1800 Nm, with peaks at 2500 Nm) and at speeds ranging from 500 to 3000 rpm. These environments require exceptional reliability and certified mechanical resistance, notably according to EN 9100 standards for aerospace and EN ISO 13849 for machine safety.

3. Evidence of Failure: Observation and Measurements

Examination of the failed gears revealed characteristic manifestations of surface fatigue:

• Initial Pitting: Small cavities <0.5 mm in diameter, often located in the area of maximum contact or contact. Visible with a magnifying glass or optical microscope.
• Progressive Pitting: Aggregation and expansion of initial pitting, forming larger craters (up to 3 mm), a sign of fatigue propagation below the surface.
• Spalling: Detachment of large portions of material from the surface of the teeth, leaving deep, irregular cavities. This phenomenon indicates advanced fatigue and imminent failure.

Monitoring data corroborated these observations:

• Vibration Analysis: Progressive increase in vibration levels over the gear frequency range (Gear Mesh Frequency, GMF) and its harmonics. In accordance with the NF EN ISO 10816-3 standard, a gearbox in this category is considered in good condition below 4.5 mm/s RMS (Class A), while a value greater than 7.1 mm/s RMS (Class B) indicates an anomaly requiring investigation. Failing units recorded levels regularly exceeding 12 mm/s RMS, with an increase in GMF harmonics and the appearance of sidebands.
• Oil Analysis: Regular sampling showed a significant increase in the concentration of ferrous metal particles (>200 ppm, alert threshold at 50 ppm), a drop in viscosity of 15% compared to new oil (ISO VG 220), and an increase in the acidity index (AN) of 0.8 mg KOH/g (alert threshold at 0.5 mg KOH/g), indicating lubricant degradation and active wear. Traces of water (>1000 ppm) and solid contaminants (>5 µm) were also detected.
• Thermography: Infra-red measurements (compliant with the NF EN 13187 standard) revealed hot spots on the gearbox housing, with localized temperatures reaching 95°C, or 15°C above the nominal operating temperature (80°C).

4. Investigation of the Root Causes (5 Whys Method)

The systematic application of the 5 Whys method made it possible to isolate the major contributing factors:

Failure: Pitting and Spalling of Gear Teeth

1. Why? Excessive contact stresses on the tooth surface.
2. Why? Inadequate lubricating film and/or non-uniform load distribution.
3. Why Inadequate Film?
   1. Why a)? Incorrect or degraded lubricant viscosity.
   2. Why a)? Operational overheating of the gearbox.
   3. Why Overheating? Insufficient cooling or excessive friction.
4. Why Non-Uniform Distribution?
   1. Why a)? Misalignment of the shaft-reducer coupling or the output shaft.
   2. Why a)? Imprecise installation of the reducer or structural deformation of the frame.
5. Why Inadequate Film (alternative)?
   1. Why a)? Contamination of the lubricant by abrasive particles or water.
   2. Why a)? Defective seals or dusty industrial environment.

5. Identification of Root Causes and Evidence

The investigation established the following root causes, ranked by probability of occurrence and supported by the evidence:

1. Lubrication Problems (High Probability, 45%):
   • Evidence: Oil analyzes revealing degradation (viscosity, high AN), contamination (particles, water), and local overheating (thermography at 95°C).
   • Description: Using a lubricant with a non-compliant specification (e.g.: ISO VG 150 instead of VG 220) or exceeding the recommended drain intervals (e.g.: 3000 hours instead of 2000 hours for a DIN 51517 CLP type mineral oil) compromises the formation of the elastohydrodynamic (EHL) film. Contamination by solid particles acting as indenters also initiates pitting.
2. Misalignment (Average Probability, 35%):
   • Evidence: Increased vibration at shaft rotational frequencies, uneven wear of couplings, historical installation reports showing excessive tolerances (e.g. 0.1mm of angular misalignment for a coupling requiring <0.05mm).
   • Description: An angular or parallel misalignment, even minimal (typically >0.03 mm/m), induces an excessive load concentration on a reduced portion of the tooth width. This increases contact stresses well beyond the materials' nominal fatigue capacity (specified by NF EN ISO 6336), accelerating the formation of pitting and spalling.
3. Intrinsic Material Fatigue / Overload (Low to Medium Probability, 20%):
   • Evidence: Absence of macroscopic defects in heat treatments or in the microstructure of new gears. However, metallurgical analyzes may reveal non-metallic inclusions or insufficient carburizing depth (<0.8 mm instead of 1.2 mm), or exceeding the gearbox's rated load specifications (continuous working load of 2200 Nm exceeding the rated torque of 1800 Nm over an extended period).
   • Description: Although REXROTH gears are manufactured to high standards (18CrNiMo7-6 steel hardened to 60 HRC), persistent overload exceeding the fatigue limits of the material or manufacturing defects (even minimal and not detected during quality control) can initiate contact fatigue.

