Accuracy classes are standardized categories that define the precision with which a mechanical component is manufactured. They specify the tolerances—the permissible deviations from ideal dimensions or functions—that a part may have. The smaller the tolerances, the higher the accuracy class. For thread drives (e.g., ball screws, trapezoidal screws, spindles, nuts), accuracy classes primarily define:
- Lead accuracy (deviation of the actual lead from the ideal lead)
- Geometric and positional tolerances (e.g., roundness, straightness, coaxiality)
- Backlash or preload accuracy
- Axial positioning accuracy
Typical Accuracy Classes for Thread Drives
Manufacturers usually use internationally standardized designations. For ball screws, for example, the following classes apply:
IT0 / IT1 extremely high precision, only a few micrometers of deviation: measuring machines, precision axes
IT3 very high accuracy: CNC machines, machine tools
IT5 medium accuracy: automation, robotics
IT7 / IT10 standard class: general linear movements, simple positioning
The smaller the number, the more precise the spindle.
Not every application requires maximum precision. For a simple lifting axis, a lower accuracy class is sufficient, whereas extremely low lead deviations are necessary in CNC machining or measuring systems. This allows designers to select components specifically based on costs and requirements.