A CNC axis that lost 0.02 mm of repeatability after 6 months. The axis used a rolled ball screw (C7 accuracy) with a single nut. The controller had backlash compensation set at 0.05 mm. The compensation worked for 6 months. Then the repeatability degraded. The customer re-measured the backlash. It had grown from 0.05 to 0.15 mm. The compensation was set for 0.05. The extra 0.1 mm was uncompensated. The ball nut wasn’t worn out — it was a single nut without preload. The balls had worn. The backlash increased. This is about ball screw preload and why a single nut can’t maintain accuracy over its life.
The three preload classes
A ball nut can be supplied with different preload levels. Preload means the balls are slightly larger than the space between the nut and screw. The balls are compressed. There’s no backlash. The screw is rigid in both directions.
| Preload class | Preload force | Backlash | Use for |
|---|---|---|---|
| Z0 (no preload) | 0 | 0.05-0.15 mm | Light load, slow, no reversal |
| Z1 (light preload, C0) | 5% of dynamic load | 0 | General automation, moderate accuracy |
| Z3 (medium preload, C3) | 10% of dynamic load | 0 | High precision, CNC, reversing loads |
The failed axis had a Z0 (unpreloaded) nut. The backlash was 0.05 mm when new. The controller compensated 0.05 mm. After 6 months of reversing loads, the balls wore by 0.05 mm. The backlash grew to 0.10 mm. The compensation was now half the actual backlash. The positioning error was 0.05 mm. The customer thought the compensation drifted. It didn’t — the nut wore.
What was changed
1. Replaced with a C3 preloaded nut. The single nut was replaced with a double-nut preloaded assembly (C3 preload). The two nuts are spring-loaded against each other. The balls are preloaded. There’s zero backlash from day one. As the balls wear, the spring compensates — the preload maintains zero backlash for the rated life (10 million revolutions). The positioning accuracy stayed at ±0.005 mm for 5 years. The C3 nut costs 2x the single nut. The accuracy it provides is worth it for a CNC axis.
2. Switched to a ground screw (C5 accuracy). The original was a rolled screw (C7 accuracy, ±0.05 mm/300 mm). A ground screw (C5, ±0.018 mm/300 mm) has better lead accuracy. With C3 preload, the axis repeatability improved to ±0.005 mm. The ground screw costs 3x the rolled one. For a CNC milling machine, the precision is required. For a simple positioning axis, C7 + C3 preload is enough.
3. Adjusted the backlash compensation to zero. With a preloaded nut, there’s no backlash. The compensation was set to 0 mm. The controller doesn’t need to compensate. A preloaded nut doesn’t need software compensation. The compensation was a band-aid for the unpreloaded nut. Removing the compensation simplified the tuning. The axis moved more smoothly (no sudden reversal step).
The preload tradeoff
Preload eliminates backlash but increases friction and heat. A C3 preloaded nut has 2-3x the friction torque of a Z0 nut. At high speeds, the preload generates heat. The screw runs hotter. The lubrication breaks down faster. For a high-speed axis (over 1000 RPM), a C0 (light preload) is the sweet spot — enough preload for accuracy, not so much that it overheats. For a low-speed positioning axis (under 500 RPM), C3 is fine.
The preload also increases the required motor torque. The preload friction torque must be added to the load torque. For a 20 mm screw with C3 preload: friction torque = 0.5 N·m. The motor must provide this torque even at no load. A motor sized for load torque only will be undersized. Always add the preload friction to the motor sizing.
The backlash compensation myth
Many machine builders use backlash compensation (software offset) instead of preloaded nuts. The compensation tells the controller “when reversing, add 0.05 mm to the command.” This works when the backlash is constant. But as the nut wears, the backlash changes. The compensation becomes wrong. The axis loses accuracy. A preloaded nut eliminates the problem entirely — there’s no backlash to compensate. Software compensation is a short-term fix. Preloaded nuts are the long-term solution. For production machines that must hold accuracy for years, specify C3 preload. Don’t rely on software compensation to hide a mechanical wear problem.
The preload rule: C3 preloaded double nut for any reversing positioning axis. The drifting axis wasn’t compensation drift — the unpreloaded nut wore and the backlash grew. Use C0 for high speed, C3 for precision. Software backlash compensation is a band-aid; a preloaded nut is the cure. Add preload friction torque to motor sizing. A CNC axis that needs to hold ±0.01 mm for years must have zero-backlash mechanics, not software patches.
