How much torque does a lead screw need?

Torque = (force × lead) ÷ (2π × efficiency), with force in newtons and lead in metres. A T8 screw with an 8 mm lead lifting 100 N at 30% efficiency needs about 0.42 N·m — well beyond a small stepper, which is why fine-lead screws are used for lifting.

Will a lead screw hold its position without power?

A screw is self-locking when its lead angle is smaller than the friction angle — in practice when efficiency is below roughly 35–40%. Standard Acme and trapezoidal screws with small leads self-lock and hold a load with the motor off. Ball screws and steep multi-start screws do not, and will back-drive.

What does T8 mean on a lead screw?

T8 is an 8 mm diameter trapezoidal screw. The lead — how far the nut travels per revolution — is quoted separately: T8×8 travels 8 mm per turn (4-start), while T8×2 travels 2 mm per turn. Same screw diameter, four times the difference in speed and torque.

Lead Screw Calculator — Torque, Speed & Self-Locking for Linear Actuators

How this calculator works

mean diameter ≈ 0.875 × nominal screw diameter
lead angle λ = atan( lead ÷ (π × mean diameter) )
effective friction µ' = µ ÷ cos 15°   (30° trapezoidal thread)
efficiency η = tan λ ÷ tan(λ + atan µ')
torque = force × lead ÷ (2π × η)
linear speed = RPM × lead ÷ 60
self-locking when tan λ < µ'

The headline is efficiency, and it's brutal: a typical trapezoidal screw with a brass nut converts only 20–40% of the motor's work into linear motion. The rest becomes heat in the thread. That's not a defect — it's the same friction that makes the screw self-locking, which is exactly why lead screws hold a Z axis up with the motor off while a ball screw (90%+ efficient) would let it sink. You are choosing between efficiency and holding, and you cannot have both from one screw.

Lead is the design knob, not diameter. Halving the lead halves the torque needed and doubles the resolution, at half the speed. A T8×2 screw lifts roughly four times the load of a T8×8 from the same motor. If your torque figure above looks impossible, drop to a finer lead before buying a bigger motor.

Two things this figure doesn't include, so add margin: friction in the guides (rails, bushings, binding from imperfect alignment) which can add 10–30%, and acceleration, since starting a load moving costs more than holding it in motion. Sizing a lift at exactly the computed torque is how axes stall halfway up. Double it and sleep well.

Driving this with a stepper — the usual choice — means checking two more things: that the motor makes this torque at the speed you want (stepper torque collapses with RPM, see the Stepper Motor Calculator) and that your driver's current limit is set to deliver it, which is the driver and Vref guide.

Common questions

Lead screw or belt for a linear axis?

Screws give force, precision and holding; belts give speed and long travel cheaply. A Z axis lifting a gantry wants a screw. A fast X axis moving a light head wants a belt — which is what the Belt & Pulley Calculator is for.

What is backlash and does it matter?

It's the play between screw and nut — the dead zone when reversing direction. For one-way lifting it's harmless; for positioning that reverses, use an anti-backlash nut (spring-loaded, takes up the slack) or always approach a position from the same direction in software.

Parts this calculation leads to

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T8 Lead Screws

Trapezoidal screws with brass and anti-backlash nuts.

Browse lead screws →

Linear Rails

Guide rails and blocks so the screw only carries axial load.

Browse linear rails →

NEMA 17 Steppers

The usual motor for a lead-screw axis.

Browse steppers →

Flexible Couplers

Absorb misalignment between motor shaft and screw.

Browse couplers →