Acme Screw Torque Calculator

Mechanical Design Tool

Estimate the torque required to raise and lower a load with an Acme, trapezoidal, or square power screw. This calculator accounts for thread friction, collar friction, lead, screw geometry, and thread form angle to produce practical engineering estimates for machine design, jacks, vises, linear actuators, and lifting assemblies.

Calculator Inputs

Results

Core Formula

T = Tthread + Tcollar

Efficiency

η = WL / 2πT

Expert Guide to Using an Acme Screw Torque Calculator

An acme screw torque calculator helps engineers, technicians, maintenance planners, machine builders, and advanced hobbyists estimate the turning effort required to move an axial load through a power screw. The calculation seems simple at first glance, but real-world performance depends on several interacting variables: the screw mean diameter, lead, thread form angle, thread friction, and collar friction. If any of those inputs are wrong, the resulting motor size, handwheel force, gearbox selection, thermal behavior, wear rate, and safety margin can also be wrong. That is why a practical calculator must do more than multiply load by radius. It must represent the mechanics of a helical inclined plane under friction.

Acme screws are widely used in jacks, presses, vises, manual positioning systems, machine tool slides, and linear actuators because they offer a good balance of strength, durability, and manufacturability. Compared with square threads, Acme threads are easier to machine and stronger at the root. Compared with ball screws, they are less efficient but often more economical, more contamination tolerant, and more likely to remain self-locking under suitable geometry and friction conditions. That self-locking behavior matters in lifting and holding applications, where back-driving can create an immediate safety risk.

What Torque the Calculator Is Actually Estimating

For a power screw, the input torque has two main components. The first is the torque needed to overcome thread friction and generate useful linear motion at the screw-nut interface. The second is the torque lost at the collar or thrust surface, where axial load is reacted into a stationary support. In many hand-calculation examples, collar losses are ignored to simplify the algebra. In working equipment, however, collar friction can be meaningful, especially at high load and large thrust-bearing diameters. A reliable calculator should therefore report:

• Thread torque to raise the load
• Collar torque loss
• Total torque to raise the load
• Total torque to lower the load
• Overall mechanical efficiency
• Self-locking or overhauling tendency

The calculator above uses established power screw relationships in which the Acme thread angle increases the effective normal force and therefore raises the friction term. For a square thread, the half-angle is zero and the correction disappears. For Acme or trapezoidal threads, the friction contribution is multiplied by the secant of the thread half-angle, which is why thread form cannot be ignored when you need realistic torque numbers.

Why Acme Thread Angle Changes Torque

Acme threads have a 29 degree included angle, or a 14.5 degree half-angle. Because the flanks are inclined, the normal force between mating threads is greater than it would be for a square thread carrying the same axial load. More normal force means more frictional resistance. In practical terms, that means an Acme screw usually requires slightly more input torque than a square thread with the same load, diameter, lead, and coefficient of friction. The difference is not enormous, but it matters in motor sizing and handwheel force calculations, especially near the limits of a design.

Thread Form	Included Angle	Half-Angle	Relative Friction Factor	Typical Design Implication
Square	0 degrees	0 degrees	1.000	Lowest thread friction, highest theoretical efficiency, harder to manufacture.
Acme	29 degrees	14.5 degrees	1.033	About 3.3% higher friction term than square for the same nominal coefficient.
ISO Trapezoidal	30 degrees	15 degrees	1.035	Very similar behavior to Acme; commonly used in industrial linear drives.

The relative friction factor shown above is the secant of the half-angle. It is not a guess. It comes directly from the geometry of the thread flanks and the normal force relationship used in power screw analysis. While the percentage increase may seem small, the effect can be material when combined with high loads or modest motor torque reserve.

The Most Important Inputs and How to Choose Them

1. Axial load: Use the maximum realistic operating load, not merely the average load. If shock loading, vibration, or startup stiction occurs, the peak requirement may be much higher than the steady-state value.
2. Mean diameter: Torque depends directly on the screw radius, so using major diameter instead of mean diameter will bias the result. Always confirm the actual thread geometry from the screw specification.
3. Lead: Lead is the axial travel per revolution. A higher lead increases speed, but it also changes the helix angle and can reduce self-locking. Multi-start screws may have much larger lead than pitch.
4. Thread friction coefficient: This is one of the least certain inputs and often the largest source of error. Lubrication, surface finish, contamination, speed, and wear all influence the final value.
5. Collar friction and collar diameter: If a thrust bearing is plain rather than rolling, collar losses may be significant. Designers often underestimate this term.

Typical Friction Values Used in Preliminary Design

Exact friction must be measured or validated from supplier data, but preliminary design still needs realistic ranges. The following values are commonly used as starting points for engineering estimates. They are representative planning values, not universal constants. Actual results depend on load, lubrication regime, materials, temperature, and surface condition.

Contact Condition	Typical Coefficient of Friction	Common Use Case	Design Impact
Steel screw on bronze nut, well lubricated	0.08 to 0.12	Industrial lead screws, moderate duty	Good compromise of efficiency, wear, and anti-seize behavior.
Steel on bronze, lightly lubricated or aging grease	0.12 to 0.16	General maintenance reality in older equipment	Higher torque demand and more heat generation.
Dry sliding metal contact	0.15 to 0.25	Low duty manual systems or neglected assemblies	Strong self-locking tendency but poor efficiency and faster wear.
Rolling thrust bearing at collar location	0.01 to 0.03 equivalent	Motorized lifting or positioning systems	Can substantially reduce total torque compared with a plain collar.

