KEVOS® MAKEOS · Fasteners
Thread Length of Engagement
How long must the thread engagement be so the bolt breaks before the threads strip?
Minimum length of engagement for a metric or unified thread so that the external thread does not strip before the screw breaks, with the extra length needed when the tapped material is weaker.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Basic major diameter | mm | Nominal size of the thread. |
| Pitch | mm | Thread pitch (for unified threads, 25.4 ÷ threads per inch). |
| Maximum minor diameter of internal thread (0 = basic) | mm | From the thread limits table for your thread class (e.g. 6H); 0 uses the basic minor diameter, which is slightly optimistic. |
| Minimum pitch diameter of external thread (0 = basic) | mm | From the thread limits table for the bolt class (e.g. 6g); 0 uses the basic pitch diameter. |
| Minimum major diameter of external thread (0 = basic) | mm | Minimum major diameter of the bolt thread from the limits table; 0 uses the basic size. |
| Maximum pitch diameter of internal thread (0 = basic) | mm | Maximum pitch diameter of the tapped thread from the limits table; 0 uses the basic pitch diameter. |
| Tensile strength of the screw | MPa | Minimum tensile strength of the screw (e.g. 800 MPa for property class 8.8). |
| Tensile strength of the tapped material | MPa | Minimum tensile strength of the nut or tapped component material. |
Method and formulas
- At = π/4 (d − 0.938194P)² (or π(Es min/2 − 0.16238P)² above 1240 MPa).
- External thread: Le = 2At ÷ [π Kn max (½ + 0.57735 n (Es min − Kn max))], with n = 1/P.
- Internal thread shear area per unit length: π n Ds min [½ + 0.57735 n (Ds min − En max)]; J = (As external × Su screw) ÷ (As internal × Su tapped); required length = J × Le when J > 1.
- Example: M10 × 1.5 (8.8) into 310 MPa material needs about 16.332 mm (1.6332 × d) of engagement.
Limitations
- Basic dimensions (inputs left at 0) overstate the shear areas slightly; use the limits for your thread class for a design check.
- Assumes full-form threads over the whole engaged length; countersinks, chamfers and incomplete threads at the hole bottom do not count.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set tapped hole depth in aluminium, cast iron or soft steel
- Check a short nut or thin plate thread
- Decide whether a thread insert is needed
Who it is for
- Design engineers
- Maintenance fitters
- Quality inspectors
What you get
- Tensile stress area
- Engagement to protect the external thread
- Strength ratio J and engagement for the internal thread
- Required engagement in diameters
Interpreting the result
- If the required length is impractical, use a thread insert, through bolt and nut, or a stronger tapped material.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1536–1537. Topic reference; no source tables or recommended settings are reproduced.
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1528. Topic reference; no source tables or recommended settings are reproduced.
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