Ball Screw Accuracy Grades: C3, C5, C7 and C10
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Ball screws are graded by lead accuracy: how closely the nut's actual travel matches what the lead says it should be. The grades come from the JIS and ISO standards and run from C0, the most accurate, to C10.
What the grades allow
Values per 300 mm of travel, from THK's catalog. The precision grades are specified as a variation band, and C7 and coarser as travel error.
| Grade | Allowed error per 300 mm | Usually |
|---|---|---|
| C3 | 8 µm | ground |
| C5 | 18 µm | ground |
| C7 | ±50 µm | rolled |
| C10 | ±210 µm | rolled |
For context, ±50 µm over 300 mm is a lot for a machine tool but fine for a conveyor or a positioning stage that homes to a sensor.
Rolled vs ground
- Rolled screws have the thread formed by rolling dies. They are much cheaper, come in long lengths and stock sizes, and are accurate enough for most automation.
- Ground screws are machined and precision ground. They hold the tighter grades and run smoother, at a much higher price and a longer lead time.
Accuracy isn't backlash
Lead accuracy describes travel error in one direction. Backlash is the play when the direction reverses, and it's controlled separately by preload: a single nut with oversized balls, a nut with a shifted lead in the middle, or two nuts pushed against each other. All of them remove play at the cost of some friction and heat. Machine tools use preloaded nuts; many automation axes don't need them.
Heat and length
A steel screw grows about 12 µm per meter for each degree Celsius. A 1 m screw that warms up 5 °C grows about 0.06 mm, more than a C5 screw's whole error over 300 mm. Machines deal with it through pretensioned or cooled screws, linear scales and software compensation. Long, slender screws also have a critical speed: double the unsupported length and the speed at which the screw starts to whip drops to about a quarter.
Which grade
- CNC machine tools: C3 or C5 ground, preloaded, often with linear scales.
- Precision automation and inspection equipment: C5 to C7.
- General automation, conveyors and positioning with homing: C7 to C10 rolled.
We supply ball screws and linear rails, and remake worn or obsolete screws. See our bearings, ball screws and linear rails and the ball screw case study.