Introduction: Failure Symptoms and Investigation Conditions
On one of the heavy machinery enterprises in Ukraine, during a planned inspection of the main drive of a technological mill, a critical level of vibration and an increased noise level (tonal drop at the gear mesh frequency) was recorded. Analysis of the vibration spectrogram showed the appearance of sidebands around the gear mesh frequency, indicating load modulation and the presence of local defects on the working surfaces of the gear teeth and wheel. Stopping the equipment and endoscopic inspection revealed developed pitting and macro-spalling on the working surfaces of the cylindrical reducer teeth. To prevent a catastrophic failure of the drive unit, which is supplied through the frequency converter Telemecanique ATV66U72N4G, a detailed engineering investigation was conducted using the root cause analysis (RCA) method.
Component Overview and Operating Conditions
Gear unit operates in a heavy drive with nominal power of 160 kW at the speed of the high-speed shaft of 1480 rpm. The gear ratio is i = 5.4. The gear material is alloyed steel 18ХГТ (18CrMnTi) with carburizing and hardening to hardness 58-62 HRC. Operating conditions correspond to heavy duty with cyclic peak loads exceeding nominal by 160%. The reducer is integrated into the motor control system based on the Telemecanique ATV66U72N4G inverter, which provides smooth start-up and speed regulation.
Working parameters of the node:
- Working oil temperature in the sump: 68-75 °C (at maximum allowable 85 °C).
- Lubricant cleanliness class for ISO 4406: 18/16/13 (actual condition before the failure corresponded to 20/18/15).
- Type of lubricant: Synthetic reducer oil ISO VG 220 based on polyalphaolefins (PAO) with a package of anti-scuff additives (EP).
- Design life (MTBF): 50 000 hours. Actual failure time was 21 400 hours.
Failure Evidence and Measurement Results
Visual and metrological inspection revealed the following defects:
- Пітинг (тобір): На ділянках поблизу полюсної лінії та у зоні ніжки зуба провідної шестерні утворилися мікрократери глибиною до 0.4 мм та площею покриття понад 25% робочої поверхні зубів.
- Spalling: Cracking of the surface layer of metal up to 1.5 mm in depth and up to 12 mm² in area has been detected on three teeth of the driven gear. The edges of the spalls have a fatigue character.
- Contact Spot Measurement: The paint test showed edge contact. The spot occupies only 45% of the tooth length instead of the required 85% according to the requirements of DSTU GOST 1643:2004.
- Вібродіагностика: Середньоквадратичне значення (RMS) швидкості вібрації у діапазоні 10-1000 Гц досягло 7.8 мм/с (граничне значення за ISO 10816-3 для жорстких фундаментів класу II становить 4.5 мм/с).
Systematic analysis of root causes (Isikawa and 5 Whys)
A Ishikawa diagram was constructed to determine the source of failure (factors: material, lubricant, installation/alignment, operational loads). The application of the "5 Why" method allowed to establish the sequence of events:
- Why did pitting occur? Due to exceeding the contact stresses above the contact fatigue limit of the case hardened layer.
- Why are the contact stresses exceeding the calculated values? Due to uneven load distribution along the tooth length.
- Why was the load distributed unevenly? Due to the angular error of parallelism of the shafts (misalignment of the reducer relative to the motor by 0.18 mm per 100 mm length).
- Why did the shaft misalignment occur? Due to thermal deformations of the frame and loosening of anchor bolts during previous high-torque start cycles.
- Чому процес руйнування прискорився? Забруднення мастила твердими частинками та зниження в'язкості при перегріві спричинили гідродинамічне пробиття мастильної плівки (коефіцієнт товщини мастильної плівки λ < 1.2).
Defined root causes
As a result of the investigation, three key causes with different likelihood of influence have been identified:
- Incorrect shaft alignment (Misalignment) — Probability 55%. Angular shaft displacement caused concentration of load on the tooth edges, resulting in local exceedance of contact stresses above 1450 MPa (allowable 1200 MPa).
- Lubricant degradation and contamination — Probability 30%. Deviation from filtration intervals and ingress of abrasive particles led to abrasive wear and accelerated occurrence of microcracks.