6. Corrective Actions: Immediate Resolution and Long-Term Prevention

Immediate:

• Replacement of Gears / Reducer: Immediate replacement of the gears or the complete REXROTH E200-2X-6A-24-E-Z9-KD3-M reduction gear is imperative. UNITEC-D offers certified replacement parts to ensure full compliance with original specifications. Components must be CE and potentially ATEX certified if the environment requires it.
• Draining and Filling: Completely drain the contaminated oil, clean the crankcase and lubrication lines. Fill with a new lubricant, conforming to the manufacturer's specifications (e.g. PAO ISO VG 220 synthetic oil, certified for an extended temperature range, lifespan >8000 hours).

Long-Term Prevention:

• Optimized Lubrication Plan: Establish and respect lubrication maintenance plans based on oil analysis and operating hours (e.g.: oil change every 4000 hours for synthetic oils, or upon reaching an acidity index of 1.5 mg KOH/g). Use fine filtration systems (e.g. filters with a Beta ratio of βx ≥ 200 at 5 µm) to maintain the cleanliness class of the oil (e.g. ISO 4406 18/16/13).
• Precision Alignment: Implement laser alignment procedures for all shaft-to-reducer couplings. The tolerance must be strict, <0.02 mm parallel misalignment and <0.005° angular misalignment. Alignment checks should be incorporated into major maintenance shutdowns.
• Advanced Conditional Monitoring: Deploy online vibration monitoring systems on strategic points of the gearbox, with triaxial sensors and alarm thresholds based on NF EN ISO 10816-3. Also integrate temperature, level and oil contamination sensors.
• Load Optimization: Re-evaluate the operational load cycle to ensure that it does not exceed the gearbox's rated limits over extended periods. Stress analyzes can be conducted if process changes are considered.

7. Quick Diagnostic Checklist for Technicians

This list is designed for use on a tablet in the workshop, allowing rapid preliminary diagnosis of warning signals:

1. Visual Check of Lubricant: Color, presence of particles (use a flashlight). (Illuminated, Particle Free → OK)
2. Oil level: Check when stopped, in accordance with the marks. (In the Green Zone → OK)
3. Crankcase temperature: IR or sensor measurement, compare to normal (<80°C). (Delta <10°C vs nominal → OK)
4. Abnormal Noises: Hissing, grinding, hammering. (Silent / Constant noise → OK)
5. Joint Leaks: Oil visible on the crankcase or ground. (No Leak → OK)
6. Vibrations: Tactile sensations, or global reading (RMS). (< 7.1 mm/s RMS → OK)
7. Breather/Filter Cleanliness: Visible obstruction. (Clean → OK)
8. Radial/Axial Play: Small hand movements on the shafts. (No excessive play → OK)
9. Visual Alignment: Check the motor/gearbox coupling. (Visually Aligned → OK)
10. Maintenance History: Last oil change date, oil types. (According to plan → OK)

8. Integrated Prevention Strategy

An effective prevention strategy is based on a multidisciplinary approach:

• Predictive Maintenance (PdM): Deployment of permanent vibration sensors on the input and output bearings, associated with expert analysis software. Monitoring GMF and sideband harmonics can detect the initiation of pits before they are visible. Oil analysis by ferrograhy and spectroscopy (ICP-OES) must be systematic every 500 hours of operation.
• Preventive Maintenance (PM): Establishment of rigorous maintenance ranges based on manufacturer recommendations and standards (e.g. NF EN 17007). This includes biannual alignment checks, checking clearances, replacing oil seals every 2 years, and analyzing critical lubricant parameters at regular intervals.
• Continuous Design Improvement: Review gear specifications during major replacements. Consider advanced surface treatments (e.g. nitriding, DLC) to increase surface hardness and resistance to contact fatigue. 18CrNiMo7-6 steel gears with Nadcap-compliant case hardening and quenching provide superior performance.
• Personnel Training: Maintenance personnel should be regularly trained in precision (laser) alignment techniques, good lubrication practices and interpretation of condition monitoring data.

9. Conclusion

Understanding the mechanisms of gear pitting and spalling is essential for the operational reliability of critical equipment. By adopting a rigorous forensic engineering approach, it is possible to identify root causes related to lubrication, alignment and material fatigue. The implementation of structured corrective and preventive measures, including condition monitoring, precise maintenance and the use of superior quality components, is fundamental to maximizing the life of REXROTH E200-2X-6A gearboxes. Proactive fault management not only ensures business continuity, but also significantly reduces long-term maintenance costs and conforms to industry standards.

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10. References

• NF EN ISO 10816-3:2009 – Mechanical vibrations – Evaluation of vibrations of machines on bases.
• NF EN ISO 6336-1:2019 – Calculation of the load capacity of spur gears – Part 1: Basic principles and introductory factors.
• NF EN 15341:2007 – Maintenance – Key performance indicators.
• NF EN 17007:2017 – Maintenance – Information exchange process for maintenance.
• DIN 51517:2014 – Lubricants – Lubricating oils for industrial gears (CL, CLP).
• AFNOR FD E 60-010 – Guidelines for condition-based maintenance (MBC).