BIZOMAT MAKEOS · Fasteners
Bolt Preload Verification from Measured Stretch (and Nut Factor)
From the bolt stretch I measured, what preload did this bolt actually get, how close is it to the target, and what nut factor does the tightening torque imply?
Verification of bolt tightening from a measured elongation: bolt stiffness from shank and thread lengths and the member stiffness (as the bolt stretch tool), preload from the measured stretch, deviation from the target preload as a share of proof load, the nut factor implied by the applied torque (as the torque–preload relation of the bolt torque–preload tool), the torque that would give the target with that nut factor, and the turn angle that corresponds to the preload.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bolt diameter | mm | Nominal bolt diameter. |
| Thread pitch | mm | Thread pitch. |
| Grip length | mm | Total thickness clamped. |
| Threaded length in the grip | mm | Thread length within the grip. |
| Bolt modulus | GPa | Elastic modulus of the bolt. |
| Clamped part modulus | GPa | Elastic modulus of the clamped parts. |
| Proof strength | MPa | Proof strength of the bolt grade. |
| Target preload as share of proof load | % | Preload you aimed for as a share of proof load. |
| Accepted deviation from target | % | Plus-or-minus deviation from the target you accept. |
| Measured bolt stretch | mm | Elongation of the bolt measured (ultrasonically or by micrometer) between loose and tight. |
| Applied tightening torque | N·m | Torque applied to reach that stretch. |
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: Bolt diameter
- What it is
- The nominal (major) diameter of the bolt. It sets the shank area, the tensile stress area and the implied nut factor.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Thread pitch
- What it is
- The axial distance between thread crests, used for the tensile stress area and to convert stretch into a nut turn angle.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Grip length
- What it is
- The total thickness of the parts clamped between the bolt head and the nut, which is the length of bolt that stretches.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Include washers and gaskets in the clamped thickness.
What this means: Threaded length in the grip
- What it is
- The length of threaded bolt inside the grip, from the nut face to where the thread runs out; the rest of the grip is plain shank.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It cannot exceed the grip; 0 means the whole grip is plain shank.
What this means: Bolt modulus
- What it is
- The elastic (Young’s) modulus of the bolt material, from the material data. It converts the measured stretch into load.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 300 GPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 207 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The preload worked out from a given stretch rises in direct proportion.
- Get it right
- Enter it in GPa, not MPa: 1 GPa = 1000 MPa, so divide a data-sheet value given in MPa by 1000.
What this means: Clamped part modulus
- What it is
- The elastic modulus of the clamped parts. It is used only for the member stiffness, which affects the turn angle, not the preload from the stretch.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 400 GPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 207 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Proof strength
- What it is
- The proof strength of the bolt grade, from the bolt standard or the supplier certificate; the proof load is this stress times the tensile stress area.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2000 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.
What this means: Target preload as share of proof load
- What it is
- The preload you aimed for when tightening, as a share of the bolt’s proof load.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% 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.
- Get it right
- A share of the proof load, not of the yield or tensile strength.
What this means: Accepted deviation from target
- What it is
- The plus-or-minus band around the target preload within which you accept the joint, as a share of the target preload.
- 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 50%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Measured bolt stretch
- What it is
- How much the bolt length grew between the loose and the tightened state, measured end to end with an ultrasonic gauge or a micrometer.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.0001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The preload is directly proportional to the measured stretch.
- Get it right
- Take both readings at the same temperature; a temperature change can be as large as the stretch itself.
What this means: Applied tightening torque
- What it is
- The torque wrench setting used to reach the measured stretch. With the preload it gives the nut factor that the joint actually showed.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0 N·m to 100000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 480 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- 0 = not used: the implied nut factor and the torque for the target are then not reported.
Method and formulas
- k_b = 1 ÷ [(grip − L_t) ÷ (E A_d) + L_t ÷ (E A_t)]; preload F = k_b × stretch (as the bolt stretch tool).
- Target F_t = share × S_p A_t; deviation = (F − F_t) ÷ F_t.
- Nut factor K = T ÷ (F d); torque for the target = K F_t d (as the bolt torque–preload tool); turn angle = 360° (δ_bolt + δ_member) ÷ P.
- Example: 0.15 mm of stretch means 113.87 kN (3.3672% from target); the torque implies K = 0.21077.
Limitations
- Elastic stretch only; the measurement must exclude temperature change and bending.
- Stiffness uses the simplified shank-and-thread model; the nut and head add a little compliance.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check critical bolts after tightening
- Calibrate a tightening torque from stretch measurements
- Find the real nut factor for a lubricant
Who it is for
- Maintenance engineers
- Design engineers
- Quality inspectors
What you get
- Bolt stiffness and preload from the stretch
- Deviation from target
- Implied nut factor, torque for the target and turn angle
Interpreting the result
- If K is well above the assumed value, the lubricant or thread condition is soaking up torque: preload will be low.
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
Stretch and angle: setting preload closer to the bolt, and checking it
In Bolted Joints by the Numbers
What it covers: How bolt stretch and nut rotation relate to preload, why the snug point matters, how torque-angle tightening narrows scatter, and how measured stretch verifies preload.
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Open the Bolt Preload Verification from Measured Stretch (and Nut Factor) calculator