Power Screw and Screw Jack Calculator: Torque to Raise and Lower a Load, Efficiency and Self-Locking
Torque and efficiency of a square-thread screw jack.
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How this calculator works
Enter the load and the thread size.
Enter the friction at the thread and the collar.
The calculator gives the torques, the efficiency and the self-locking check.
Formulas used
- T_R = \frac{Fd_m}{2}\,\frac{l + \pi\mu d_m}{\pi d_m - \mu l} + \frac{F\mu_cd_c}{2}
- T_L = \frac{Fd_m}{2}\,\frac{\pi\mu d_m - l}{\pi d_m + \mu l} + \frac{F\mu_cd_c}{2}
- \eta = \frac{Fl}{2\pi T_R}
Worked example
With the default numbers in the calculator above:
- Torque to raise the load: 56.358 N m
- Torque to lower the load: 36.828 N m
- Efficiency of raising the load: 16.94 %
- Self-locking (thread only): Yes, the thread alone will hold the load
- Torque from the collar friction alone: 24 N m
Change the numbers in the calculator to see the full working, every step, for your own case.
Common questions
When is a power screw self-locking?
When the friction coefficient times pi times the mean diameter is bigger than the lead. Then the load will not run down on its own.
Why is the efficiency of a screw jack low?
Most of the torque goes into overcoming friction in the thread and at the collar. A self-locking screw always has an efficiency under 50 percent.
How do I allow for an Acme thread?
Divide the thread friction coefficient by the cosine of the half thread angle (14.5 degrees for Acme).
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Further reading: Any machine design text (power screws).
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