Ball Screws: Eight guides from selection entry to troubleshooting
Eight guides from selection entry to troubleshooting
A ball screw is not chosen by diameter alone. These five fix the model:
Accuracy is stated as travel deviation over 300 mm, not as a vague "precision" level:
| Grade | Deviation / 300 mm | Process | Typical use |
|---|---|---|---|
| C3 | ≈ 0.008 mm | Ground | Semiconductor handling, metrology |
| C5 | ≈ 0.018 mm | Ground | CNC machining centres, laser processing |
| C7 | ≈ 0.050 mm | Rolled | General automation, lifting, feed |
| C10 | ≈ 0.210 mm | Rolled | Rough positioning, low-cost transport |
A smaller number is more precise, but cost climbs steeply. If your positioning tolerance is ±0.05 mm, C7 is the economical choice — spend the difference on preload and support units.
Preload removes axial clearance and raises stiffness; the cost is higher friction torque, more heat and shorter life. Preload and accuracy are independent — a C5 screw can run Z0 or Z3.
| Class | Meaning | Use |
|---|---|---|
| ZF | Slight clearance | Light load, lowest friction, longest life |
| Z0 | No preload (standard) | General feed, some backlash acceptable |
| Z1 | Light preload | Positioning axes needing low backlash |
| Z2 | Medium preload | Machining axes needing stiffness |
| Z3 | Heavy preload | Maximum stiffness, accepting heat and life penalty |
Rated life is expressed first as total revolutions, then converted to hours:
Ca is the dynamic load rating (kgf) and Fm the equivalent axial load (kgf). In hours:
Real duty cycles vary, so work out the equivalent load across segments before substituting. Life scales with the cube of load: 26% more load halves the life, which explains most premature failures.
A slender screw resonates above its critical speed regardless of load; long strokes must also be checked for buckling. Both are governed by the support arrangement:
Above roughly 1 m of stroke or 1000 rpm, run both checks. Most "correctly sized but vibrating" screws fail here.
Shaft ends are machined to drawing: keys to GB/T 1095, retaining-ring grooves to GB/T 894, fits to GB/T 1800, screw acceptance to GB/T 17587.3.
Fixed ends use the BK / EK / FK series, supported ends the BF / EF / FF series, selected by shaft diameter. Where the shaft diameter does not match the housing bore, machine the shaft end — do not shim it.
| Symptom | Likely cause | Action |
|---|---|---|
| Noise, rough travel | Poor lubrication, contamination, bad ball circulation | Clean, re-lubricate, check seals |
| Position drift | Lost preload, loose locknut | Re-torque the locknut, re-check preload |
| Overheating | Excess preload, poor lubrication, over-speed | Lower the preload class or improve lubrication |
| Early pitting | Overload or shock loading | Re-select against the dynamic load rating |
Record load, speed, stroke and lubrication before diagnosing — swapping parts on symptoms alone rarely fixes the cause.
Go by real positioning tolerance. C5 is ground at about 0.018 mm per 300 mm; C7 is rolled at about 0.050 mm. For general automation within ±0.05 mm, C7 is the economical pick — spend the difference on preload and support units.
No. Accuracy (C3/C5/C7/C10) describes travel deviation; preload (ZF/Z0/Z1/Z2/Z3) describes axial clearance and stiffness. They are chosen independently, so a high-accuracy screw can run low preload.
Usually critical speed or buckling. Both depend on stroke, shaft diameter and support arrangement — not on the motor or the load. Fixed–supported gives the highest coefficient and is the default for long strokes.
Yes. We machine steps, keyways to GB/T 1095, ring grooves to GB/T 894 and special nut flanges, and issue a part drawing for approval before cutting.