1. Introduction
Ball screw pair (BSP) backlash is a critical issue for CNC machine tools, machining centers, and automated lines. Symptoms are manifested in the form of reduced positioning accuracy (error more than 0.02 mm per 300 mm stroke), vibrations during reversing (amplitude > 0.15 mm/s² at 50-100 Hz) and increased noise (> 85 dB). At metalworking enterprises of Ukraine, such failures are recorded in 18% of cases after 8,000-12,000 hours of operation (UNITEC-D data for 2023).
2. Overview of the component
Ball-screw pair Parker G01284 (screw diameter 40 mm, pitch 10 mm, accuracy class C5 according to ISO 3408-3) is designed to transfer rotary motion to linear motion with an efficiency of up to 90%. Working conditions:
- Axial load: up to 12 kN (nominal 6 kN);
- Rotation speed: up to 2,500 rpm;
- Temperature range: -10°C to +80°C (short-term up to +120°C);
- Humidity: up to 90% without condensation;
- Grease: plastic grease Klüber Isoflex NBU 15 (NLGI class 2).
Structurally, the KGP consists of:
- Hardened steel screw (hardness 58-62 HRC according to EN ISO 6508-1);
- Nuts with integrated ball return channels;
- Chrome steel ball (diameter 6.35 mm, accuracy class G10 according to ISO 3290-1);
- Polyurethane gaskets (hardness 90 Shore A for ISO 868).
3. Evidence of refusal
Technical data collected during diagnostics:
3.1 Vibration analysis
| Parameter | Nominal value | Actual value | Limit value (for ISO 10816-3) |
|---|---|---|---|
| General vibration level (RMS), mm/s | 0.8 | 2.1 | 1.8 |
| Amplitude during reversal, mm/s² | 0.1 | 0.35 | 0.25 |
| Frequency of the dominant harmonic, Hz | 50 (main) | 75, 150 (multiples) | - |
3.2 Backlash measurement
A watch-type indicator (accuracy class 0.001 mm according to DSTU EN ISO 463:2015) and a torque wrench (moment 5 N·m) were used. Results:
- Backlash in the new KGP: 0.005 mm;
- Backlash in the failed KGP: 0.042 mm (excess by 740%);
- Pre-tension force: 180 N (nominal 350 N).
3.3 Lubricant analysis
The lubricant sample was taken after 10,000 hours of operation. Laboratory analysis (ASTM D7412 method):
- Content of metal particles: 0.42% (limit value 0.1%);
- Viscosity at 40°C: 120 mm²/s (nominal 150 mm²/s);
- Acid value: 1.8 mg KOH/g (limit 1.0);
- Water content: 0.15% (limit 0.05%).
3.4 Visual inspection
- The presence of abrasive particles with a size of 5-50 μm on the raceways (microscopic analysis according to ISO 4406);
- Corrosion spots on the screw (depth up to 0.02 mm);
- Wear of seals (decrease in thickness by 30%);
- Deformation of balls (ovality up to 0.003 mm).
4. Study of root causes
The method of fault tree analysis (Fault Tree Analysis) according to the EN 61025 standard was used with a probability estimate:
Upper level: Increased backlash of KGP (> 0.03 mm)
Intermediate events:
- Loss of pretension (probability 0.45);
- Increasing the gap between balls and tracks (0.35);
- Lubricant contamination (0.20).
4.1 Analysis according to the "5 Why" method
Problem: 0.042mm backlash after 10,000 hours.
- Why? Wear of balls and raceways.
Evidence: Ovality of balls 0.003 mm, microcracks on the raceways. - Why? Insufficient lubrication and abrasive contamination.
Evidence: The content of metal particles is 0.42%, a decrease in the viscosity of the lubricant. - Why? Seal damage and irregular oil change.
Evidence: Seal wear by 30%, replacement interval exceeded by 30%. - Why? Lack of oil condition monitoring system.
Evidence: No contamination sensors or real-time oil analysis. - Why? The requirements of the ISO 18436-4 standard regarding lubrication monitoring are not taken into account.
4.2 Ishikawa diagram
The main categories of reasons:
- Materials: Low-quality seals (hardness 85 Shore A instead of 90), grease contamination;
- Car: Absence of vibration sensors, irregular maintenance;
- Methods: Non-observance of oil change intervals (recommended 4,000 hours, actually 6,000);
- Person: Insufficient qualification of personnel (lack of training for ISO 18436-7);
- Environment: High humidity (90%), presence of abrasive dust (pollution class 19/16/13 according to ISO 4406).
