If you don’t know the actual torque your screw jack needs, it’s easy to pick a motor that’s too small or stress your drive system. The Screw Jack Calculator lets you directly estimate the raising torque, lowering torque, efficiency, and whether the screw will self-lock based on load, screw mean diameter, lead, and thread friction. These numbers make the difference in real-world design for automotive lifts, construction jacks, ground support rigs, and industrial stops. On this page you’ll find full formulas with step-by-step math, practical design notes, and a concise FAQ.
What is a Screw Jack?
A screw jack turns rotational input into a direct lifting force. Turn the handle, the load moves up or down. The threads create a mechanical advantage, so even a moderate torque can move heavy loads—though you’ll need more turns for small thread leads.
Simple Explanation
Imagine wrapping a ramp around a post. Turning the screw is like walking up that ramp in a circle—your circular effort lifts a weight vertically. Finer threads (smaller lead) increase mechanical advantage but mean you’ll need extra turns to lift the same height.
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Screw Jack Mechanism Diagram
Screw Jack Calculator Interactive Visualizer
You can directly see how load, diameter, lead, and friction affect raising torque, efficiency, and whether the screw will self-lock. Adjust the sliders to visualize how practical design choices impact your requirements.
RAISING TORQUE
4.32 N⋅m
EFFICIENCY
29.8%
LOWERING TORQUE
0.58 N⋅m
SELF-LOCKING
YES
FIRGELLI Automations — Interactive Engineering Calculators
How to Use This Calculator
- Enter the load you need to lift in Newtons (N).
- Enter the mean screw diameter (dm) in millimeters — this is the average of the major and minor thread diameters.
- Enter the thread lead in millimeters and the friction coefficient for your screw-nut material pair.
- Click Calculate to see your result.
Screw Jack Calculator Lifting Force
This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.
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Mathematical Equations
Raising Torque
Use the formula below to calculate raising torque.
Lowering Torque
Use the formula below to calculate lowering torque.
Efficiency
Use the formula below to calculate mechanical efficiency.
Self-Locking Condition
Use the formula below to calculate the self-locking condition.
Where:
- F = Applied load (N)
- dm = Mean diameter of screw thread (m)
- l = Lead of screw thread (m)
- μ = Coefficient of friction between screw and nut
- T = Required torque (N⋅m)
- η = Mechanical efficiency (%)
Simple Example
Load: 1000 N, mean diameter: 20 mm, lead: 5 mm, friction coefficient: 0.15.
Raising torque ≈ 2.04 N⋅m. Lowering torque ≈ 0.27 N⋅m. Efficiency ≈ 34.6%. Self-locking: Yes (μ × π × dm = 0.00942 m > l = 0.005 m).
Technical Guide to Screw Jack Calculator Lifting Force
Understanding Screw Jack Mechanisms
A screw jack is just a wedge spun into a spiral. When you turn the screw, the threads translate that rotation into a straight push or pull. This calculator is built for engineers who want to quickly check if their input torque and thread design will get the job done—not just in theory, but in a real shop-floor context.
The core move is using thread geometry as a “mechanical lever.” Small leads give you lots of leverage but slow travel. Large leads are quicker, but you’ll work harder or sacrifice holding power. Both mean diameter and lead are key inputs. Don’t neglect friction: it can dominate your results.
Key Performance Parameters
Raising Torque
Raising torque is the minimum you’ll need to turn the screw against both load and friction. As friction climbs, torque goes up, and efficiency drops. But higher friction also makes the system harder to back-drive, which can be good for safety if you want self-locking.
Lowering Torque
Lowering torque changes sign depending on your friction-to-lead ratio. If friction dominates, you have to keep turning the jack to lower the load—the system is self-locking and can’t run backwards by itself. If not, the load could start moving down with only minimal resistance.
Mechanical Efficiency
Efficiency is simply how much input energy gets to your load. Numbers here are mostly low (20–60%) due to thread friction. It isn’t a flaw—most jacks sacrifice efficiency for self-locking and simplicity. Tuning parameters (lead, diameter, lube) makes a real difference, but so does your maximum acceptable power loss and motion speed.
Practical Applications
Screw jacks show up wherever you want controlled, high-force linear movement with mechanical locking as a bonus, including:
- Automotive Industry: Lifting vehicles, changing tires, precise assembly adjustments
- Construction: Shoring, leveling, formwork, and support structures
- Manufacturing: Leveling machines and setting heights
- Aerospace: Equipment lifting, support stands, and cargo handling
- Entertainment: Lifting stages and adjusting truss or lighting platforms
If you want rapid actuation or electronic control, electric actuators might be more suitable than traditional screw jacks.
Worked Example Calculation
Here’s how an engineer might size a jack for a 5000 N load with:
- Load (F) = 5000 N
- Mean screw diameter (dm) = 20 mm = 0.020 m
- Thread lead (l) = 4 mm = 0.004 m
- Friction coefficient (μ) = 0.15
Step 1: Calculate Raising Torque
Plug the values into the raising torque formula:
Traise = (5000 × 0.020 / 2) × ((0.004 + π × 0.15 × 0.020) / (π × 0.020 - 0.15 × 0.004))
Traise = 50 × ((0.004 + 0.00942) / (0.06283 - 0.0006))
Traise = 50 × (0.01342 / 0.06223) = 10.78 N⋅m
Step 2: Calculate Efficiency
η = 0.004 / (0.004 + π × 0.15 × 0.020) × 100%
η = 0.004 / 0.01342 × 100% = 29.8%
Step 3: Check Self-Locking
Self-locking condition: μ × π × dm > l
0.15 × π × 0.020 = 0.00942 m
Since 0.00942 > 0.004, the jack is self-locking.
Design Considerations and Best Practices
Thread Selection
Fine threads mean more holding force and less risk of running backwards, but you’ll wait longer for actuation. Coarse threads speed things up, but you lose some self-locking and may need to brace your system for back-driving.
Material and Surface Treatment
Friction values come down to material choice and finish. Steel-on-steel usually runs 0.15 to 0.25 (dry), while bronze-on-steel is a little lower—especially with lube. But reducing friction also makes it easier to back-drive, and more lubrication isn’t always better if you want a screw to hold its load without constant attention.
Safety Factors
Most engineers use a torque safety factor of two to four in basic machinery. More if lives or expensive hardware are at stake. Think not just about load, but shock, fatigue, and what happens if something fails—especially if you run close to ratings.
Maintenance Considerations
Threads wear; friction changes. Regular checks for excessive play or metal dust can save headaches later. Lubricate threads as recommended, but also understand that cleaner isn’t always better if it causes unwanted back-driving. Severe environments and heavy usage call for more frequent inspection.
Alternative Solutions
If your project needs faster travel, automated movement, or feedback control, screw jacks aren’t always the best fit. Electric linear actuators can be easier for rapid or programmable setups. Choose based on real-world constraints—there’s no “best” solution, only what actually works for your payload, speed, environment, and power availability.
This tool gives you a quick read on the basic capabilities and limitations of screw jack systems, helping you decide if they’re the right fit for your project, or if you should be looking elsewhere.
Frequently Asked Questions
What is the typical friction coefficient for screw jack threads?
How do I determine if my screw jack will be self-locking?
What safety factor should I apply to screw jack torque calculations?
Why is my screw jack efficiency so low compared to other mechanisms?
How does thread lead affect screw jack performance?
Can I use this calculator for ball screw applications?
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About the Author
Robbie Dickson
Chief Engineer & Founder, FIRGELLI Automations
Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.
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