- Nominal Diameter
The nominal diameter is the shaft diameter of the ball screw. Generally, a larger shaft diameter supports a greater load.
The load capacity for different shaft diameters is listed in the manufacturer's catalog under two key parameters: dynamic load rating and static load rating.
The static load rating is the maximum load the ball screw can withstand while stationary. In most applications, the screw is in motion, so the dynamic load rating is the more relevant parameter. A lower actual load relative to the dynamic load rating results in a longer theoretical service life.
- Lead
Lead is the linear distance the nut travels when the screw completes one full revolution. Common lead values (in mm) include: 1, 2, 4, 5, 6, 8, 10, 16, 20, 25, 32, and 40. Lead directly affects both the nut's linear speed and the screw's available thrust.
A larger lead produces a higher linear speed at the same rotational speed. The relationship is:
v = r × i
- v: Nut linear speed (mm/s)
- r: Screw rotational speed (r/s)
- i: Lead (mm)
The relationship between lead and screw thrust is:
F = (2π × T × η) / i
- F: Screw thrust (N)
- T: Motor torque (N·m)
- η: Transmission efficiency (ball screw efficiency is typically 85%–95%)
- i: Lead (m)
Note: In the speed formula, lead is expressed in mm; in the thrust formula, lead must be converted to meters for consistent SI units.
- Pitch
Pitch is the axial distance between corresponding points on adjacent thread teeth. Lead is a multiple of pitch. For single-start screws, lead equals pitch (a 1:1 ratio). The ball screws offered by STEPPERONLINE are single-start, so pitch and lead are numerically identical. In practice, lead is the primary parameter used for selection and discussion.
- Nut Compatibility
Ball screw nuts from different manufacturers are not interchangeable, even when the pitch and lead values are identical. Thread profiles, tolerances, and ball groove geometries vary between manufacturers. Always use a nut matched to its original screw to ensure proper fit and performance.