- Defects of the case hardened layer - Probability 15%. Local reduction of the effective hardened layer depth on individual teeth below the normative 1.2 mm according to the requirements of EN ISO 6336.
Corrective and preventive actions
To eliminate the detected defects and prevent recurring failures, a comprehensive set of measures has been developed:
- Immediate repair: The reducer was disassembled. The damaged gear pair was replaced with new original components of accuracy class 6 according to DSTU GOST 1643:2004.
- Вирівнювання: Виконано лазерне вирівнювання осей двигуна та редуктора за допомогою високоточних систем. Допуск на паралельність осей знижено до рівня менше 0.03 мм на 100 мм довжини. Анкерні болти замінено на міцніші (клас міцності 10.9) із застосуванням динамометричного затягування.
- Lubrication system modernization: An additional oil filtration station with 5 micron filtration precision and a humidity sensor has been installed. The type of oil has been replaced with a synthetic analog having an increased viscosity index.
- Drive settings: The acceleration and deceleration time (S-shaped ramp) in the frequency converter Telemecanique ATV66U72N4G has been optimized to minimize impact dynamic torques in the coupling.
Express diagnostic card for field engineers
Table 1 contains the algorithm for quick condition check of the reducer directly in the workshop using portable equipment.
| Step | Check parameter | Tool / Method | Normative value | Diagnostic feature of deviation |
|---|---|---|---|---|
| 1 | Vibration of the housing | Vibration analyzer (FFT spectrum) | RMS < 4.5 mm/s (ISO 10816-3) | Pickups at engagement frequency with side strips |
| 2 | Bearing temperature/case temperature | Pyrometer / Infrared thermometer | < 80 °C | Local overheating in the bearing support zones |
| 3 | Lubricant Condition | Sample Analysis / Laboratory Analysis | ISO 4406: 18/16/13 | Presence of metallic powder, water > 500 ppm |
| 4 | Tooth contact spot | Control paint | At least 85% by length | End or point contact |
| 5 | Play in engagement | Clock-type indicator | According to the reducer passport | Increase of side clearance by 30% from the norm |
| 6 | Integrity of gear tooth surfaces | Industrial Endoscope | Smooth surface without cavities | Presence of microcraters, pitting |
| 7 | Mounting position of shafts | Laser Centering System | Error < 0.05 mm | Offset of axes above manufacturer's tolerances |
| 8 | Noise background of the drive | Acoustic Sound Level Meter | < 85 dB(A) at a distance of 1 m | Metallic screech, uneven humming |
Failure Prevention Strategy
To ensure uninterrupted operation of critical drives in the black and colored metallurgy and mining industries in Ukraine, the following maintenance strategy should be implemented:
- Condition Monitoring: Continuous measurement of vibration using impact pulses (spike energy) to detect defects in bearings and gearings at early stages (particularly according to standard ISO 13373).
- Oil Analysis: Conducting spectrographic oil analysis every 1000 hours of operation to monitor the concentration of iron, chromium, and silicon wear products.
- Регламентні заміни: Обов'язкова заміна ущільнень, фільтруючих елементів та ревізія муфт кожні 12 000 годин згідно з рекомендаціями виробників промислового обладнання.
Conclusion
The root cause analysis confirmed that pitting and tooth chipping are caused by the combined effect of angular misalignment of shafts and contamination of lubricant. Strict adherence to alignment standards according to EN ISO 10816, quality control of lubricant materials, and proper adjustment of frequency converters allow extending the service life of gear units to nominal MTBF values. For selection of certified spare parts, drive components, and automation tools, please refer to the product range on page UNITEC-D E-Catalog.
Normative references and literature
- ISO 6336: Calculation of load capacity of spur and helical gears.
- DSTU GOST 1643:2004: Cylindrical gear transmissions. Tolerances.
- ISO 10816-3: Mechanical vibration — Evaluation of machine vibration by measurements on non-rotating parts.
- ISO 4406: Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles.
- EN ISO 13373: Condition monitoring and diagnostics of machines — Vibration condition monitoring.
