Screw Jack Load Capacity Calculator
Using this screw jack axial load capacity calculator
A mechanical screw jack changes rotary input into controlled vertical movement, making it useful for machine leveling, maintenance lifts, fixture adjustment, and other shop tasks where precise travel matters. This calculator provides a first-pass axial yield-capacity estimate for the screw using its effective diameter and material yield strength. It can help compare candidate screws, identify an obviously undersized size, or turn a known material property and screw dimension into a consistent preliminary load figure.
The result is deliberately limited to the screw's simplified axial yield condition. A jack assembly can fail before that value is reached: a long screw can buckle, threads can strip, a nut can deform, the body can tilt, the base can slip, or the load can be eccentric. Treat the displayed number as a theoretical indicator for the screw under ideal axial loading, not as a certified rating for a complete jack. It is still useful for screening options and deciding when a detailed design review is needed.
What the screw-jack inputs mean in practice
Screw Diameter (mm) is the diameter used in the load-carrying area term. Since cross-sectional area varies with diameter squared, this is the input with the strongest effect on the estimate. For a conservative calculation, use the smallest effective diameter carrying compression—often the root or core diameter rather than the thread's outside major diameter. A nominal outside diameter can make a threaded screw look stronger than its actual core section.
Thread Pitch (mm) is the axial spacing between thread crests and, for a single-start thread, equals the advance per turn. Pitch is retained as a useful hardware reference, but it does not enter this axial yield-capacity calculation. It remains important when evaluating travel per revolution, torque and efficiency, thread engagement, self-locking behavior, and the distinction between pitch and lead on multi-start screws. Do not use this page's result as a torque calculation.
Material Yield Strength (MPa) is the stress at which the screw material begins to plastically deform. One MPa equals one newton per square millimeter, which fits directly with a diameter entered in millimeters. Use a verified value from a material certificate, drawing, or manufacturer data whenever possible. If the grade or condition is uncertain, selecting a lower supported value is more conservative than assuming a best-case strength.
How the screw-jack yield formula works
This screw-jack calculator applies the standard axial-yield relationship: cross-sectional area multiplied by material yield strength.
Here F is the approximate axial yield load, d is the effective screw diameter, and σy is material yield strength. The circular-area term πd2/4 gives the section area, and multiplying it by yield strength produces force in newtons when millimeters and MPa are used. Diameter therefore has a squared effect, while yield strength changes the result proportionally. Pitch does not belong in this yield-stress relationship because no torque, efficiency, or thread-force model is being calculated.
The displayed capacity is not automatically a recommended working load. Lifting and support equipment normally requires a safety factor between a theoretical limit and an allowable service load. After calculation, the page divides the estimated axial yield load by several common safety factors so you can compare the theoretical figure with more conservative working-load values.
Worked example for the default screw-jack inputs
With the default effective diameter of 30 mm and yield strength of 250 MPa, the simplified axial yield estimate is about 176.7 kN. The calculator converts that theoretical figure to working-load comparisons: approximately 117.8 kN at a safety factor of 1.5, 88.4 kN at a safety factor of 2, and 58.9 kN at a safety factor of 3. These are arithmetic comparisons, not equipment certifications.
This screw-jack example also illustrates a useful output check. A 30 mm effective steel section should not return a result measured in a few newtons, yet the figure does not guarantee that every jack using a similar-looking screw can safely lift that load. Unsupported length, thread engagement, housing stiffness, base condition, and alignment can substantially reduce the governing limit. If an output seems surprising, verify the effective diameter first; substituting major diameter for core diameter is a common source of overstatement.
How screw diameter changes axial yield capacity
The table below holds yield strength at 250 MPa to show how capacity changes when only the effective screw diameter changes. The relationship is not linear: a modest diameter increase produces a larger area increase because the calculation uses diameter squared.
| Scenario | Effective diameter | Estimated axial yield capacity | Working load at safety factor 2 | Why it changes |
|---|---|---|---|---|
| Smaller screw | 24 mm | 113.1 kN | 56.5 kN | Less cross-sectional area means substantially less axial yield capacity. |
| Baseline | 30 mm | 176.7 kN | 88.4 kN | This matches the default worked example. |
| Larger screw | 36 mm | 254.5 kN | 127.2 kN | The diameter-squared area term drives the increase. |
When comparing screw-jack designs, diameter can be an efficient way to increase this simplified yield margin, although it also affects material use, mass, cost, nut geometry, and packaging. Pitch is a separate design tradeoff: it affects travel and torque behavior rather than the axial-yield number calculated here. A design that looks strong in this tool can still be unsuitable if buckling, thread stripping, drive torque, or stability governs.
