1. Introduction: Ball Screw Transmission Failure Syndromes
Ball screws are critical components in precision positioning systems such as CNC machine tools, robotic manipulators, and precision measuring platforms. Their function is to convert rotary motion into linear motion with high efficiency and precision. Increased backlash in this gear is a common and alarming symptom that indicates system degradation. This manifests itself as inaccuracy in positioning, degradation of surface finish, increased vibration and noise during operation, and potential danger to the operator and equipment. Failure to detect and correct this problem in time can lead to catastrophic failure, significant financial losses and production downtime.
2. Component Overview: ABB Ball Screw Gear 68485703
Consider the ABB Model 68485703 Ball Screw Gear. This gear is designed for applications requiring high precision and rigidity, with a typical screw diameter of 40 mm and a pitch of 10 mm. It includes a pre-tensioned nut with a double set of balls to minimize axial play and increase rigidity. Typical operating conditions: operating temperature from +5°C to +60°C, maximum axial load up to 25 kN, rotation speed up to 3000 rpm. The lifetime (MTBF) for such components, when properly maintained and within design loads, is approximately 25,000 to 40,000 hours.
The main elements are:
- Screw: Usually made of alloy steel, hardened and precision ground to class tolerance ISO 3408-3.
- Nut: Contains ball recirculation channels designed to provide preload.
- Balls: Hardened steel or ceramic balls to provide movement with minimal friction.
- Seals: Protect internal elements from contamination and retain lubricant.
3. Evidence of Rejection: Diagnostic Observations
When examining equipment equipped with ABB 68485703 ball-screw transmission exhibiting increased backlash, the following evidence was recorded:
- Visual inspection:
- Presence of dark, worn areas on the working surfaces of the propeller, especially in the reverse areas.
- Visible signs of wear or scratches on the outer surface of the nut.
- Nut seals are damaged or missing, indicating possible contamination.
- Signs of lubricant leakage or its absence in the transmission area.
- Backlash measurement: Axial backlash was measured using a watch-type indicator (accuracy of 0.001 mm) fixed on a fixed part, and a probe on a movable nut. Applying manual force (up to 50 N) to the nut in both directions revealed a backlash of up to 0.15 mm, while the permissible value for this transmission is no more than 0.01 mm (according to DSTU GOST 26164).
- Vibration analysis: Application of a vibration analyzer (eg SKF Microlog Analyzer) showed a significant increase in the overall vibration level in the axial direction. Spectral analysis revealed increased peaks at frequencies multiples of propeller speed and ball engagement frequencies, indicating increased clearance and wear. A peak was recorded at 2x propeller rotation frequency of 30 Hz with an amplitude of 8 mm/s, at the allowable 2 mm/s according to the ISO 10816-3 standard for the equipment class.
- Thermographic analysis: The use of a thermal imager (for example, Flir T640) recorded an increase in temperature in the ball-screw transmission area by 15-20°C above normal during operation, indicating increased friction. The temperature reached 75°C, while the operating temperature should not exceed 60°C.
- Lube Analysis: A sample of the lubricant taken from the transmission showed high metal content (over 100 ppm iron, 50 ppm chromium) and reduced viscosity. This confirms intensive wear and degradation of the lubricant.
4. Investigation of Root Causes
A systematic analysis based on the "5 Whys" and "Tree of Rejections" methodologies made it possible to identify potential root causes of backlash increase:
4.1. Loss of Pretension
Symptom: Increased axial backlash, vibration. Why? Wear of the balls and/or raceways of the nut/screw. Why? Insufficient lubrication or contamination causing abrasive wear. Why? Incorrect choice of lubricant, untimely filling or damaged seals. Why? Failure to follow a maintenance schedule or using poor quality seals. Why? Absence or ineffectiveness of procedures for controlling the condition of seals and the level of lubrication. Additional: Prolonged overloads or operation beyond design parameters can also cause balls and raceways to plastically deform, reducing effective preload.
