As an efficient and precise transmission element, ball screw selection is a critical step in mechanical design. The following are key selection factors:
Key Parameter Matching:
Axial Load: Determine the maximum dynamic and static load requirements, and accordingly select the screw’s nominal diameter (which determines load capacity) and nut type. Load is the foundation of ball screw selection.
Accuracy Grade: Based on the equipment’s positioning accuracy and repeatability requirements (e.g., C5 or C7), select a screw with the appropriate accuracy grade. Precision equipment requires a higher grade.
Operating Speed: Determine the maximum speed (rpm) and required linear speed (m/min). For high speeds, ensure the screw’s dN value (nominal diameter x speed) is within a safe range. If necessary, select a screw with a larger lead.
Lead Selection:
Lead directly affects linear speed (speed = speed x lead) and motor torque requirements (under the same load, a larger lead reduces the required torque). A balance must be struck between speed requirements and torque limits.
Rigidity and Preload:
System rigidity is critical to positioning accuracy and vibration resistance. Select the appropriate nut preload level (light, medium, or heavy) based on application requirements. Higher preload increases rigidity, but also increases friction and temperature rise.
Support Method:
The support bearing configuration at each end of the screw (fixed-supported, fixed-fixed, etc.) directly affects the screw’s critical speed, compression rod stability, and system rigidity. The selection should be based on length, speed, and accuracy requirements.
Verification Calculation:
After the initial selection, a life calculation (usually based on the rated dynamic load), a critical speed check (to avoid resonance), and a compression rod stability check (to prevent compression instability in long screws) must be performed to ensure reliability and safety. An appropriate safety factor (usually 1.2-1.5) should be included.
Example: A CNC engraving machine requires high accuracy (±0.01mm), medium speed operation, and a 500kg load. When selecting a ball screw, prioritize C5-grade precision. Based on load calculations, choose a nominal diameter of 32mm and a lead of 10mm based on speed requirements. Use medium-preload nuts for increased rigidity, fixed-fixed supports for stability, and rigorously complete all verification calculations.
Only by systematically considering these factors and conducting rigorous calculations can we match our equipment with a ball screw system that delivers reliable performance and a long lifespan.