Screw Jack Efficiency Calculator

Formula

• Lead angle λ = atan(lead / (π × dm))
• Friction angle φ = atan(μ / cos(α))
• T_raise = F × (dm/2) × tan(λ + φ)
• η = tan(λ) / tan(λ + φ) × 100%
• Self-locking when λ < φ

About this calculator

Part of our Mechanical Calculators collection. Beams, bolts, springs, gears, bearings, and core ME calculations.

How it works

Lead angle comes from thread lead and mean diameter. Raising and lowering torque include thread angle and friction effects. Efficiency compares useful work to input work. Self-locking occurs when friction angle exceeds lead angle on the raising thread.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

Mean diameter 50 mm, lead 10 mm, μ = 0.12, load 10 kN, arm 300 mm.

1. λ = 3.64°, φ = 6.84°
2. λ < φ → self-locking
3. T_raise = 10000 × 25 × tan(10.48°) = 46269 N·mm
4. Effort = 46269 / 300 = 154 N

Result: Self-locking, efficiency 34.4%, effort 154 N.

When to use

• Checking whether a screw jack holds load without back-driving
• Sizing hand-wheel effort for a given load
• Comparing lead and friction effects on efficiency

Limitations

• Square-thread model with thread half-angle input (0° for square thread)
• Does not include nut material wear or multiple-start threads separately
• Static friction assumed; dynamic conditions may differ

FAQ

When is a screw self-locking?

Self-locking occurs when the lead angle is less than the friction angle (λ < φ). The load cannot back-drive the screw without applied torque.

Why was self-locking logic fixed?

Self-locking depends on lead angle versus friction angle, not on efficiency being below 50%. The calculator now uses λ < φ directly.