Precision Lead Accuracy, Friction Torque, and Tolerances

Lead accuracy describes how precisely the actual travel per revolution of the shaft corresponds to the nominal lead value. Manufacturing-related deviations accumulate over the entire travel length, resulting in cumulative lead deviation, which directly affects the positioning accuracy of the system along the entire travel range. 

Positioning accuracy refers to the maximum deviation between the target position and the actually achieved position. Repeatability, on the other hand, measures how precisely a position can be reproduced when approached repeatedly from the same direction. A system can exhibit excellent repeatability even if there is a constant systematic deviation (offset) from the target position – this offset can be corrected through calibration or software compensation. 

Preload reduces or completely eliminates backlash and significantly increases the axial stiffness of the ball screw assembly. This improves controllability, repeatability, and dynamic response of the drive system. At the same time, higher preload can slightly increase friction torque and heat generation. The optimal preload is therefore a targeted compromise between stiffness, friction, smooth running, and service life – tailored to the specific requirements of the application. 

System accuracy is influenced by numerous factors beyond the manufacturing precision of components: 

• Mounting accuracy and precise alignment 
• Bearing preload and bearing quality 
• Lubrication (type, viscosity, temperature dependency) 
• Temperature drift and thermal expansion 
• Drive control (backlash compensation, friction compensation) 
• Resolution of feedback system (encoders, measuring systems) 

In practice, thermal expansion of the shaft and nut is often the dominant factor for positioning errors – thermally stable design and maybe even active temperature compensation are therefore crucial for highest accuracy. 

Form tolerances (e.g., roundness, straightness of the shaft) and position tolerances (e.g., parallelism, perpendicularity, concentricity of bearing surfaces) define the geometric quality of shaft, nut, and bearing arrangement – for example according to DIN ISO 3408. 

Deviations from ideal geometry increase friction, generate tilting moments, and negatively affect accuracy, smooth running, wear, and service life. 

Application-appropriate design is key: Form and position tolerances should be matched to specific system requirements. Quality is defined by reliable adherence to specified tolerances. 

Heating of the shaft, nut, or surrounding machine structure leads to thermal expansion and also changes the friction torque in the system. Without active compensation, the position drifts continuously – especially with long travel distances and high rotational speeds. 

Effective measures for temperature control: 

• Thermally stable design with symmetrical structure 
• Controlled and temperature-stable lubrication 
• Break-in phases before precision operation 
• Active temperature management (cooling, heating) 
• Software-based temperature compensation 
• Use of temperature-stable materials 

The overall accuracy of a linear positioning system depends on the precise interaction between mechanical components (ball screw, guides, bearings) and electronic control. Modern drive systems contribute significantly to precision: 

• High-resolution encoders enable fine position detection (typically 0.1 µm and better) 
• Zero-backlash mechanical coupling between motor and shaft eliminates lost motion 
• Optimally tuned controllers (PID, cascade control) ensure fast and stable settling behavior 
• Backlash compensation compensates for mechanical backlash 
• Friction models and feedforward control reduce velocity-dependent positioning errors 
• High-dynamic servo motors enable rapid acceleration with simultaneous precise positioning 

The result: minimized positioning errors, optimized path accuracy for contouring, and maximum dynamics with highest repeatability.