These ranges line up with common machine design assumptions used in preliminary calculations. When a project is safety-critical, repeatedly cycled, or tightly power-limited, replace estimate values with test-backed friction data whenever possible.

Example Calculation and Interpretation

Assume a 10,000 N load on a screw with 40 mm mean diameter, 6 mm lead, an Acme profile, thread friction coefficient of 0.12, collar friction coefficient of 0.08, and 30 mm mean collar diameter. The calculator will estimate the torque to raise the load, the torque to lower it, and the efficiency. In a case like this, the raising torque is often several times larger than the ideal no-friction torque, because both thread friction and collar losses consume input energy. If the lowering torque remains positive, the assembly requires turning effort even to descend, which indicates self-locking behavior under the assumed friction state. If the lowering torque trends toward zero or negative, the screw may back-drive or overhaul.

That distinction is operationally important. A self-locking screw can hold position without a brake under the specific assumptions used in the model. But engineers should not treat self-locking as a permanent guarantee. Lubrication changes, vibration, wear polishing, contamination removal, temperature shifts, and production tolerances can all alter effective friction. A design intended to protect people or expensive equipment should use a positive holding brake or mechanical lock when the risk of back-driving is unacceptable.

Efficiency, Heat, and Duty Cycle

The overall efficiency reported by the calculator is based on useful output work per revolution divided by input work per revolution. In plain language, it shows how much of your applied torque is turned into lifting the load, rather than being lost as friction. Acme screws are usually far less efficient than ball screws, but that is not automatically a disadvantage. Lower efficiency can provide beneficial self-locking and more predictable holding behavior in static positioning tasks.

Still, low efficiency means higher motor current, greater hand effort, more heating, and more wear. In continuous-duty applications, that wasted energy becomes a thermal management issue. If the screw is part of a high-cycle machine, a packaging line, or a motorized lift operating many times per hour, efficiency deserves special attention. The same machine may function perfectly in intermittent use and overheat in continuous use if the torque model is too optimistic.

Practical rule: If your design is power-limited, speed-sensitive, or high-cycle, evaluate whether a lower-friction nut material, improved lubrication, a reduced collar loss, or an alternate linear drive technology would reduce required torque enough to justify the change.

Self-Locking Versus Overhauling

A classic self-locking criterion compares the helix angle with the friction angle. When the friction angle is greater than the helix angle, the screw resists back-driving under static assumptions. When the helix angle becomes too large, especially on high-lead screws, back-driving becomes more likely. This is why a fast-travel screw can suddenly behave very differently from a slow-travel screw even when the diameter and load remain unchanged.

For one representative case with 40 mm mean diameter and a thread friction coefficient of 0.12, the self-locking threshold lead for an Acme thread is about 15.6 mm per revolution using the simple criterion based on effective friction. A 6 mm lead is comfortably below that threshold, so self-locking is expected. But if the lead were increased into the mid-teens while friction fell because of excellent lubrication, the design could approach the back-driving boundary.

Comparing Acme Screws With Other Linear Drive Options

• Acme versus square thread: Square threads are theoretically more efficient because the flank angle is zero, but they are harder to manufacture and often less practical in general equipment.
• Acme versus trapezoidal thread: These are very close in behavior. The final selection is often driven by regional standards, availability, and mating component supply.
• Acme versus ball screw: Ball screws can exceed 85% to 90% efficiency in many applications, but they usually back-drive easily and cost more. Acme screws commonly trade efficiency for simplicity, robustness, and holding capability.

Common Mistakes When Using an Acme Screw Torque Calculator

1. Using pitch when the equation requires lead.
2. Ignoring collar friction in a plain thrust interface.
3. Confusing major diameter with mean diameter.
4. Choosing friction coefficients that are too optimistic for the maintenance reality of the machine.
5. Assuming self-locking in all service conditions without checking sensitivity to lubrication and wear.
6. Applying static torque results directly to dynamic startup, shock, or reversing service without added margin.

How to Validate Your Final Design

After using the calculator for preliminary sizing, validate the design in three steps. First, compare the predicted torque with available motor, gearbox, or operator input capability and include a service factor. Second, estimate duty-cycle heating, especially if the screw will run frequently or continuously. Third, verify the screw and nut against buckling, bearing stress, thread pressure, wear life, and column stability. Torque is only one part of a complete power screw design.

For units and measurement consistency, the National Institute of Standards and Technology metric and SI guidance is a valuable reference. For broader mechanical design study, MIT OpenCourseWare offers engineering course resources, and Purdue Engineering provides access to respected mechanical engineering content and educational materials.

Bottom Line

An acme screw torque calculator is most useful when it reflects the actual physics of the threaded system rather than an oversimplified force-times-radius estimate. If you supply realistic load, lead, mean diameter, thread friction, and collar friction values, the resulting torque and efficiency estimates become far more actionable for selecting motors, handwheels, gearboxes, bearings, and safety devices. Use the calculator above as a design-grade screening tool, then confirm the final configuration with manufacturer data, testing, and a full machine design review.