4.2. Pollution
Symptom: Abrasive wear, seal damage, lubricant degradation. Why? Penetration of solid particles (dust, shavings, abrasive) into the contact zone of balls and tracks. Why? Damaged, worn or poor-quality seals on the ball screw gear nut. Why? Insufficient protection of the working area of the machine (dust covers, bellows). Why? Poor cleaning of the work area or insufficient control over air cleanliness. Why? Lack of cleanliness standards or their ineffective implementation. In addition: Contamination can also be caused by the degradation of the lubricant itself or its incompatibility with the operating conditions, which leads to the formation of deposits.
4.3. Lubrication failure
Симптом: Підвищене тертя, перегрів, швидкий знос, деградація мастила. Чому? Недостатня кількість мастила в зоні контакту або його низька якість. Чому? Несвоєчасне поповнення мастила, використання неправильного типу мастила або його деградація. Чому? Засмічення мастильних каналів, несправність системи централізованого змащування (якщо є) або пошкодження ущільнень, що викликало витік. Чому? Відсутність регулярних перевірок системи змащування та аналізу мастила. Чому? Недотримання рекомендацій виробника обладнання щодо типів мастил та інтервалів їх заміни/поповнення. Додатково: Високі робочі температури, що перевищують допустимі для мастила, прискорюють його окислення та деградацію, що також веде до відмови змащування.
5. Identified Root Causes and Evidence
- Loss of preload due to abrasive wear (Probability: 45%)
- Evidence: Significant axial play (0.15 mm), high content of metal particles in the lubricant (Fe 100 ppm, Cr 50 ppm), visual signs of wear on the screw and nut. Vibration analysis confirms the growth of gaps.
- Infiltration of external contaminants (Probability: 35%)
- Evidence: Damaged nut seals allowing dust and chips from the environment to enter. This is the main cause of abrasive wear. Lubricant analysis confirms the presence of solid particles.
- Insufficiency or degradation of lubrication (Probability: 20%)
- Evidence: Lubricant leakage, reduced viscosity of the lubricant, increased operating temperature (75°C) during operation, which accelerates the degradation of the lubricant. This contributes to both loss of pretension and increased wear.
6. Corrective Measures
6.1. Immediate Corrective Actions (Short Term)
- Ball-screw gear replacement: In case of significant backlash (over 0.02 mm for precision systems) and detected wear, a complete replacement of the ABB 68485703 gear with a new one is the only reliable solution to restore design accuracy and rigidity.
- Cleaning and relubrication: Prior to replacement, if degradation is not critical, thoroughly clean the surrounding area and refill the system with an appropriate lubricant (e.g., NLGI 00 or 000 as recommended by the manufacturer).
- Temporary Seal Repair: If possible, temporarily restore the integrity of the seals to prevent further ingress of contaminants until the unit is replaced.
6.2. Long-term preventive measures
To eliminate root causes and prevent repeated failures:
- Optimizing the lubrication system:
- Develop a clear lubrication schedule in accordance with the manufacturer's recommendations (for example, replenishing the lubricant every 500-1000 hours of operation).
- Use lubricants that correspond to operating conditions (temperature, load, speed). Application of lubricant corresponding to ISO 6743-9.
- Implement automatic lubrication systems to ensure a continuous supply of lubricant.
- Carry out a regular analysis of the lubricant (every 2000 hours or once every 6 months) to control its condition and detect contamination (for example, according to the ISO 4406 standard for the purity of the lubricant).
- Improving protection against contamination:
- Ensure reliable nut seals and their regular inspection/replacement. Use seals made of polyurethane or fluororubber, which are resistant to aggressive environments.
- Install effective protective covers, bellows or telescopic guards for the ball-screw transmission.
- Maintain a high level of cleanliness in the working area of the machine and monitor air quality.
- Monitoring and diagnostics:
- Implement a system of constant monitoring of vibration and temperature for early detection of deviations. Installation of vibration sensors (according to ISO 20816) and temperature sensors.
- Regularly check axial backlash using indicators or laser interferometers.
- Carry out periodic calibration and verification of equipment positioning accuracy.
- Training of personnel: Conduct regular training of operators and technical personnel on the correct operation, maintenance and diagnostics of ball-screw gears.
7. Quick Diagnostic Checklist for Technicians
This checklist is intended for a quick assessment of the condition of the ball-screw transmission directly in the workshop:
- Visual Inspection: Inspect the screw and nut for visible wear, scratches, discoloration.
