Screw Jack Load Capacity Calculator
How Screw Jacks Carry Loads
Screw jacks transform rotational motion into powerful linear force. When you turn the handle, the screw thread pushes against a nut, gradually lifting a heavy object. To avoid failure, engineers determine how much load the jack's screw can safely sustain. The key variables are thread diameter, pitch, and the yield strength of the material. A simplified equation for the load capacity is:
Where is the allowable load, is screw diameter, is thread pitch, and y is yield strength. This equation treats the screw as if it experiences simple tension. Real designs also account for shear stresses and buckling.
Thread Geometry Impact
Pitch determines how steeply the thread rises. A smaller pitch results in a greater mechanical advantage but slower lifting. Diameter contributes to the surface area carrying the load. Table 1 shows how different diameters and pitches affect theoretical capacity using a fixed material strength of 250 MPa:
| Diameter (mm) | Pitch (mm) | Approx. Capacity (kN) |
|---|---|---|
| 25 | 5 | 49 |
| 30 | 5 | 71 |
| 35 | 7 | 85 |
Remember that these values ignore friction and are purely theoretical. In practice, jacks are rated far below the raw yield limit to prevent deformation or sudden failure.
Safety Factors and Standards
Engineering codes recommend safety factors ranging from 1.5 to 3 or more, depending on application. For a jack used to lift a vehicle, you might apply a factor of two. That means you only load the jack to half its theoretical capacity. Some jurisdictions require additional certification for equipment used in commercial settings, so check local regulations if you plan to design or sell jacks.
Design Considerations
Aside from screw strength, designers consider nut material, lubrication, and overall jack stability. For heavy industrial jacks, you might use ball bearings or worm gears to reduce friction. Portable jacks usually prioritize weight and cost, so they rely on simple steel screws. Keep in mind that repeated overloading can cause the screw to stretch or threads to strip.
Using This Calculator
Enter the screw diameter and pitch in millimeters along with the yield strength of the material. The calculator outputs a rough maximum load in kilonewtons. You can then divide by a chosen safety factor to get a recommended working load. This approach is useful for quick comparisons between jack designs or for educational purposes.
Applications
Screw jacks aren't just for lifting cars. They're used in industrial presses, adjustable platforms, and even stage equipment. Some high-capacity jacks integrate multiple screws connected by gears or linkages. Understanding how thread size and pitch affect load helps you select the right jack for the job.
Experiment with different values to see how each variable changes the capacity. For example, doubling the diameter roughly quadruples the theoretical load, while doubling the pitch cuts it in half. Balance your need for mechanical advantage with the available space and material cost.
Maintenance and Inspection
Like any mechanical device, a screw jack requires regular lubrication and inspection. Clean the threads to remove debris and check for signs of rust or fatigue. If your jack will hold heavy loads for long periods, consider routine nondestructive testing to catch small cracks early.
Historical Background
The concept of lifting with a screw dates back to ancient times when simple wooden jacks were used in construction. Modern steel jacks follow the same principle but rely on improved metallurgy and standardized threads. Appreciating this history reveals how even straightforward machines can evolve dramatically over centuries.
Worked Example
Consider a jack with a 30 mm diameter screw, 5 mm pitch, and a steel yield strength of 250 MPa. Plugging these numbers into the formula yields an ideal capacity of roughly 71 kN, as shown in the table above. If you apply a safety factor of two to account for real-world conditions, the recommended working load becomes about 35 kN. This example demonstrates why manufacturers often rate consumer jacks well below their theoretical limits.
Safety Factor Comparison
| Ideal Capacity (kN) | Safety Factor | Working Load (kN) |
|---|---|---|
| 71 | 1.5 | 47 |
| 71 | 2.0 | 35 |
| 71 | 3.0 | 24 |
Buckling and Friction
The simplified formula assumes the screw experiences pure tension. In reality, long slender screws can buckle under compressive load before reaching the yield limit. Euler’s buckling equation offers a way to evaluate this risk by incorporating effective length and moment of inertia. Additionally, thread friction reduces efficiency; if the coefficient of friction is high, more input torque is required and the effective load capacity falls. Lubrication, thread shape, and surface finish all influence friction losses.
Limitations and Assumptions
This calculator treats material yield strength as a single value, yet many alloys exhibit different properties in tension versus compression. It also ignores stress concentrations at thread roots and assumes uniform loading. Real jacks may include ball bearings, worm gears, or trapezoidal threads that alter mechanics. For critical applications—such as lifting structural components during building repairs—engineers should perform detailed finite element analysis and consult applicable standards.
Related Calculators
If your project involves supporting beams or columns, try the Beam Load Calculator for bending analysis. To evaluate fastener performance in your assembly, visit the Bolt Clamp Force Calculator.
Conclusion
The Screw Jack Load Capacity Calculator demystifies the mechanics behind one of the most common lifting tools. By factoring in basic thread geometry, safety factors, and potential failure modes, you can estimate safe loads quickly. Always apply appropriate safety margins and consult detailed design standards for precise calculations.