KEVOS® MAKEOS · Threads
Thread Helix Angle and Screw Efficiency
What is the helix angle of this thread, how efficient is it, and will it self-lock?
Lead, helix (lead) angle, efficiency raising a load and self-locking check for a power or fastening screw with a given flank angle and friction coefficient.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Pitch diameter | mm | Pitch (mean) diameter of the thread. |
| Pitch | mm | Pitch: axial distance between adjacent threads. |
| Number of starts | starts | Number of starts; lead = starts × pitch. |
| Thread form | choice | Thread profile; the half-angle is used for the friction wedge effect. |
| Coefficient of friction | Thread friction coefficient for the materials and lubrication (about 0.1–0.2 for lubricated steel on bronze). |
Method and formulas
- Lead L = starts × P; tan λ = L ÷ π d₂.
- Normal flank angle αₙ = atan(tan α cos λ).
- Efficiency η = tan λ (cos αₙ − μ tan λ) ÷ (cos αₙ tan λ + μ).
- Self-locking when μ ≥ cos αₙ tan λ.
- Example: 20 mm pitch diameter with 4 mm pitch has a 3.6426° helix angle and 28.838% efficiency.
Limitations
- Collar or thrust-bearing friction is not included and lowers overall efficiency.
- Vibration can back-drive a nominally self-locking screw.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a jack or lead screw will back-drive
- Compare single and multi-start screws
- Find the helix angle for tool clearance
Who it is for
- Design engineers
- Maintenance engineers
- Automation engineers
What you get
- Lead and helix angle
- Efficiency raising a load
- Self-locking or back-driving
Interpreting the result
- If the screw must hold a load safely, use a brake; do not rely on self-locking under vibration.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1921–1928, 2061–2062. Topic reference; no source tables or recommended settings are reproduced.
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