KEVOS® MAKEOS · Fasteners
Bolted Joint Design Check (Preload, Separation, Proof and Fatigue)
Does this bolt size and tightening torque keep the joint closed and the bolt safe under the working load?
Complete check of a preloaded tension joint: stress area, bolt and member (frustum) stiffness, joint constant, preload and tightening torque from the target share of proof load, preload scatter from the nut factor tolerance, and factors against separation, exceeding proof load and fatigue (Goodman) under a load cycling from zero to the working value.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bolt | ||
| Nominal diameter | mm | Nominal bolt diameter. |
| Thread pitch | mm | Thread pitch. |
| Proof strength | MPa | Proof strength of the property class (from the bolt standard). |
| Tensile strength | MPa | Minimum tensile strength. |
| Endurance limit of the threaded bolt | MPa | Fully corrected endurance limit of the threaded bolt (rolled or cut threads, from fatigue data). |
| Bolt modulus | GPa | Elastic modulus of the bolt. |
| Joint | ||
| Grip length | mm | Total thickness clamped. |
| Threaded length within the grip | mm | Length of thread inside the grip. |
| Member modulus | GPa | Elastic modulus of the clamped members (the frustum estimate assumes one material). |
| Tightening and load | ||
| Target preload (share of proof load) | % | Preload as a share of proof load (75% is common for reusable joints). |
| Nut factor | Nominal nut factor for the finish and lubrication. | |
| Nut factor scatter (±) | % | Uncertainty in the nut factor, which scatters the preload for a given torque. |
| Working tension load per bolt | kN | External tension load per bolt, cycling from zero. |
| Required factor (separation and fatigue) | Factor you require against separation and fatigue. | |
Method and formulas
- A_t = π ÷ 4 (d − 0.9382 P)²; k_b from the shank and threaded lengths; k_m by the 30° frustum: 0.5774 π E d ÷ 2 ln[5 (0.5774 l + 0.5 d) ÷ (0.5774 l + 2.5 d)].
- C = k_b ÷ (k_b + k_m); F_i = share × S_p A_t; torque T = K F_i d; scatter gives F_i ÷ (1 ± scatter).
- Separation factor = F_i,min ÷ [(1 − C) P]; proof factor = (S_p A_t − F_i,max) ÷ C P.
- Fatigue (Goodman): n_f = S_e (S_ut − σ_i) ÷ [σ_a (S_ut + S_e)], σ_a = C P ÷ 2A_t, σ_i = F_i,max ÷ A_t.
- Example: an M12 bolt at 70% of proof needs 84.941 N·m and holds 3.6253 times the separation load.
Limitations
- Concentric tension joint; prying, bending and thermal loads are excluded.
- Frustum stiffness assumes members of one material wider than about three bolt diameters.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose bolt size, grade and torque for a joint
- Check a joint after a load increase
- Decide whether torque control is good enough
Who it is for
- Design engineers
- Assembly engineers
- Maintenance engineers
What you get
- Stress area, stiffnesses and joint constant
- Preload range and tightening torque
- Factors against separation, proof and fatigue
Interpreting the result
- If separation fails, raise preload or use more bolts; if fatigue fails, use rolled threads, a longer bolt or a stiffer joint.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1536. Topic reference; no source tables or recommended settings are reproduced.
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Open the Bolted Joint Design Check (Preload, Separation, Proof and Fatigue) calculator
