When you need to turn a motor's rotation into precise linear motion — CNC axes, 3D printer Z-stages, semiconductor stages, valve actuators — you reach for a screw drive. The two dominant choices are lead screws (also called ACME or trapezoidal screws) and ball screws. They look similar but behave very differently.

Lead screws use a threaded rod running through a nut with matching threads. The nut and screw slide against each other — pure sliding friction. This makes them:

• Cheap and simple — often under $20 for a meter of rod
• Self-locking when the lead angle is shallow (typically under ~5°): back-driving is impossible, so a Z-axis won't fall when power dies
• Inefficient — typically 20–50% mechanical efficiency. Most torque becomes heat in the nut
• Wear-limited — the nut is usually bronze or PTFE and slowly grinds away, developing backlash

Ball screws replace sliding friction with rolling contact. Hardened steel balls circulate through a helical race between screw and nut, returning via a recirculation tube. This changes everything:

• Efficient — typically 85–95%. A 1 Nm motor can push loads a lead screw would need 3 Nm to move
• Long-lived — rated in millions of revolutions, not thousands
• Precise — preloaded double-nuts eliminate backlash to under 5 µm
• NOT self-locking — a vertical ball screw will happily drop its load if the motor loses holding torque. You need a brake
• Expensive — $200+ per meter, plus precision mounting

Rule of thumb for sizing: Linear force F from motor torque T is roughly:

F ≈ (2π · T · η) / L

where L is the screw lead (linear distance per revolution) and η is efficiency. For a 5 mm lead ball screw (η ≈ 0.9) with 1 Nm torque: F ≈ (2π × 1 × 0.9) / 0.005 = 1130 N (about 250 lbf). The same screw as an ACME lead screw at η = 0.35 yields only ~440 N.

Real-world example: Consumer 3D printers use lead screws on the Z-axis — cheap, self-locking, and Z moves slowly enough that friction losses don't matter. But every serious CNC machining center uses ball screws on X and Y axes: they move at 30+ m/min under heavy cutting loads, and a lead screw would burn up in weeks while robbing the servo of most of its torque.

The choice is almost always about duty cycle and speed: intermittent, slow, vertical → lead screw. Continuous, fast, precise → ball screw with a brake.