How do you calculate the torque to raise a load with a power screw?

The torque to raise a load W with a power screw is T = (W × dm / 2) × [(μπdm + L cos θ) / (πdm cos θ − μL)] + (W × μc × dc / 2), where dm is the mean diameter, L is the lead, μ is the thread friction coefficient, θ is the thread half-angle, μc is the collar friction coefficient, and dc is the mean collar diameter.

What is the self-locking condition for a power screw?

A power screw is self-locking when the friction is large enough to prevent the load from driving the screw backward. The condition is μ ≥ L cos θ / (πdm), which means the torque to lower the load is positive. If this condition is not met, the screw will back-drive (overhaul) under the applied load.

What is the difference between ACME, square, and buttress thread forms?

Square threads (θ = 0°) have the highest efficiency but are difficult to manufacture. ACME threads (29° included angle, θ = 14.5°) are easier to machine and are the most common power screw thread. Buttress threads (45° included angle, θ = 22.5° on the load side) are designed for heavy loads in one direction only. Trapezoidal threads (30° included angle, θ = 15°) are the metric equivalent of ACME threads.

Power Screw Calculator

Raising & Lowering Torque • Efficiency • Self-Locking — Simulate • Explore • Practice • Quiz

Mode

Thread Form

Major Diameter 30 mm

Pitch 6 mm

Starts (n) 1

Load (W) 10 kN

μ Thread 0.15

μ Collar 0.12

Collar Dia. (dc) 45 mm

Torque (Raise)

0 N·mm

Torque (Lower)

0 N·mm

Efficiency

0 %

Lead Angle

0°

Lead (L)

0 mm

Mean Dia. (dm)

0 mm

Self-Locking

Thread Angle (θ)

0°

Power Screw Calculator — Lead Screw Design & Torque Analysis

Power screws (also called lead screws or translation screws) are mechanical devices that convert rotary motion into linear motion to transmit power. They are fundamental components in machine design, appearing in screw jacks, vises, C-clamps, presses, machine tool lead screws, and valve stems. Understanding the torque required to raise and lower a load, the efficiency of the mechanism, and whether the screw is self-locking are essential skills for mechanical engineering students and designers.

A power screw works by applying a torque to rotate the screw against a nut that is either fixed or constrained from rotating. The load moves along the screw axis. The friction between the thread surfaces and at the collar (thrust bearing) determines how much torque is needed. The lead of the screw (axial advance per revolution) equals the pitch multiplied by the number of starts: L = n × p. The mean diameter dm = d − p/2 is used in all torque calculations.

Thread Forms — Square, ACME, Buttress, Trapezoidal

Four main thread profiles are used in power screws. Square threads have zero thread angle (θ = 0°), giving the highest mechanical efficiency, but they are expensive to manufacture and cannot be adjusted for wear. ACME threads (29° included angle, θ = 14.5°) are the most common industrial standard — they balance efficiency, strength, and ease of manufacturing. Buttress threads (45° included angle, θ = 22.5° on the load flank) are designed for heavy axial loads in one direction, such as in presses and artillery breeches. Trapezoidal threads (30° included angle, θ = 15°) are the ISO metric equivalent of ACME threads and are widely used in Europe. The thread angle increases the effective friction, reducing efficiency compared to square threads.

Self-Locking vs Overhauling

A power screw is self-locking when the friction is high enough to prevent the load from driving the screw backward without applied torque. The condition is μ ≥ L·cos(θ) / (π·dm). When this condition is satisfied, the torque to lower the load is positive, meaning external torque must be applied to lower the load. If the screw is not self-locking (the lowering torque is negative), the screw will overhaul or back-drive — the load will push the screw and nut apart without any applied torque. Self-locking is essential in screw jacks and vises for safety; overhauling is desirable in some automatic feed mechanisms.

Who Uses This Simulator?

This power screw calculator is designed for mechanical engineering students studying machine design, TVET trainees learning about fasteners and power transmission, manufacturing engineers designing screw jacks and presses, and instructors teaching lead screw mechanics. It provides a visual, interactive way to understand how thread form, friction, lead angle, and collar friction affect torque requirements and efficiency — no complex software or lab equipment needed.

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