Root Cause Analysis of Oil Leakage in Reducers: Seals, Ventilation and Assembly Errors

1. Introduction

Oil leakage through the shaft seal of an industrial reducer is a symptom that is often regarded as a minor cosmetic issue. At Ukrainian enterprises in the mining and cement industries, machinists are often limited to merely adding oil or simply replacing the seal without identifying the actual cause of the failure. However, such a superficial approach inevitably leads to recurrence after 200–500 hours of operation. In this technical report, we will conduct a technical expert analysis of a typical case of oil leakage in a cylindrical-conical heavy-duty reducer.

The subject of our study is the main drive of the belt conveyor at the processing plant. Oil leakage was detected on the output shaft three weeks after the planned maintenance. The vibration monitoring station reported an increase in high-frequency vibration acceleration levels in the bearing unit, and the temperature of the housing in the seal area reached +82°C, while the nominal value is +65°C. Our goal is to analyze the chain of events leading to the premature failure of the SKF 1009103 seal, and to propose engineering solutions according to the ISO and EN standards.

2. Component Overview

Radial shaft seal (reinforced lip) performs a critical function in retaining lubricating fluid inside the reducer housing and protecting the internal cavity from abrasive dust, moisture, and debris. In our case, the component SKF 1009103 is used, which is designed for operation under high axial and radial loads.

• Landing dimensions: according to ISO 6194 and DIN 3760.
• Material of the working seal lip: fluorocarbon rubber (FKM / Viton), capable of prolonged operation in a temperature range from -40°C to +200°C.
• Робоче середовище: редукторна синтетична олива на основі поліальфаолефінів (PAO) класу в'язкості ISO VG 220 з протизадирними присадками (EP).
• Sliding speed on the shaft surface: 12.5 m/s.
• Working pressure in the housing: from -0.05 to +0.1 bar.

The seal is installed in the original cover of the reducer immediately before the shaft exit to the drive coupling. Operating conditions on the site are severe: air dustiness reaches 45 mg/m³, and daily temperature fluctuations of the surrounding environment range from -15°C in winter to +35°C in summer in an unheated workshop.

3. Failure Evidence

During disassembly of the unit on the machine tool and inspection under magnifying glass (10x), the engineering team recorded the following actual data:

• Lip condition: Thermal hardening and microcracks are observed on the fifth lip of the SKF 1009103 seal. The lip has even wear across the face, but its elasticity is lost by 70%.
• Стан шийки вала: На валу в місці контакту з робочою кромкою ущільнення виявлено кільцеву канавку глибиною 0.22 мм і шириною 3.5 мм. Шорсткість поверхні вала в зоні зносу перевищувала допустиме значення і становила $R_a = 0.95$ мкм замість нормативних $R_a = 0.20 \dots 0.40$ мкм (згідно з ISO 4287).
• Bearing spring: Spiral compression spring displaced from the housing groove and partially deformed. The spring tension has decreased by 40% from the initial force.
• Oil Analysis: The content of solid particles (silica from ore dust) was 0.18% by mass, which is 6 times higher than the permissible limit for this unit.

Vibration control data prior to shutdown showed spectral peaks at the shaft rotation frequency (1X) and harmonics associated with local imbalance and shaft beating (radial beating amounted to 0.08 mm with a tolerance of 0.03 mm).

4. Root Cause Analysis

For systematic analysis, we applied the Ishikawa diagram ("fishbone") method and the "5 Why" technique. Four main categories of factors were identified: material, assembly method, equipment condition, and external environment.

1. Why has the seal lost its integrity? Because the working edge has lost its elasticity and a gap has appeared between it and the shaft.
2. Чому кромка деградувала? Через перегрів у зоні контакту до температури понад +90°C.
3. Why did local overheating occur? Due to the absence of sufficient heat dissipation and increased friction resulting from wear of the bearing and scoring.
4. Why did a groove form on the shaft? Abrasive particles of dust penetrated under the lip of the seal and acted as a grinding paste, wearing down the metal.
5. Чому абразив потрапив під кромку? Сапун редуктора був забитий технологічним пилом, що призвело до створення надлишкового тиску гарячих картерних газів (до +0.4 бар), виштовхування мастила назовні та створення ефекту підсмоктування бруду при охолодженні.

5. Identified root causes (by probability ranking)

6. Corrective and Preventive Actions

To eliminate the detected defects and prevent equipment stoppages on the production line, a plan of technical measures has been developed:

• Urgent Repair (Immediate Remediation): Disassembly of the reducer, grinding the shaft journal to the repair size or installation of a quick repair sleeve (speedi-sleeve). Replacement of the SKF 1009103 seal with a new one according to the SKF mounting instruction. Replacement of the breather with a model equipped with an integrated fine air filter (filter rating 3 µm).
• Довгострокова профілактика: Впровадження регулярного контролю перепаду тиску в картері редуктора. Перехід на синтетичні мастильні матеріали з високим індексом в'язкості та стійкістю до деструкції згідно з ISO 12925-1 (категорія CKD).

7. Quick Diagnostic Checklist for Technicians

This checklist is intended for use by fitters and reliability engineers directly in the workshop using a tablet or mobile terminal:

1. [ ] Check the temperature of the housing in the bearing and seal area with a pyrometer (norm: up to +70°C).
2. [ ] Assess the level and condition of the oil through the observation window (presence of emulsion, foam, dark sediment).
3. [ ] Check free air movement through the plug (absence of blockage by sludge or dust).
4. [ ] Make vibration measurements using a portable vibrometer in three mutually perpendicular axes (high-frequency spectrum check).
5. [ ] Inspect the output shaft for the presence of grease marks, dust around the cover.
6. [ ] Check the alignment of the reducer shaft and the connected machine using a laser alignment system (allowable offset less than 0.05 mm).
7. [ ] Check the torque of the cover fastening bolts of the bearings and the reducer housing using a torque wrench.
8. [ ] Carry out spectral analysis of the oil sample for the presence of wear products (Fe, Cu, Cr) every 6 months.

8. Future Prevention Strategy

To achieve the design reliability indicator of the reducer (MTBF not less than 40 000 hours of operation), it is necessary to integrate the following practices:

• Maintenance intervals: Full seal replacement and bearing seat inspection every 12 000 hours of operation or once every two years.
• Condition monitoring: Installation of fixed vibration and temperature sensors with signal output to the company's SCADA system for early detection of bearing defects.
• Construction improvement: Application of two-stage seals with a protective lip (dust seal) and a barrier grease between the joints for operation in abrasive dust conditions of cement production plants.

9. Conclusion

Oil leakage in heavy machinery reducers is always a result of imbalance in the triangle of "pressure-temperature-mechanical clearance". Replacing the seal alone without checking the vent and controlling the shaft runout is an ineffective expense. Adhering to installation schedules according to ISO standards and using certified components guarantee uninterrupted operation of industrial equipment.

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10. References

• ISO 6194-1:2007 — Rotary shaft lip-type seals incorporating elastomeric sealing elements.
• DIN 3760 — Radial shaft seal rings for industrial applications.
• ISO 4287:1997 — Geometrical Product Specifications (GPS) — Surface texture: Profile method.
• ISO 12925-1:2018 — Lubricants, industrial oils and related products (class L) — Family C (Gears).
• SKF General Catalogue: Industrial Shaft Seals and Mounting Instructions.