Ball Screw Accuracy Grades C0–C10: Why the Numbers Skip, and How to Choose
Nahshon Tech Column #03 | Selection Guide
Open an NSK ball screw catalog and the accuracy grades read C0, C1, C2, C3, C5, Ct7, Ct10 — no C4, no C6, no C9. That is not a misprint. Under JIS B 1192 these are two different grade systems: the C series for positioning and the Ct series for transport.
What the grades actually measure
Accuracy grades govern lead accuracy — how far the nut actually travels versus theory. JIS B 1192 defines four characteristics (NSK catalog E3162, Tables 1.1–1.3): ±ep (tolerance on specified travel over the full thread), υu (travel variation over the full thread), υ300 (variation within any 300 mm), and υ2π (variation within any single revolution). Allowable values of υ300 and υ2π:
| Item | C0 | C1 | C2 | C3 | C5 | Ct7 | Ct10 |
|---|---|---|---|---|---|---|---|
| υ300 (µm/300 mm) | 3.5 | 5 | 7 | 8 | 18 | 52 | 210 |
| υ2π (µm/rev) | 2.5 | 4 | 5 | 6 | 8 | — | — |
Why the numbers skip
C0–C5 are precision-ground positioning screws: all four characteristics are guaranteed, typically with zero-backlash preload. Ct7/Ct10 are rolled transport screws: only υ300 is specified, and the cumulative error is derived as ep = ±(lu/300) × υ300 (NSK Table 1.4). The numbers are step codes in two separate ladders, not a continuous series — and the tolerance roughly doubles at each step (8 → 18 → 52 µm from C3 to C5 to Ct7), matching real jumps in grinding, inspection, and thermal-control cost. There is no natural process step for a "C4". Note also that higher grades are limited in length: C0 is guaranteed up to 1,600 mm of effective thread, C5 up to 12,500 mm (NSK Table 1.2).
Selection guide
| Grade | Typical applications | Notes |
|---|---|---|
| C0–C2 | Optics, wafer-level ultra-precision positioning | Made to order, long lead time |
| C3 | Semiconductor/LCD equipment, inspection and metrology | Positioning workhorse, usually zero-backlash preload |
| C5 | General automation, machine-tool feed axes | Best value, widest stock availability |
| Ct7 | Transfer and loading mechanisms | Rolled screws, lowest cost |
A practical example from the catalog: a 1,000 mm stroke axis requiring ±0.035 mm positioning accuracy has roughly 1,300 mm of thread; C5 (±0.054 mm for 1,250–1,600 mm) fails, C3 (±0.029 mm) passes. Calculate first, then pick the grade — and remember each grade up is a step up in price and lead time. Over-specifying a transfer axis with C3 wastes money; under-specifying a positioning axis with Ct7 is a disaster.
Nahshon is an NSK distributor in Taiwan: Compact FA-PSS (C5, axial play ≤ 0.005 mm) from standard stock with short lead times, C3-grade USS (zero backlash, cleanroom-ready) available to order, plus custom shaft-end machining. Try our AI selection assistant or contact us for a quote.
Allowable values cited from NSK catalogs E3162 (Tables 1.2–1.4) and E3239 (Compact FA series).