BIZOMAT MAKEOS · Fasteners
Bolt Stretch and Turn-of-Nut Angle
How much will this bolt stretch at the target preload, and how far must the nut turn from snug?
Bolt elongation, member compression and the nut rotation from snug-tight needed to reach a target preload, from bolt and joint stiffness.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal diameter | mm | Nominal bolt diameter. |
| Pitch | mm | Thread pitch. |
| Grip length | mm | Total clamped thickness. |
| Threaded length within the grip | mm | Length of thread inside the grip. |
| Bolt elastic modulus | GPa | Bolt modulus. |
| Member elastic modulus | GPa | Modulus of the clamped parts (one material assumed for the stiffness estimate). |
| Target preload | kN | Target preload from the joint design (see the bolt torque and preload tool). |
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: Nominal diameter
- What it is
- The nominal (major) diameter of the bolt, for example 20 for an M20; it sets the shank area 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 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the nominal size, not the measured minor or pitch diameter.
What this means: Pitch
- What it is
- The axial distance from one thread crest to the next on the bolt; for a single-start thread it is also the distance the nut advances in one turn.
- 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 2.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The nut turn needed for the same stretch is inversely proportional to the pitch, so a finer pitch needs more rotation.
- Get it right
- Coarse and fine series differ for the same diameter; check which the bolt uses.
What this means: Grip length
- What it is
- The total thickness of all the parts clamped between the bolt head and the nut, including washers.
- 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 80 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- This is the clamped thickness, not the overall bolt length.
What this means: Threaded length within the grip
- What it is
- The part of the grip length where the bolt is threaded rather than plain shank; the rest of the grip is taken as full-diameter shank.
- 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 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More thread in the grip makes the bolt more flexible, so it stretches further for the same preload.
- Get it right
- It cannot be longer than the grip. For a fully threaded bolt, set it equal to the grip length.
What this means: Bolt elastic modulus
- What it is
- The elastic (Young’s) modulus of the bolt material, used for the stiffness of the shank and threaded portions.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 1000 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
- A higher modulus makes the bolt stiffer, reducing its stretch and the nut turn.
- Get it right
- Enter gigapascals, not megapascals. Take it from the bolt material data.
What this means: Member elastic modulus
- What it is
- The elastic modulus of the clamped parts, used in the cone (frustum) estimate of joint stiffness; one material is assumed for all the parts.
- Units and format
- Enter a number in GPa: gigapascals (1 GPa = 1000 MPa). The form accepts 0.000001 GPa to 1000 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
- Stiffer members compress less, reducing the nut turn needed.
- Get it right
- For mixed materials, the single value is an approximation; joints with soft parts or gaskets need testing.
What this means: Target preload
- What it is
- The clamp force you want in the bolt after tightening, from your joint design.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0.000001 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Bolt stretch, member compression and nut turn all rise in proportion to preload.
- Get it right
- Enter kilonewtons, not newtons. It is the load in one bolt, not the total for the joint.
Method and formulas
- kb = 1 ÷ (Ls ÷ (E Ad) + Lt ÷ (E At)); km by the 30° frustum estimate.
- Bolt stretch δb = F ÷ kb; member compression δm = F ÷ km.
- Nut turn from snug = 360° × (δb + δm) ÷ P.
- Example: M20 with an 80 mm grip at 120 kN stretches 158.08 µm and needs about 27.696° from snug.
Limitations
- The snug point is uncertain and adds scatter; turn-of-nut procedures usually come from tests or the code (e.g. AS 4100 / RCSC tables).
- Elastic stiffness only; yield-controlled tightening goes beyond this calculation.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set a turn-of-nut or angle-tightening procedure
- Check ultrasonic or micrometer bolt stretch readings
- Compare joint stiffness options
Who it is for
- Design engineers
- Maintenance fitters
- Quality inspectors
What you get
- Bolt and member stiffness
- Bolt elongation at preload
- Member compression
- Nut turn from snug
Interpreting the result
- Validate any angle procedure on the actual joint with a tension-indicating method.
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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