Lead: The linear distance (mm/rev) the nut travels per one screw rotation (360°). It is the primary parameter that determines travel speed and resolution.
Small lead: High resolution, high thrust (at the same torque), and low travel speed.
Large lead: High travel speed, low resolution, and low thrust (at the same torque).
Nominal diameter: The outer diameter of the screw (mm). This primarily affects the screw’s rigidity and load-bearing capacity. A larger diameter indicates greater rigidity and load-bearing capacity.
Accuracy grade: Indicates the manufacturing accuracy of the screw, typically measured in terms of cumulative travel error (e.g., the error range per 300mm of travel, in μm). Common grades include C0, C1, C3, C5, C7, and C10 (the smaller the number, the higher the accuracy). The selection depends on the application requirements.
Dynamic Load Rating: The maximum axial load a screw can withstand during its rated life (typically 1 million revolutions). This is the core factor in the selection calculation.
Static Load Rating: The maximum axial load a screw can withstand without permanent deformation when stationary or at very low speeds.
Preload: An internal preload (interference fit) is applied to the balls within the nut to eliminate axial play and improve rigidity and positioning accuracy. The preload level (light, medium, or heavy) should be selected based on the application load, accuracy, and rigidity requirements. Excessive preload increases friction, heat, and reduces life.
Critical Speed: The upper limit of the speed at which a screw may experience resonance (bending vibration) at high speeds. When selecting a screw, ensure that the operating speed is well below the critical speed (typically 60%-80%). If necessary, increase the critical speed by using additional screw supports (fixed-supported, fixed-fixed) or reducing the span.