BIZOMAT MAKEOS · Mechanics
Power Screw Raise and Lower Torque
What torque raises and lowers this load with a screw jack or lead screw, including collar friction?
Torque to raise and to lower a load with a square, Acme or trapezoidal power screw, including thrust-collar friction, with efficiency and self-locking.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Axial load | N | Load to raise or lower. |
| Mean (pitch) diameter of thread | mm | Mean thread diameter = nominal − pitch ÷ 2 (approximately). |
| Lead | mm | Axial advance per turn (pitch × starts). |
| Thread form | choice | Thread profile. |
| Thread friction coefficient | Thread friction for the materials and lubrication (about 0.1–0.15 lubricated steel on bronze). | |
| Mean collar diameter (0 = thrust bearing ignored) | mm | Mean diameter of the thrust collar or washer; 0 if a rolling bearing makes collar friction negligible. |
| Collar friction coefficient | Friction coefficient at the collar. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Axial load
- What it is
- The axial load the screw must raise or lower, acting along the screw axis.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both torques rise in direct proportion.
- Get it right
- Include the weight of any carriage or platform moved with the load.
What this means: Mean (pitch) diameter of thread
- What it is
- The mean, or pitch, diameter of the screw thread, roughly halfway between the major and minor diameters, from thread tables.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger mean diameter raises the friction torque.
- Get it right
- Use the pitch diameter, not the nominal or major diameter.
What this means: Lead
- What it is
- How far the nut travels along the screw for one turn: the thread pitch multiplied by the number of starts.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger lead raises the raising torque and efficiency and makes self-locking less likely.
- Get it right
- For a multi-start screw the lead is larger than the pitch.
What this means: Thread form
- What it is
- The thread profile, which sets the flank angle used to allow for the extra friction on sloping thread flanks.
- Units and format
- Choose one of 3 options from the list: Square; Acme (29°); Trapezoidal (30°).
- Example
- The form opens with “Trapezoidal (30°)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Choose the profile actually cut on the screw; a square thread has no flank angle.
What this means: Thread friction coefficient
- What it is
- The coefficient of friction between the screw and nut threads, for the materials and lubrication used, from data or tests.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.12 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher friction raises both torques and lowers efficiency.
- Get it right
- Starting friction is higher than running friction; the tool gives running torque.
What this means: Mean collar diameter (0 = thrust bearing ignore…
- What it is
- The mean diameter of the thrust collar or washer that carries the axial load where the screw or nut bears against the frame.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger collar diameter adds collar friction torque to raising and lowering.
- Get it right
- 0 means collar friction is ignored, for example when a rolling thrust bearing is fitted.
What this means: Collar friction coefficient
- What it is
- The coefficient of friction at the thrust collar or washer, for its materials and lubrication.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.12 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher collar friction adds torque to both raising and lowering.
- Get it right
- Ignored when the collar diameter is 0.
Method and formulas
- Raise: T = F dm/2 × (L + π μ dm sec α) ÷ (π dm − μ L sec α) + F μc dc/2.
- Lower: T = F dm/2 × (π μ dm sec α − L) ÷ (π dm + μ L sec α) + F μc dc/2.
- Efficiency = F L ÷ (2π T_raise); self-locking when the thread part of the lowering torque is positive.
- Example: 20000 N on a 30 mm, 6 mm lead trapezoidal screw needs 110.82 N·m to raise.
Limitations
- Uses the flank half-angle in the axial plane (sec α approximation).
- Starting friction is higher than running friction.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size the handle or motor for a screw jack
- Compare collar bearings and lubrication
- Check whether a jack holds its load
Who it is for
- Design engineers
- Maintenance engineers
- Machine builders
What you get
- Torque to raise
- Torque to lower
- Efficiency raising
- Self-locking
Interpreting the result
- If the screw must hold a load reliably, add a brake; vibration can back-drive a self-locking screw.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1921–1928, 2561–2568. Topic reference; no source tables or recommended settings are reproduced.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.