Thread Engagement Length
Verify thread-flank shear and bearing stress at engagement length L and compute required minimum length (ISO 261 / GB metric threads).
Inputs
Formula
τ = F/(0.5πdL); σc = F/(0.75πdL); L_req = F/(0.5πd·[τ])
Fundamentals
Engagement length is the effective axial length where internal and external threads mesh, governing joint strength and locking.
Engagement $L_e$ must give enough thread-flank area to transfer load; too short shears, too long adds little strength but costs machining. Typically $L_e\ge0.8d$ adjusted by duty.
History
The effect of engaged threads on joint strength came from early thread-failure statistics; ISO 965 defines deviations and engagement-length series.
Engineering applications
Used for threaded pairs under alternating or impact load, e.g. lifting-eye bolts and hoist studs, to prevent stripping.
Glossary
| Engagement $L_e$ | Axial length of effective thread mesh. |
| Pitch $P$ | Axial distance between adjacent thread crests. |
| Nominal dia $d$ | Major diameter defining the thread size. |
How to use
- Fill in Axial force F, Major dia. d, Engagement L, Allow. shear [τ], Allow. bearing [σc] in the Inputs section (watch the unit on each field).
- Click Calculate; the tool evaluates the formula shown above.
- Read Actual shear τ, Bearing stress σc, Required length L_req in the results area.
Formula notesThread engagement Le sets joint strength; typically Le ≥ 0.8 d (steel–cast iron) or ≥ 1.0 d (steel–aluminium).
Formula · Worked Example · Knowledge
Formula
Thread shear $\tau=F/(0.5\pi d L)$, bearing stress $\sigma_c=F/(0.75\pi d L)$; min engagement $L_{req}=F/(0.5\pi d[\tau])$.
Worked Example
F=10000 N, d=10 mm, [τ]=120 MPa → L_req≈5.3 mm (15 mm used for margin).
Key Points
- Short engagement shears the flank or crushes the nut.
- Coarse ISO 261 threads often use L≥0.8d.
- Check the weaker of internal/external thread materials.
Parameters
Inputs
| Parameter | Symbol | Unit | Default |
|---|---|---|---|
| Axial force F (N) | F | N | 10000 |
| Major dia. d (mm) | d | mm | 10 |
| Engagement L (mm) | L | mm | 15 |
| Allow. shear [τ] (MPa) | tau_allow | MPa | 120 |
| Allow. bearing [σc] (MPa) | sigc_allow | MPa | 200 |
Outputs
| Result | Symbol | Unit |
|---|---|---|
| Actual shear τ | tau | MPa |
| Bearing stress σc | sigc | MPa |
| Required length L_req | L_req | mm |
Applications
- Common engineering use cases
FAQ
What formula does this tool use?
This tool computes per ISO / AGMA / ASME standard formulas: τ = F/(0.5πdL); σc = F/(0.75πdL); L_req = F/(0.5πd·[τ])
How accurate are the results?
Results match input precision, based on SI units and common engineering approximations; for critical duty re-check with a safety factor.
Where is it used?
Common engineering use cases