BIZOMAT MAKEOS · Fasteners
Screw Clamp Force Check (Handle Torque, Clamping Force, Screw Stress)
For a screw clamp, vice or press screw turned by a handle, what clamping force does the handle effort produce, and is the screw stressed within limits?
Check of a hand-operated clamping screw: the torque from the handle effort and its radius, the clamping force that torque produces through the thread and the swivel pad or collar friction (the power screw torque relation solved for the load, as the power screw torque tool), the compressive stress at the screw root, and whether the screw stays put, against the clamping force you need and the allowable stress.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Handle effort | N | Force applied at the end of the handle or tommy bar. |
| Handle radius (screw axis to hand) | mm | Distance from the screw axis to where the hand pushes. |
| Mean (pitch) diameter of thread | mm | Mean diameter of the screw thread. |
| Lead | mm | Lead of the screw (pitch × starts). |
| Thread form | choice | Thread form. |
| Thread friction coefficient | Friction in the thread (dry steel about 0.15–0.2, oiled about 0.1). | |
| Mean pad (collar) diameter | mm | Mean diameter of the swivel pad or thrust collar where the screw bears. |
| Pad friction coefficient | Friction at the pad or collar. | |
| Duty | ||
| Screw root diameter | mm | Root (minor) diameter of the screw. |
| Allowable compressive stress | MPa | Allowable compressive stress in the screw. |
| Clamping force needed | N | Clamping force the job needs. |
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: Handle effort
- What it is
- The push or pull a person applies at the end of the clamp handle or tommy bar while tightening.
- Units and format
- Enter a number in N: newtons. The form accepts 0.000001 N to 100000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamping force rises in direct proportion to the handle effort.
- Get it right
- Use a force one operator can repeat comfortably, from your ergonomic guidance, not a one-off maximum.
What this means: Handle radius (screw axis to hand)
- What it is
- The lever arm of the handle: the distance from the screw axis to where the hand applies the force.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Clamping force rises in direct proportion to the handle radius.
- Get it right
- Measure to the middle of the hand grip, not to the end of the bar.
What this means: Mean (pitch) diameter of thread
- What it is
- The mean or pitch diameter of the clamp screw thread, halfway between crests and roots, where the thread force is taken to act.
- 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 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the pitch diameter, not the nominal outside diameter.
What this means: Lead
- What it is
- How far the screw advances along its axis in one full turn. It equals the pitch times the number of thread 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 2 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 advances faster but gives less clamping force for the same handle torque.
- Get it right
- For a multi-start thread, multiply the pitch by the number of starts.
What this means: Thread form
- What it is
- The shape of the clamp screw thread. Inclined flanks press harder on the nut than a square thread, which adds friction.
- 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
- Check the thread standard on the drawing; Acme and trapezoidal look alike but differ in flank angle.
What this means: Thread friction coefficient
- What it is
- The friction coefficient between the screw thread and the nut thread.
- 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 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Higher thread friction lowers the clamping force for the same handle effort.
- Get it right
- Use a value for the actual materials and lubrication from a power-screw reference; oiling changes it a lot.
What this means: Mean pad (collar) diameter
- What it is
- The mean diameter of the swivel pad or collar where the screw end turns against the pad, which adds friction torque.
- 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 8 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 pad diameter adds friction torque and lowers the clamping force.
- Get it right
- Measure to the middle of the bearing ring, halfway between its inner and outer edges.
What this means: Pad friction coefficient
- What it is
- The friction coefficient where the screw end turns against the swivel pad or collar.
- 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 0.15 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A ball end or thrust washer gives much less friction than a flat dry face.
Duty
What this means: Screw root diameter
- What it is
- The root (minor) diameter of the screw thread, the smallest section that carries the clamping force in compression.
- 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 8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Screw stress falls with the square of the root diameter.
- Get it right
- Use the minor diameter from the thread standard, not the pitch or nominal diameter.
What this means: Allowable compressive stress
- What it is
- The compressive stress you allow on the root section of the screw.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 200 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Set it from the screw material’s strength and your design code; long, slender screws also need a buckling check.
What this means: Clamping force needed
- What it is
- The clamping force the job needs to hold the work against machining, assembly or test forces.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Work it out from the forces on the part and the friction holding it, with your own safety margin.
Method and formulas
- Torque = handle effort × radius.
- Torque to raise load F = F [d_m ÷ 2 × (L + π μ d_m sec α) ÷ (π d_m − μ L sec α) + μ_c d_c ÷ 2] (as the power screw torque tool), solved for F.
- Root stress = F ÷ (π d_r² ÷ 4).
- Example: 11.25 N·m at the handle gives 6,595.5 N of clamping force and 131.21 MPa in the screw.
Limitations
- Static tightening; impact from hammering the handle multiplies the force.
- Long, slender screws can buckle; check them as columns.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a fixture or welding clamp screw
- Find the clamping force of a vice
- Size the handle for a needed clamping force
Who it is for
- Fixture designers
- Toolmakers
- Design engineers
What you get
- Handle torque
- Clamping force and efficiency
- Screw stress and decision
Interpreting the result
- Most of the handle effort goes into friction: lubricating the thread and pad can nearly double the clamping force.
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.
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Open the Screw Clamp Force Check (Handle Torque, Clamping Force, Screw Stress) calculator