Robotic systems often need to turn motor rotation into straight line motion for lifting, pushing, positioning, or clamping. Lead screws and ball screws are two common mechanisms that do this inside linear actuators, CNC machines, 3D printers, and robot joints. Both use a threaded shaft and a moving nut, but their friction, accuracy, cost, and maintenance needs are very different.
Understanding the difference helps engineers choose the right actuator for speed, force, precision, and budget.
In a lead screw, the nut slides along the screw threads, so the motion depends on sliding friction between surfaces. In a ball screw, small hardened balls roll between the screw and nut, often circulating through return channels, which greatly reduces friction. Lower friction makes ball screws more efficient and better for high speed or high precision motion, while lead screws can be simpler, cheaper, quieter, and sometimes self locking.
Backlash, efficiency, lead, pitch, and load capacity are the main ideas used to compare these mechanisms.
Key Facts
- Linear travel per revolution is x = L N, where L is screw lead and N is number of revolutions.
- Linear speed is v = L f, where f is rotational speed in revolutions per second.
- Ideal axial force from torque is F = 2 pi eta T / L, where eta is efficiency, T is torque, and L is lead.
- Lead screws usually use sliding contact and often have efficiency from about 20% to 50%.
- Ball screws use rolling contact with recirculating balls and often have efficiency from about 85% to 95%.
- Backlash is lost motion between the screw and nut, and it reduces positioning accuracy unless compensated or preloaded.
Vocabulary
- Lead screw
- A threaded shaft that converts rotation into linear motion using sliding contact with a nut.
- Ball screw
- A screw mechanism that uses recirculating balls between the screw and nut to convert rotation into low friction linear motion.
- Lead
- The axial distance a nut moves along a screw during one full revolution.
- Backlash
- The small amount of lost motion caused by clearance between mechanical parts when direction reverses.
- Preload
- A controlled internal force or adjustment that removes clearance and reduces backlash in a screw mechanism.
Common Mistakes to Avoid
- Confusing pitch with lead is wrong because pitch is the spacing between thread peaks, while lead is the linear travel per revolution.
- Ignoring efficiency is wrong because the same motor torque can produce very different linear forces in a lead screw and a ball screw.
- Assuming all screws are self locking is wrong because many ball screws and high lead screws can backdrive when a load pushes on them.
- Forgetting backlash during direction changes is wrong because the motor may rotate before the carriage actually moves, causing position error.
Practice Questions
- 1 A lead screw has a lead of 4 mm per revolution. How far does the nut move after 25 revolutions?
- 2 A ball screw has a lead of 10 mm per revolution and rotates at 300 rpm. What is the linear speed of the actuator in mm/s?
- 3 A robot gripper must hold a vertical load in place with power off, move slowly, and be low cost. Would a lead screw or ball screw usually be the better choice, and why?