5. Established root causes
Ranking by probability and criticality (according to the FMEA method, RPN = Severity × Occurrence × Detection):
| The root cause | Probability | Criticality (1-10) | RPN | The evidence |
|---|---|---|---|---|
| Loss of pretension due to ball wear | 0.45 | 9 | 81 | Reduction of tension force from 350 N to 180 N, ball ovality 0.003 mm |
| Lubricant contamination with abrasive particles | 0.35 | 8 | 56 | The content of metal particles is 0.42%, the presence of quartz dust (size 5-50 microns) |
| Lubricant degradation (oxidation, loss of viscosity) | 0.20 | 7 | 28 | Acid number 1.8 mg KOH/g, viscosity 120 mm²/s (nominal 150) |
| Seal damage (wear, cracks) | 0.15 | 6 | 18 | Seal thickness reduction by 30%, 85 Shore A hardness |
| Corrosion of the screw due to moisture | 0.10 | 5 | 10 | Corrosion spots up to 0.02 mm deep, water content in lubricant 0.15% |
6. Corrective actions
6.1 Emergency measures
- Replacing the ball-screw pair:
- Use the original KGP Parker G01284 or an analogue with accuracy class C5 (for example, UNITEC-D article 4010-10-C5);
- Check the tightening torque of the nut (30 N·m ± 2 N·m per EN 1090-2);
- Install a new set of seals (item UNITEC-D 4010-SEAL-KIT).
- Restoration of previous tension:
- Use a torque wrench to set a force of 350 N ± 10 N;
- Measure the backlash with a watch-type indicator (tolerance 0.005-0.01 mm);
- Fix the nut with a lock nut (moment 25 N·m).
- Replacement of lubricant:
- Remove the old lubricant by washing with kerosene (purity class according to ISO 4406 not lower than 15/12);
- Fill with new grease Klüber Isoflex NBU 15 (volume 80 g for nut G01284);
- Check the oil level after 24 hours of operation.
6.2 Long-term measures
- Oil condition monitoring:
- Install an oil contamination sensor (for example, Parker icountPD);
- Conduct a laboratory analysis of the lubricant every 2,000 hours (ASTM D7412 method);
- Replace the lubricant when the limit values are exceeded (metal particles > 0.1%, water > 0.05%).
- Improving operating conditions:
- Install additional seals made of fluororubber (hardness 90 Shore A) to protect against dust;
- Reduce the humidity in the area of operation to 60% (using dehumidifiers);
- Ensure a temperature regime of 20-25°C (avoid overheating > 80°C).
- Modernization of the maintenance system:
- Implement a vibration monitoring system (Wilcoxon Research 786A sensors);
- Train personnel according to the ISO 18436-7 program (vibration analysis);
- Reduce the oil change interval to 3,000 hours.
- Design improvements:
- Replace standard seals with seals with integrated magnetic filters (item UNITEC-D 4010-MAG-SEAL);
- Use grease with solid additives (for example, Klüber Isoflex Topas NB 52);
- Install an automatic lubrication system (for example, Lincoln Quicklub).
7. Quick diagnostic checklist for technicians
Use this checklist on your tablet or smartphone during your PCP review:
| № | Verification | Tool | Limit value | Action in case of excess |
|---|---|---|---|---|
| 1 | Backlash measurement | Clock type indicator (0.001 mm) | > 0.02 mm | Replace the KGP or restore tension |
| 2 | Pretensioning force | Torque wrench (5 N·m) | < 250 N | Adjust the tension to 350 N |
| 3 | Vibration when reversing | Vibrometer (e.g. Fluke 805) | > 0.25 mm/s² | Conduct an analysis of lubricants and seals |
| 4 | Noise level | Noise meter (class 2 for IEC 61672-1) | > 80 dB | Check lubrication and ball wear |
| 5 | Nut temperature | Infrared thermometer (eg Fluke 62 MAX+) | > 60°C | Reduce the load or replace the lubricant |
| 6 | Condition of seals | Microscope (10x magnification) | Cracks > 0.5 mm or wear > 20% | Replace the seal (item UNITEC-D 4010-SEAL-KIT) |
| 7 | The content of metal particles in the lubricant | Ferrograph (e.g. Spectro Scientific) | > 0.1% | Change the oil and flush the system |
| 8 | Lubricant viscosity at 40°C | Viscometer (ASTM D445 method) | < 130 mm²/s | Replace the lubricant with Klüber Isoflex NBU 15 |
| 9 | The acid value of the lubricant | Titrator (ASTM D664 method) | > 1.0 mg KOH/g | Replace the oil |
| 10 | The water content of the lubricant | Coulometric titrator (ASTM D6304 method) | > 0.05% | Dry the system and replace the oil |
| 11 | The ovality of the balls | Micrometer (accuracy class 0.001 mm) | > 0.002 mm | Replace the balls or the entire KGP |
| 12 | Corrosion on the screw | Magnifying glass (20x magnification) | Spots > 0.01 mm | Polishing or replacing the screw |
Red flags (early warnings)
- An increase in vibration by 20% from the nominal level;
- The temperature of the nut exceeds 50°C under normal load;
- A change in the color of the lubricant (darkening or a gray shade);
- Appearance of metal dust on seals;
- Reversal time increase by 10% (for systems with CHPK).
