A lead screw turns rotation into a straight push. Each turn advances the nut by one lead, and the torque needed to drive a load is just the work done per turn — load times lead — spread over the 2π radians of a revolution and inflated by the screw's efficiency loss. That is why a hand-cranked screw jack can lift a car: a fine lead and heavy friction mean a small torque produces an enormous axial force. The same friction makes most power screws self-locking, so the load stays put when you let go.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: power-screw work–energy relations.
The lead-screw equations
Keep the lead in metres when working in SI so torque comes out in newton-metres. Efficiency carries everything: a sliding ACME screw at 30% needs three times the ideal torque, while a ball screw at 90% needs barely more than the frictionless minimum. The flip side is holding: low efficiency makes a screw self-locking (no brake needed), while a ball screw will back-drive under load and must be braked.
Worked example — a 500 kg actuator
Scenario: Lift 5,000 N on a 5 mm-lead ACME screw at 30% efficiency, driven at 200 rpm.
The motor must supply ~13.3 N·m and ~278 W to lift the load at 16.7 mm/s. Because efficiency is below 50%, the screw is self-locking — remove the torque and the load holds without a brake. Swapping to a 90% ball screw would cut the torque to ~4.4 N·m and the power to ~93 W, but you would then need a holding brake because a ball screw back-drives.
Frequently Asked Questions
T = F·lead/(2π·η): load × lead over one turn, divided by efficiency. 5000 N on a 5 mm/30% screw ≈ 13.3 N·m.
Pitch = thread-to-thread spacing; lead = travel per turn. Single-start: equal. Multi-start: lead = pitch × starts. Lead drives travel.
Sliding ACME 20–40%; ball screws 85–90%. Higher efficiency = less torque and heat, but ball screws aren't self-locking.
The load can't back-drive the screw — it holds with no torque. Happens when lead angle < friction angle, i.e. efficiency below ~50%.
v = lead × rpm. A 5 mm lead at 200 rpm = 1000 mm/min (16.7 mm/s). More lead = faster but more torque per load.