Interpreting a screw-jack capacity result carefully
Read the calculated result in two layers. The bold capacity is a simplified theoretical axial-yield indicator for the selected effective diameter and material strength. The working-load table divides that indicator by selected safety factors, making it easier to compare conservative operating values across alternatives. When testing combinations, diameter should have a strong squared effect and yield strength should change the result proportionally; changing pitch should not change this particular output.
The result does not estimate handle torque, motor torque, screw efficiency, lift speed, or self-locking behavior. It does not establish stability on an uneven base, determine whether nut threads or the jack frame govern, or determine whether a long extended screw will buckle before reaching yield. The calculator addresses one part of a jack assessment and does not replace a full jack design check or lifting plan.
Screw-jack assumptions and limitations
This screw-jack model is most useful when its boundaries are clear. It assumes a representative material yield strength, an effective diameter that reflects the real load-carrying section, and a centered axial load. It treats the screw section as reaching yield; it does not model the rest of the jack or the mechanics needed to turn it.
- Buckling is not included: a long, slender screw can lose stability before its calculated yield capacity is reached.
- Thread stripping is not included: weak nut material, short engagement length, or damaged threads can govern first.
- Off-center loading is not included: eccentric loads create bending and local stress beyond a simple axial model.
- Body and base limits are not included: a strong screw cannot compensate for a weak jack frame or unstable support surface.
- Torque and efficiency are not included: pitch, friction, lead, and drive geometry need a separate screw-mechanics calculation.
- Standards and regulations are not included: use applicable design rules and manufacturer data for certified or regulated lifting work.
These limits define the calculator's proper role. Use it to compare effective diameters and material choices, identify unrealistic assumptions, and prepare for a more detailed review. If a reduction in effective diameter sharply erodes the calculated margin, that is a valuable warning before performing a buckling, thread, housing, and drive analysis.
Before relying on a number for a real lift, inspect the hardware and review the load path. Confirm that the screw is straight, threads are clean and adequately engaged, the base is supported, and the load is centered. Treat this calculator as a disciplined preliminary estimate rather than permission to override visible damage, poor setup, manufacturer limits, or site conditions.
Screw-jack field checklist before a real lift
Before applying a screw-jack result in a shop or on site, confirm that the load is centered over the jack, contact surfaces are flat, the base is fully supported, and the screw is not extended so far that buckling becomes controlling. Inspect for worn threads, corrosion, bent screws, and signs that the nut or housing has yielded. A calculated axial yield figure does not override visible damage or poor setup.
Also examine the entire load path. When several jacks lift one machine, a single screw may carry more than a simple share of the weight because the structure may not distribute force evenly. Dynamic effects such as impact, sudden release, or an uneven floor can raise force far above a slow static condition. Use the calculator to compare screw sections and choose a conservative working load, not as a guarantee of a complete lifting arrangement.
Screw jack axial yield capacity results
After calculation, this area lists working-load comparisons for common safety factors, helping distinguish the theoretical screw yield estimate from more conservative operating values.
Mini-game: Safe Lift Window
This optional mini-game turns the same idea behind the calculator into a quick timing challenge. Each crate has a required load in kN. The gauge needle swings as if you were turning a screw jack through different pitch and material conditions. Your job is to engage the lift when the current capacity lands inside the safe window: above the crate's demand, but not so far above it that you drift into overstress. The round lasts about a minute, later phases speed up or wobble, and a saved best score lets you chase cleaner runs. It is separate from the calculator result, but it teaches a useful instinct: in lifting work, the sweet spot is a comfortable margin, not a dramatic near miss.