8. Prevention strategy
8.1 Maintenance intervals
| Operation | Periodicity | Method | Standard |
|---|---|---|---|
| Backlash check | 1,000 hours | Clock type indicator | DSTU EN 13018:2015 |
| Vibration analysis | 500 hours | Vibrometer (ISO 10816-3) | ISO 18436-2 |
| Lubricant replacement | 3,000 hours | Rinsing + refueling | ISO 4406 |
| Lubricant analysis | 2,000 hours | Laboratory analysis (ASTM D7412) | ASTM D7412 |
| Inspection of seals | 1,500 hours | Visual inspection + microscope | ISO 3601-1 |
| Comprehensive diagnostics | 6,000 hours | Checklist from Chapter 7 | EN 13306 |
8.2 Condition monitoring
- Vibration monitoring:
- Install sensors on the nut and screw supports;
- Limit values: 1.5 mm/s (general level), 0.2 mm/s² (when reversing);
- Software: SKF @ptitude or equivalent.
- Temperature monitoring:
- Use infrared sensors (for example, Optris PI 450);
- Limit value: 60°C (long-term excess);
- Emergency shutdown at 80°C.
- Oil monitoring:
- Contamination sensors (for example, Parker icountPD);
- Limit values: metal particles > 0.05%, water > 0.03%;
- Automatic notification when exceeding.
8.3 Structural Improvements
- Using seals with magnetic filters:
- UNITEC-D 4010-MAG-SEAL article traps metal particles > 5 μm in size;
- Reduces oil contamination by 40%;
- Compatible with Parker G01284.
- Transition to grease with solid additives:
- Klüber Isoflex Topas NB 52 contains molybdenum disulfide (MoS₂);
- Reduces the coefficient of friction by 25% at high loads;
- Service life increases by 30%.
- Automatic lubrication system:
- Lincoln Quicklub provides metered supply of lubricant every 50 hours;
- Excludes the human factor;
- Reduces oil consumption by 15%.
- Use of KGP with pretension:
- Model Parker G01284-P (with pretension of 2% of dynamic load);
- Reduces backlash by 50% compared to the standard model;
- Recommended for high precision applications.
9. Conclusion
The increase in the backlash of the ball-screw pair is the result of the complex effect of loss of pretension, contamination of the lubricant and its degradation. A systematic approach to diagnosis and prevention, based on ISO 3408, EN 1090 and ASTM D7412 standards, allows you to reduce the probability of failure by 70% and increase the service life of the KGP to 15,000-20,000 hours. To implement the proposed measures, we recommend using certified components and consumables from the UNITEC-D E-Catalog catalog, including seals, lubricants, and monitoring systems.
10. Sources
- ISO 3408-3:2018. Ball screws — Part 3: Acceptance conditions and acceptance tests.
- EN 1090-2:2018. Execution of steel structures and aluminum structures — Part 2: Technical requirements for steel structures.
- ASTM D7412-18. Standard Test Method for Condition Monitoring of Phosphate Antiwear Additives in In-Service Petroleum and Hydrocarbon Based Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry.
- 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.
- ISO 4406:2021. Hydraulic fluid power — Fluids — Method for coding the level of contamination by solid particles.
- DSTU EN 13018:2015. Non-destructive testing. Visual control. General principles.
- Klüber Lubrication. Technical Data Sheet: Isoflex NBU 15. 2022.
- Parker Hannifin. Ball Screw Catalog: G-Series. 2021.
- UNITEC-D GmbH. Failure Analysis Report: Ball Screw Backlash Increase. 2023.
- ISO 18436-4:2014. Condition monitoring and diagnostics of machines — Requirements for qualification and assessment of personnel — Part 4: Field lubricant analysis.
