KEVOS® MAKEOS · Mechanics
Screw Jack and Power Screw Design
What torque and lever force raise this load, will the screw self-lock, and are the threads, nut and screw strong enough?
Complete check of a screw jack or power screw with Acme or trapezoidal thread: raising and lowering torque with collar friction, efficiency and self-locking, the force at the lever, nut bearing pressure from the engaged threads, combined direct and torsional stress at the screw root, and Euler buckling of the screw column.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load | kN | Axial load lifted. |
| Major diameter | mm | Major diameter of the screw. |
| Pitch | mm | Thread pitch. |
| Number of starts | Number of thread starts. | |
| Flank half-angle (15° for Acme/trapezoidal) | ° | Half the thread angle (15° for 29° Acme and 30° trapezoidal threads; 0 for square threads). |
| Thread friction coefficient | Thread friction coefficient (about 0.1–0.15 lubricated steel on bronze). | |
| Mean collar diameter (0 = none) | mm | Mean diameter of the thrust collar or bearing face. |
| Collar friction coefficient | Collar friction coefficient (about 0.01 with a thrust ball bearing). | |
| Lever arm length | mm | Length of the lever or handle. |
| Nut length | mm | Length of thread engaged in the nut. |
| Allowable thread bearing pressure | MPa | Allowable bearing pressure between screw and nut threads (low for slow-moving bronze nuts). |
| Screw yield strength | MPa | Yield strength of the screw. |
| Unsupported screw length | mm | Unsupported length of the screw acting as a column. |
| Effective length factor | Effective length factor (2 for a jack with a free top). | |
| Screw modulus | GPa | Elastic modulus of the screw. |
Method and formulas
- Pitch diameter d₂ ≈ d − p ÷ 2, root d₃ ≈ d − p; lead l = starts × p.
- Raise: T = W d₂ ÷ 2 × (l + π μ d₂ sec α) ÷ (π d₂ − μ l sec α) + W μ_c d_c ÷ 2; lower with the signs reversed (handbook jack formulas with collar friction).
- Self-locking when π μ d₂ sec α > l; efficiency = W l ÷ 2π T_raise.
- Nut pressure = W ÷ (π d₂ (p ÷ 2) n), n = nut length ÷ pitch; root stress by von Mises from W ÷ A₃ and the thread torque; buckling P_cr = π² E I ÷ (K L)².
- Example: 20 kN on a Tr40×7 screw needs 128.14 N·m to raise, or 427.15 N on a 300 mm lever.
Limitations
- Approximate thread diameters; use the thread standard for final dimensions.
- Static load; wear life of the nut depends on speed and lubrication.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design a lifting screw or press screw
- Check a jack for self-locking and lever effort
- Size the nut length for wear
Who it is for
- Machine designers
- Automation engineers
- Design engineers
What you get
- Raising and lowering torque, efficiency and lever force
- Self-locking and nut bearing pressure
- Screw stress and buckling factor
Interpreting the result
- A thrust ball bearing at the collar can halve the raising torque.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 171, 2561. Topic reference; no source tables or recommended settings are reproduced.
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