1. Introduction
On one of the heavy machinery enterprises in Ukraine, during planned technical maintenance of the main drive of the technological line, an unusual high-frequency noise and increased vibration of the reducer were detected. Inspection of internal surfaces using an endoscope revealed local damage to the working surfaces of the gear teeth and wheel. The main cause of the equipment stoppage was progressive pitting (pitting corrosion) followed by transition to microspalling (spalling). The mean time between failures (MTBF) for this unit was expected to be 45 000 hours, but the failure state occurred already after 14 200 hours of operation. This required conducting a thorough technical investigation to determine the root causes in accordance with the requirements of standard ISO 10816 and failure analysis methodologies.
2. Component Overview
The under test node is a high-precision gear reducer, in which the key element for torque transmission is the component with the article number FIBRO 2480.022.10000. This node operates under high cyclic loads, ensuring power transmission within the kinematic chain of a machine tool or machining complex.
- Nominal torque: up to 2400 Nm
- Робоча температура мастила: 65 °C – 85 °C (гранична 95 °C)
- Gear material: Alloyed casehardened steel 18ХГТ (or equivalent 18CrNiMo7-6 according to EN 10084), surface hardness 58-62 HRC, depth of casehardening layer 1.2-1.6 mm.
- Rotational speed: 1500 rpm (input shaft)
The component is located inside a sealed cast iron housing with a lubrication sump system. The working medium involves the use of synthetic gear oil of ISO VG 220 class based on polyalphaolefins (PAO) with anti-scuff additives (EP).
3. Failure Evidence
Visual inspection shows a classical picture of surface fatigue. Numerous small pits (pitting) with diameters ranging from 0.5 to 2.0 mm and depth up to 0.8 mm are observed along the pitch line and below the tooth root. On the loaded side of the teeth of component FIBRO 2480.022.10000, these pits are found to merge into continuous areas of spalling.
| Measurement parameter | Normative value (ISO / DIN) | Actual value on the object |
|---|---|---|
| Vibration (RMS velocity) | < 2.8 mm/s (ISO 10816-3) | 7.4 mm/s (predominant engagement frequencies) |
| Oil Cleanliness Class | ISO 4406: 16/14/11 | ISO 4406: 20/18/15 (high solid particle contamination) |
| Viscosity of oil at 40 °C | 220 mm²/s ± 10% | 185 mm²/s (destruction of the thickener and slip) |
| Contact spots per tooth | 70% along the length and height | < 35% (edge contact on one side) |
4. Root Cause Analysis
To determine the chain of events leading to the premature failure of FIBRO 2480.022.10000, the "5 Whys" method was applied in combination with an Ishikawa diagram, focusing on four main factors: lubricant, geometry/alignment, materials, and operating environment.
- Why did pitting and spalling occur? Due to exceeding the contact stresses on the tooth surface beyond the material's contact fatigue limit.
- Why did excessive contact stresses occur? Due to uneven load distribution along the tooth length (end contact).
- Why is the load distributed unevenly? Due to angular misalignment of the shafts (shaft tilt) within 0.15 mm over a length of 100 mm.
- Why did the shaft misalignment occur? Due to thermal deformations of the foundation and insufficient accuracy of alignment during installation after the previous repair.
- Яку роль зіграло мастило? В'язкість робочої рідини впала нижче критичного мінімуму через локальний перегрів, що унеможливило створення масляної плівки мінімальної товщини ($h_{min} > \sigma_1 + \sigma_2$), необхідної за стандартами ISO 281 та AGMA 9005-E02.
5. Identified Root Causes
As a result of the comprehensive analysis, three key causes have been identified with different probabilities of contribution to the accident:
- Angular misalignment and edge contact (Probability: 50%): Confirmed by painting method and contact stain analysis. The load was concentrated at the edge of the tooth, where the stresses exceeded the calculated values by 2.3 times.
- Lubricant degradation and contamination (Probability: 30%): Oil sample analysis showed the presence of wear products (cast iron and steel dust) and reduction in the thickness of the hydrodynamic film ($EHD$), causing metallic contact of roughness.
- Misalignment parameters (Probability: 20%): Deviation from perpendicularity tolerances of bearing support mounting locations during assembly of the unit.
6. Corrective Actions
To eliminate the detected defects and prevent recurring failures, a comprehensive set of immediate and long-term measures has been developed:
Увага: Експлуатація обладнання з вібрацією понад 7.0 мм/с заборонена згідно з правилами промислової безпеки України (НПАОП 0.00-1.81-18). Необхідна негайна заміна пошкодженого компонента.
- Urgent repair: Disassembly of the reducer, complete cleaning of the housing, replacement of the damaged gear element with an original component from the UNITEC-D catalog in accordance with DIN 3962 (accuracy class not lower than 6).
- Вирівнювання та юстування: Лазерне вирівнювання валів із дотриманням допуску паралельності осей $\le 0.02$ мм та кутового перекосу $\le 0.05$ мм/м.
- Lubricant optimization: Switching to high-quality synthetic oil ISO VG 220 with an improved EP additive package and installation of a two-stage air filter system for the sump (cleanliness class no worse than ISO 4406 17/15/12).
7. Quick Diagnostic Card for Technical Specialists
This checklist table is designed for field engineers and can be used directly in the workshop using a tablet:
- Visual inspection of the casing: Check for cracks, assess the level and color of the oil through the inspection window.
- Vibration measurement: Remove the bearing support indicators in three mutually perpendicular directions ($V_{RMS}$).
- Thermometry: Measure the temperature of the housing in the engagement zone with an infrared thermometer (limit up to 85 °C).
- Frequency spectrum analysis: Search for gear mesh frequency ($GMF$) harmonics and sidebands of rotational frequency.
- Oil sample: Take a sample from the lower drain point for laboratory analysis to check for ferromagnetic particles ($Fe$ > 100 ppm — warning).
- Checking the breather: Clean or replace the air filter breather to prevent dust accumulation.
- Bolt Tightening Check: Check the tightening torques of the reducer mounting bolts to the frame according to the technical manual.
- Grip inspection endoscopy: Inspect at least 4 teeth around the perimeter for the presence of microcracks, pitting, or scoring.
8. Prevention Strategy
To ensure uninterrupted operation of industrial equipment at Ukrainian enterprises, the following maintenance strategy should be implemented:
- Condition Monitoring (PdM): Installation of stationary vibration sensors and continuous spectral analysis of acceleration shock pulses ($ shock\ pulses $).
- Lubrication schedule: Laboratory analysis of the lubricant every 2000 hours of operation or once every 6 months. Complete replacement of synthetic oil every 8000–10000 hours.
- Alignment audit: Scheduled check of alignment of the drive every 12 months after completion of the cycle of seasonal temperature fluctuations in the workshop.
9. Conclusion
Premature failure of gearboxes due to pitting and scuffing is always the result of a combined effect of mechanical deviations (misalignment) and deterioration of lubrication conditions. Strict adherence to installation schedules, regular vibration monitoring, and the use of certified spare parts allow avoiding emergency stoppages and achieving the equipment's design life. Selection of quality components for replacement is available in UNITEC-D E-Catalog.
10. References
- ISO 10816-3: Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts.
- ISO 281: Rolling bearings — Dynamic load ratings and rating life.
- DIN 3962: Tolerances for cylindrical gear teeth.
- AGMA 9005-E02: Industrial Gear Lubrication.
- EN 10084: Case hardening steels — Technical delivery conditions.
