KEVOS® 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. | |
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
WORKOS 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
