BIZOMAT MAKEOS · Fasteners
Bolt Tightening Torque and Preload
What preload does this torque give, or what torque gives the target preload?
Relation between tightening torque and bolt preload for metric bolts using the nut factor (T = K F d), with stress in the tensile stress area and the share of proof load.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Calculate | choice | Choose torque from a preload target, or preload from a torque. |
| Nominal diameter | mm | Nominal (major) diameter of the bolt. |
| Thread pitch | mm | Thread pitch (coarse M12 is 1.75 mm). |
| Nut factor K | Nut factor for the actual surface finish and lubricant, from the fastener supplier or a torque–tension test (about 0.2 for plain dry steel, lower when lubricated). | |
| Proof strength of the bolt | MPa | Proof stress for the property class from the bolt standard (e.g. ISO 898-1) or the supplier. |
| Target preload as % of proof load | % | Target preload as a percentage of proof load from your joint design (often 65–75%). |
| Tightening torque | N·m | Specified tightening torque (preload mode). |
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: Calculate
- What it is
- Which way round the relation is solved: the torque needed to reach a preload set as a share of proof load, or the preload a known tightening torque produces.
- Units and format
- Choose one of 2 options from the list: Torque for a target preload (% of proof); Preload from a tightening torque.
- Example
- The form opens with “Torque for a target preload (% of proof)” selected as a worked example. Choose the option that matches your case.
- Get it right
- In torque mode the tightening torque input is ignored; in preload mode the target percentage is ignored.
What this means: Nominal diameter
- What it is
- The nominal (major) diameter of the bolt thread, the number after the M in the size designation. It sets the stress area and the lever arm in the torque relation.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For the same preload the torque rises in proportion to the diameter.
- Get it right
- Use the thread size, not the shank diameter of a reduced-shank bolt or the hole size.
What this means: Thread pitch
- What it is
- The axial distance from one thread crest to the next, the number after the × in a designation such as M12 × 1.75. It is used only to work out the tensile stress area.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 20 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.75 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A finer pitch gives a larger stress area, so a higher proof load.
- Get it right
- Coarse and fine threads of the same size have different pitches; a plain M designation without a pitch means the coarse series.
What this means: Nut factor K
- What it is
- The nut factor: an empirical coefficient that lumps thread friction, under-head friction and geometry into one number linking torque to preload. It depends on plating, lubricant and surface condition.
- Units and format
- Enter a plain number with no unit. The form accepts 0.05 to 0.5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque for a given preload rises in proportion to K; preload from a given torque falls as K rises.
- Get it right
- Take it from the fastener supplier for the actual finish and lubricant, or measure it with a torque–tension test; a lubricated bolt has a lower K than a dry one.
What this means: Proof strength of the bolt
- What it is
- The proof strength (proof stress) of the bolt: the stress it must carry without permanent set, set by its property class. Multiplied by the stress area it gives the proof load.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher proof strength raises the proof load, so the target preload and torque in torque mode.
- Get it right
- Use the proof stress for the property class in ISO 898-1 or the supplier’s data, not the tensile or yield strength.
What this means: Target preload as % of proof load
- What it is
- The preload you want to reach, expressed as a share of the bolt’s proof load, chosen in your joint design. Used only in torque mode.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Preload and tightening torque rise in proportion.
- Get it right
- Type the percentage itself, not a fraction. A value over 100 means loading the bolt past its proof load.
What this means: Tightening torque
- What it is
- The tightening torque applied to the nut or bolt head, as specified on the drawing or set on the torque tool. Used only in preload mode to find the resulting preload.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 1000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Preload rises in proportion to torque.
- Get it right
- Make sure the value is in newton metres; a torque wrench set in ft·lbf or in·lbf must be converted first.
Method and formulas
- Tensile stress area As = π/4 (d − 0.938194P)².
- Proof load = Sp × As; target preload F = % × proof load.
- Torque T = K F d (d in metres gives N·m); preload from torque F = T ÷ (K d).
- Example: M12 × 1.75 at 75% of a 600 MPa proof stress needs 37.92 kN, about 91.008 N·m with K = 0.2.
Limitations
- The nut factor scatters widely (often ±25%); preload from torque is approximate. Use tension control, turn-of-nut or direct tension indicators for critical joints.
- Coarse and fine metric threads only; prevailing-torque nuts add torque that does not produce preload.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set a tightening torque for a joint
- Check what preload a specified torque produces
- Compare lubricated and dry assembly
Who it is for
- Design engineers
- Maintenance fitters
- Quality inspectors
What you get
- Tensile stress area
- Preload and bolt stress
- Share of proof load
- Tightening torque
Interpreting the result
- Specify torque with the lubricant condition; changing lubrication changes the preload at the same torque.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Tightening by torque: the nut factor and its scatter
In Bolted Joints by the Numbers
What it covers: How tightening torque becomes preload through the nut factor, why friction makes that uncertain, how lubricants change it, and how to correct a torque wrench for an adapter.
Method explained here · BIZOMAT original book
Apply loads and restraints honestly
In Design Simulation by the Numbers
What it covers: Represent bolted supports, motor mass and torque realistically, prove equilibrium with a reaction check and catch loads that were applied several times over.
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