Common Issues with Ball Screws: Cause Analysis and Solutions

I. Excessive Backlash Issues

1. Insufficient Preload

• Cause: Lack of preload or inadequate preload leads to nut slippage due to self-weight in vertical installations, resulting in significant backlash under no-load conditions.
• Analysis: Unpreloaded ball screws can exhibit backlash exceeding 0.05mm, compromising positioning accuracy and limiting use to low-load, low-precision applications.
• Solutions:
  ▶ Apply preload equivalent to 1–3% of the rated dynamic load using dual-nut shim or spring preloading;
  ▶ Choose single-nut structures with built-in preload (precision class C5 or higher) for high-accuracy scenarios.

2. Excessive Torsional Deflection

• Cause: Improper heat treatment (insufficient hardness, uneven hardness distribution, or soft material) or an excessively high length-to-diameter ratio (L/D > 70) reducing rigidity.
• Analysis: An L/D ratio exceeding 70 may cause screw sagging due to self-weight, leading to nut misalignment and increased backlash; substandard material hardness accelerates wear.
• Solutions:
  ▶ Maintain L/D ≤ 60 and use dual-end fixed supports (instead of single-side supports) for heavy loads;
  ▶ Select high-strength alloy steel (e.g., SUJ2) and ensure heat treatment meets industry hardness standards (ball: HRC 62–66, nut: HRC 58–62, screw: HRC 56–62).

3. Improper Bearing Selection and Installation

• Cause: Using deep groove ball bearings instead of angular contact bearings, or misalignment during bearing installation (perpendicularity error > 0.02mm/m).
• Analysis: Deep groove ball bearings cannot withstand axial loads, causing axial play; bearing tilt leads to periodic backlash variations.
• Solutions:
  ▶ Prioritize angular contact bearings with a 60° contact angle (e.g., 7000 series), installed back-to-back;
  ▶ Ensure bearing seat perpendicularity to the screw shoulder within 0.01mm tolerance during machining, using double locknuts for anti-loosening.

4. Inadequate Support Rigidity

• Cause: Thin-walled or low-strength materials (e.g., cast iron instead of steel) for nut or bearing seats.
• Analysis: Elastic deformation under load shifts the screw axis, effectively increasing backlash.
• Solutions:
  ▶ Increase support wall thickness (recommended ≥15mm) or reinforce with ribbed structures;
  ▶ Use 45# steel with quenching and tempering (hardness HB220–250) for critical components.

II. Erratic Motion Issues

1. Machining Precision Defects

• (1) Excessive Surface Roughness
  • Cause: Insufficient grinding precision for screw/nut raceways (Ra > 0.4μm) or ball roundness error > 0.001mm.
  • Solutions: Adopt super-finishing processes to control raceway roughness at Ra ≤ 0.2μm; screen balls for roundness error ≤ 0.0005mm.
• (2) Lead/Pitch Deviation
  • Cause: Inadequate precision of thread machining tools (e.g., pitch cumulative error > ±0.015mm/300mm).
  • Solutions: Use high-precision grinders (positioning accuracy ±0.005mm) and fully inspect finished screws with laser lead measuring instruments.
• (3) Recirculation System Failure
  • Cause: Misalignment of recirculation tubes (>0.5mm offset) or burrs inside tubes causing ball jams.
  • Solutions: Use positioning fixtures to align recirculation tubes with raceways; conduct no-load running tests at speeds ≥500mm/s after assembly.

2. Foreign Material Ingress and Lubrication Failure

• (1) Raceway Contamination
  • Cause: Lack of dust protection (e.g., scrapers), allowing machining chips (>50μm) or dust to enter raceways.
  • Solutions: Install double-lip seals (IP54 protection); clean raceways with kerosene and replenish lithium-based grease (NLGI Grade 2) every 200 operating hours.
• (2) Insufficient Lubrication
  • Cause: Exceeding lubrication intervals (>200 hours) or using incorrect grease (e.g., calcium-based instead of lithium-based).
  • Solutions: Integrate automatic lubrication systems (greasing interval ≤8 hours) for automated equipment; use molybdenum disulfide grease for high-temperature environments.

3. Installation Misalignment

• Cause: Parallelism error between nut seat and guide rail > 0.1mm/m or coaxiality error between bearing seat hole and screw axis > 0.03mm.
• Analysis: Eccentric loading increases raceway friction by over 30% due to unilateral stress on balls.
• Solutions: Calibrate with a dial indicator during installation (parallelism ≤0.05mm/m, coaxiality ≤0.02mm); use shims for alignment if necessary.

III. Component Failure Issues

1. Ball Fracture

• Cause:
  ▶ Material defects (e.g., inclusions) or under-heat treatment (hardness ▶ Thermal stress concentration (temperature difference >50°C causing stress >800MPa due to coefficient of expansion mismatch).
• Solutions:
  ▶ Select SUJ2 bearing steel balls and reject defective ones via magnetic particle inspection;
  ▶ Add cooling structures (e.g., hollow screws with coolant) for high-speed applications, limiting temperature rise ≤30°C.

2. Recirculation Tube Damage

• Cause: Over-travel (nut exceeding effective stroke by >10mm) or impact during installation (force >50N).
• Analysis: Deformed tubes block ball circulation, causing local pressure surges and raceway spalling (fatigue life reduced by 70%).
• Solutions:
  ▶ Set dual limits (hard + soft) with a safety margin ≥5mm in control software;
  ▶ Use impact-resistant nylon recirculation tubes (instead of plastic) and test stroke limits after assembly.

3. Screw Shoulder Fracture

• Cause:
  ▶ Design flaws (transition radius 2.5);
  ▶ Uneven locknut torque (deviation >±10%) causing shoulder runout >0.02mm.
• Solutions:
  ▶ Optimize shoulder design with transition radii R5–R8mm and perform finite element analysis (safety factor ≥2.0);
  ▶ Tighten locknuts with a torque wrench (e.g., M20 nut: 150–180N·m) and ensure shoulder runout ≤0.01mm.

IV. Preventive Maintenance Recommendations

1. Regular Inspections:
   • Backlash Measurement: Use a dial indicator to check backlash (no-load ≤0.01mm, full-load ≤0.03mm);
   • Vibration Analysis: Monitor vibration with accelerometers (RMS value ≤1.5m/s²).
2. Lubrication Management:
   • Manual Lubrication: Apply grease (1/3 nut volume) every 100 operating hours;
   • Automatic Lubrication: Use progressive distributors with 0.5–1ml grease per injection, interval 4 hours.
3. Spare Parts Management:
   • Critical Spares: Stock balls (same batch), recirculation tubes, and seals;
   • Scheduled Replacement: Replace ball screws every 3 years or 10,000 hours under high-speed conditions.