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).
FAQ
NSK 滾珠螺桿精度等級為什麼沒有 C4、C6？
因為 C 系與 Ct 系是兩套不同體系：C0–C5 是定位用、精密研磨、四個特性值全保證；Ct7／Ct10 是搬送用、以轉造為主、只保證 υ300。相鄰等級的公差帶接近倍增，對應研磨與檢驗成本的實質分界，中間塞一個 C4 或 C6 在製程上沒有自然級距。
C3 和 C5 到底差多少？
以 υ300（任取 300 mm 行程內的變動量）看，C3 是 8 µm、C5 是 18 µm，相差超過一倍。C5 是標準庫存量最大的等級、交期以天計；C3 屬精密研磨訂製品，交期以月計。
定位精度要求 ±0.035 mm 的進給軸該選哪一級？
先把行程換算成有效螺紋長再查表。型錄示範算例是行程 1,000 mm、加上螺帽長與餘裕後螺紋長約 1,300 mm，該區間 C5 為 ±0.054 mm 不合格，C3 為 ±0.029 mm 才過關。順序是先算需求再查表，不是先挑等級。