BIZOMAT MAKEOS · Fasteners
Turn-of-Nut Preload and Bolt Fatigue Check
What preload does this nut rotation give in my joint, is it within the bolt’s proof load, and will the bolt survive the cyclic external load?
Check of a bolt tightened by the turn-of-nut method in a fluctuating-load joint: bolt stiffness from the shank and threaded lengths in the grip and the preload from the nut rotation past snug with the joint stiffness (as the turn-of-nut preload tool), the joint constant, the preload as a share of proof load, and the fatigue safety factor of the bolt under the cyclic external load by the Goodman line (as the bolted joint fatigue tool).
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Turn angle past snug | ° | Nut rotation after the joint is snug. |
| Thread lead | mm | Lead (pitch for single-start) of the thread. |
| Nominal (shank) diameter | mm | Nominal diameter of the bolt shank. |
| Tensile stress area | mm² | Tensile stress area of the thread. |
| Unthreaded shank length in the grip | mm | Length of plain shank within the clamped grip. |
| Threaded length in the grip | mm | Length of thread within the grip. |
| Bolt elastic modulus | GPa | Elastic modulus of the bolt. |
| Joint (clamped parts) stiffness | kN/mm | Stiffness of the clamped parts (from a frustum estimate or test). |
| Load and strength | ||
| Maximum external load (cycling from zero) | kN | External tensile load on the joint, cycling between zero and this value. |
| Proof strength | MPa | Proof strength of the bolt grade. |
| Largest preload as a share of proof load | % | Highest preload you accept as a share of the proof load. |
| Bolt endurance strength (fully corrected) | MPa | Fully corrected endurance strength of the bolt thread. |
| Bolt tensile strength | MPa | Tensile strength of the bolt. |
| Required fatigue safety factor | Fatigue safety factor you need. | |
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: Turn angle past snug
- What it is
- The angle the nut is turned after the joint is snug, the part of tightening that stretches the bolt and compresses the joint.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0.000001° to 3600°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25° 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 direct proportion to the turn angle.
What this means: Thread lead
- What it is
- The axial advance of the nut for one full turn: the thread pitch for an ordinary single-start bolt.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Nominal (shank) diameter
- What it is
- The nominal diameter of the bolt shank, used for the cross-section of the unthreaded part within the grip.
- 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 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Tensile stress area
- What it is
- The tensile stress area of the bolt thread from the thread standard, used for the stiffness of the threaded length and for the bolt stresses.
- Units and format
- Enter a number in mm²: square millimetres. The form accepts 0.000001 mm² to 1000000 mm². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 58 mm² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the stress area from the thread table, not the area of the nominal diameter.
What this means: Unthreaded shank length in the grip
- What it is
- The length of plain, unthreaded shank lying within the clamped grip between the bolt head and the nut.
- 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 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Threaded length in the grip
- What it is
- The length of threaded bolt lying within the grip, from the end of the shank to the face of the nut.
- 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 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Shank length plus threaded length should equal the total grip; together they set the bolt stiffness.
What this means: Bolt elastic modulus
- What it is
- The elastic modulus of the bolt material, from the bolt or material data sheet.
- 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 205 GPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter GPa, not MPa: the tool multiplies by 1000 internally.
What this means: Joint (clamped parts) stiffness
- What it is
- The axial stiffness of the clamped parts between bolt head and nut: the force per millimetre of compression, from a frustum estimate or test.
- Units and format
- Enter a number in kN/mm: kN/mm. The form accepts 0.1 kN/mm to 1000000 kN/mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1200 kN/mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter kN/mm, not N/mm; a stiffer joint takes more preload per degree of turn and a smaller share of the external load.
Load and strength
What this means: Maximum external load (cycling from zero)
- What it is
- The external tensile load pulling the joint apart at this bolt, cycling between zero and this value.
- Units and format
- Enter a number in kN: kilonewtons (1 kN = 1000 N). The form accepts 0 kN to 1000000 kN. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 kN as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter the load on one bolt, not the total load on the joint.
What this means: Proof strength
- What it is
- The proof strength of the bolt property class or grade from the bolt standard; used to express preload as a share of proof load.
- 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 600 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Largest preload as a share of proof load
- What it is
- The highest preload you accept as a percentage of the bolt’s proof load, set by your tightening specification.
- 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 90% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Bolt endurance strength (fully corrected)
- What it is
- The endurance strength of the bolt thread, fully corrected for size, thread forming and reliability, from your fastener fatigue data.
- 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 129 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Bolt tensile strength
- What it is
- The minimum tensile strength of the bolt grade, from the bolt standard; it anchors the Goodman line.
- 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 830 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Required fatigue safety factor
- What it is
- The least Goodman fatigue safety factor you accept for the bolt under the cyclic external load.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Bolt flexibility = (L_s ÷ A_s + L_t ÷ A_t) ÷ E; preload F = (θ ÷ 360) × lead ÷ (bolt flexibility + joint flexibility) (as the turn-of-nut preload tool).
- Joint constant C = k_b ÷ (k_b + k_j).
- σ_a = C P ÷ 2A_t; σ_i = F ÷ A_t; n = S_e (S_ut − σ_i) ÷ [S_ut σ_a + S_e (σ_m − σ_i)] (as the bolted joint fatigue tool).
- Example: 25° past snug gives 29.445 kN (84.612% of proof); the joint constant is 0.23556 and the fatigue factor 4.2702.
Limitations
- Angle measured from a true snug condition; gaps and soft gaskets take up rotation before preload builds.
- Fully corrected endurance strength from your data.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set a turn angle for a fatigue-loaded joint
- Check preload scatter from angle tightening
- Check a bolted joint under cyclic load
Who it is for
- Design engineers
- Maintenance engineers
- Fitters
What you get
- Bolt stiffness, preload and joint constant
- Preload as a share of proof load
- Fatigue safety factor
Interpreting the result
- Stiff clamped parts (low joint constant) shield the bolt from the cyclic load: keep gaskets thin and hard.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535. Topic reference; no source tables or recommended settings are reproduced.
- VDI 2230 Part 1
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
Bolts that cycle: fatigue in tension joints
In Bolted Joints by the Numbers
What it covers: How a cyclic external load becomes a small alternating stress in a well-preloaded bolt, why separation and soft members destroy that protection, and how to check bolt fatigue.
Method explained here · BIZOMAT original book
Bolted joints: preload is the defence
In Metal Fatigue by the Numbers
What it covers: Why a properly preloaded bolt sees only a fraction of a cycling load, how to check bolt fatigue by Goodman from the preload, and why a turn-of-nut rule failed on a short, stiff joint.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Turn-of-Nut Preload and Bolt Fatigue Check calculator