BIZOMAT MAKEOS · Fasteners
Torque Wrench Adapter Check (Extension Correction, Actual Torque, Bolt Preload)
When a crowfoot or extension adapter lengthens my torque wrench, what should I set the wrench to, and what torque and bolt preload does a given setting really give?
Correction of a torque wrench used with an extension adapter: the effective lever length from the wrench length and the adapter length at its angle, the actual torque at the bolt for the wrench setting, the setting that gives the target torque, and the bolt preload and share of proof load from the actual torque with the nut factor (as the bolt torque and preload tool), against the target preload and the largest share of proof load allowed.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Wrench length (hand grip centre to square drive) | mm | Length from the centre of the hand grip to the square drive of the wrench. |
| Adapter length (square drive to bolt centre) | mm | Length of the adapter or crowfoot from the square drive to the bolt centre. |
| Adapter angle to the wrench (0 = straight in line) | ° | Angle between the adapter and the wrench (0 in line, 90 at right angles, 180 folded back). |
| Wrench setting | N·m | Torque the wrench is set to. |
| Bolt | ||
| Nominal diameter | mm | Bolt nominal diameter. |
| Thread pitch | mm | Thread pitch. |
| Nut factor K | Nut factor (about 0.2 dry, 0.15 lubricated). | |
| Proof strength of the bolt | MPa | Proof strength of the bolt. |
| Target preload (% of proof) | % | Preload wanted as a share of proof load. |
| Preload tolerance (±) | % of proof | How far the preload may be from the target. |
| Largest preload (% of proof) | % | Highest preload accepted. |
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: Wrench length (hand grip centre to square drive)
- What it is
- The lever length of the torque wrench alone: from the centre of the hand grip, where the force is applied, to the centre of the square drive.
- 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 400 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- For a fixed adapter, a longer wrench makes the correction factor closer to one.
- Get it right
- Measure to the grip centre or marked load point, not to the end of the handle.
What this means: Adapter length (square drive to bolt centre)
- What it is
- The length of the crowfoot, extension spanner or adapter, from the centre of the square drive to the centre of the bolt or nut it turns.
- 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 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A longer in-line adapter increases the torque at the bolt for the same wrench setting.
- Get it right
- Measure centre to centre, not the overall length of the adapter.
What this means: Adapter angle to the wrench (0 = straight in li…
- What it is
- The angle the adapter makes with the wrench handle: in line extends the lever, at right angles adds nothing, and folded back shortens it.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 180°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The adapter length counts with the cosine of this angle, so the correction falls to nothing at a right angle.
- Get it right
- Fit a crowfoot at right angles to the wrench when you can; then no correction is needed.
What this means: Wrench setting
- What it is
- The torque the wrench is set to or reads at its click or dial, which the adapter changes before it reaches the bolt.
- 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 81 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque at the bolt, preload and share of proof rise in proportion to it.
- Get it right
- This is the wrench setting, not the torque at the bolt; the tool gives the corrected setting as a result.
Bolt
What this means: Nominal diameter
- What it is
- The nominal (major) diameter of the bolt thread, used for the nut factor relation and, with the pitch, the tensile stress area.
- 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 a given torque a larger diameter gives less preload.
- Get it right
- Use the nominal size, for example the 12 of M12, not the hexagon width across flats.
What this means: Thread pitch
- What it is
- The axial distance from one thread crest to the next, used with the nominal diameter to find the tensile stress area of the bolt.
- 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 slightly larger stress area and proof load, lowering the share of proof.
- Get it right
- Coarse and fine threads of the same size have different pitches; check the thread designation.
What this means: Nut factor K
- What it is
- The nut factor that links tightening torque to bolt preload, covering thread and under-head friction for the actual finish and lubrication.
- 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
- A higher nut factor gives less preload for the same torque.
- Get it right
- Use the factor for the actual coating and lubricant from the fastener supplier or a tightening test; it is the main source of preload scatter.
What this means: Proof strength of the bolt
- What it is
- The proof strength of the bolt material, the stress it can carry without permanent set, which with the stress area 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 target torque and lowers the share of proof for a given preload.
- Get it right
- Take it from the bolt property class standard or the supplier; proof strength is lower than yield or tensile strength.
What this means: Target preload (% of proof)
- What it is
- The preload you are aiming for, expressed as a percentage of the bolt’s proof load, which sets the target torque at the bolt.
- 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
- Raises the target torque and the corrected wrench setting in proportion.
- Get it right
- Type the percentage itself; take it from the joint design or the assembly specification.
What this means: Preload tolerance (±)
- What it is
- How far the achieved preload may be above or below the target, in percentage points of proof load, forming the accepted band.
- Units and format
- Enter a number in % of proof: % of proof. The form accepts 0 % of proof to 50 % of proof. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 % of proof as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only widens or narrows the pass band; the calculated preload does not change.
- Get it right
- It is percentage points of proof, so a target with a band either side adds and subtracts this value directly.
What this means: Largest preload (% of proof)
- What it is
- The highest preload accepted, as a percentage of proof load; the upper edge of the pass band is held at this value even if the tolerance band reaches beyond it.
- 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 90% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the over-tightened limit; the calculated preload does not change.
- Get it right
- Take it from the joint design or fastener standard; it caps the band above the target.
Method and formulas
- Effective length = L_w + E cos φ; actual torque = setting × effective length ÷ L_w.
- Setting for the target torque = target × L_w ÷ effective length, with the target torque from the nut factor (as the bolt torque and preload tool).
- Preload = actual torque ÷ (K d); share of proof = preload ÷ (S_p A_s) (as the same tool).
- Example: the setting gives 91.125 N·m at the bolt; set 80.896 N·m for the target; preload 75.097% of proof.
Limitations
- Wrench held at the grip centre; holding it elsewhere changes the effective length.
- Nut factor scatter dominates the preload accuracy, adapter or not.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set a torque wrench used with a crowfoot spanner
- Check a bolt tightened through an adapter
- Avoid overtightening with extensions
Who it is for
- Fitters
- Maintenance technicians
- Quality inspectors
What you get
- Effective length and correction
- Actual torque and the setting for the target
- Preload and share of proof
Interpreting the result
- An in-line adapter makes the wrench read low: the bolt gets more torque than the setting unless it is corrected.
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.
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Open the Torque Wrench Adapter Check (Extension Correction, Actual Torque, Bolt Preload) calculator