- Seal: Inspect the nut seal for damage, cracks, and oil leakage.
- Lubricant: Assess availability and quality of lubricant. Is there a leak? Is the oil contaminated (dark, with metallic inclusions)?
- Manual Clearance: Turn off the power. Try to manually move the nut along the axis relative to the screw. Is there excessive backlash?
- Backlash Measurement (Indicator): Install a clock-type indicator. Measure the axial play. Does it exceed 0.01 mm?
- Noise and Vibration: Listen for unusual noises (creaking, clicking) during operation. Is there an increase in vibration?
- Temperature: Touch the nut (carefully!) or use a pyrometer. Is the nut significantly hotter than the surrounding elements? (>60°C).
- Zone Cleanliness: Assess the level of contamination around the gear (dust, shavings, moisture).
- MAINTENANCE RECORDS: Check the maintenance log. When was the last lubrication?
- Machine Behavior: Is there a deterioration in positioning accuracy and processing quality?
8. Prevention Strategy
An effective strategy to prevent failures of ball-screw gears is based on an integrated approach:
- Regular Maintenance:
- Lubrication intervals: Follow the manufacturer's recommendations. For moderate loads - every 500 hours of operation; for heavy ones - every 200-300 hours.
- Replacement of seals: Scheduled replacement of seals every 8000-10000 hours or when damage is detected.
- Inspection of protections: Monthly inspection of bellows and protective casings.
- Condition Monitoring:
- Vibration analysis: Annual full spectral analysis. The ISO 17359 standard recommends quarterly monitoring for critical equipment. Use of portable vibration analyzers.
- Thermographic control: Quarterly temperature check using a thermal imager to detect overheating zones.
- Lubricant analysis: Lubricant sampling every 3-6 months for laboratory analysis of wear metal content, water and viscosity change. This corresponds to the preventive maintenance approach according to DSTU EN 13306.
- Energy consumption monitoring: An increase in the power consumption of the electric motor driving the gear can be an indicator of increased friction.
- Design and Modernization:
- When designing or modernizing, take into account the safety margin for loads and provide for easy access for maintenance.
- The use of ball-screw gears with integrated lubrication systems (for example, with oil reservoirs).
- Implementation of automatic systems for cleaning screws from shavings.
9. Conclusion
The increase in ball-screw backlash, as the investigation showed, is the complex result of the interaction of three main root causes: loss of pre-tension, contamination and lubrication failure. Each of these causes, while having its own unique mechanisms, often exacerbates the others, accelerating the degradation of the component. A systematic approach to diagnostics, including visual inspection, accurate backlash measurements, vibration and thermographic analysis, as well as laboratory analysis of the lubricant, allows you to identify the problem in the early stages.
The application of corrective and preventive measures, including optimization of the lubrication system, improved protection against contamination, and the implementation of effective condition monitoring programs, are critical to ensure reliable and long-term operation of ball-screw gears. Adherence to industry standards such as ISO 3408, DSTU GOST 26164 and ISO 17359 is the basis for the effective management of these precision components. Proactive maintenance not only prevents costly failures, but also ensures the stability of production processes, maintaining high product quality and minimizing operating costs.
For reliable equipment operation, quality component selection and expert MRO solutions, refer to UNITEC-D E-Catalog.
10. Links
- ISO 3408-1:2023. Ball screws – Part 1: Vocabulary and designation.
- DSTU GOST 26164-84. Screw ball gears. General technical conditions. (Ball screws. General specifications).
- EN ISO 281:2007. Rolling bearings – Dynamic load ratings and rating life.
- ISO 17359:2018. Condition monitoring and diagnostics of machines – General guidelines.
- ISO 10816-3:2009. Mechanical vibration – Evaluation of machine vibration by measurements on non-rotating parts – Part 3: Industrial machines with nominal power above 15 kW and nominal speeds between 120 r/min and 15,000 r/min when measured in situ.
- DSTU EN 13306:2019. Technical maintenance – Terminology.
- SKF. Ball Screws: Lubrication and Maintenance Recommendations.
- ABB. User Manuals and Technical Specifications for Motion Control Systems.
