Calculation
τ = F / (b · lü)
Soft soldering and brazing differ by roughly a factor of fifty in allowable shear stress; the two processes must never be evaluated with the same characteristic values in this formula.
τ = F/(b·lü)
Soldered/brazed lap joint shear stress and overlap length
Soft soldering and brazing differ by roughly a factor of fifty in allowable shear stress; the two processes must never be evaluated with the same characteristic values in this formula.
MINTSI
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Result
Select a target and calculate.
Understand the inputs
- Nominal shear stress τ — Mean shear stress in the solder/braze joint at the given load.
- Transmitted force F — Axial force to be transmitted through the solder/braze joint.
- Joint width b — Width of the solder/braze joint, transverse to the load direction.
- Overlap length lü — Length of the overlap in the direction of load.
Example
5,000 N through a 10 mm wide, 5 mm overlapping brazed joint gives τ = 5,000 N / (10 mm · 5 mm) = 100 MPa — already at the typical guideline value for brazed joints.
Assumptions and limits
Nominal, evenly distributed shear stress without application factor or safety margin; guideline values for static loading on structural steel are roughly 100 MPa for brazing and only about 2 MPa for soft soldering, and must not be confused.
Understand Soldered/brazed lap joint shear stress and overlap length
This calculator finds the nominal shear stress of a soldered or brazed lap joint from force, joint width and overlap length, or the overlap length required for an allowable stress.
What does this quantity describe?
As with an adhesive joint, the nominal mean shear stress of a solder/braze joint is computed from τ = F/(b·lu). Process choice matters greatly: soft soldering below about 450°C and brazing above about 450°C differ in allowable stress by roughly a factor of fifty.
Picture the difference between soft soldering and brazing like the difference between a hot-glue gun and a structural weld: both join two parts materially, but achievable load capacity differs by orders of magnitude, depending on filler metal and processing temperature.
Formula and variables
τ = F / (b · lü)
τ = F/(b·lu)F = τ·b·lulu = F/(τ·b)
| Symbol / input | Meaning |
|---|---|
| Nominal shear stress τ | Mean shear stress in the solder/braze joint at the given load. |
| Transmitted force F | Axial force to be transmitted through the solder/braze joint. |
| Joint width b | Width of the solder/braze joint, transverse to the load direction. |
| Overlap length lü | Length of the overlap in the direction of load. |
Choose the inputs correctly
Force F, joint width b and overlap length lu set the nominal shear stress. For pre-sizing, lu can also be chosen as the target when the allowable stress is entered as τ.
How to use the calculator
Enter force, joint width and overlap length to check nominal stress. For a sizing question — what overlap length an allowable stress needs — choose lu as the target and enter the allowable value as τ.
Worked example
5,000 N through a 10 mm wide, 5 mm overlapping solder joint gives τ = 5,000 N / (10 mm · 5 mm) = 100 MPa — already at the typical guideline value for brazed joints, and roughly fifty times too high for a soft-soldered joint.
Understand the result and units
A result of 100 MPa immediately shows this joint can only be made with a braze filler — a soft-soldered joint with an allowable stress of only about 2 MPa would need roughly fifty times the bonded area for the same force.
Force is given in N, joint width and overlap length in mm, and the resulting stress in MPa (N/mm²).
Soft soldering versus brazing: a factor of fifty
Per Roloff/Matek, typical allowable shear stresses for statically loaded solder joints on structural steel are roughly 100 N/mm² for brazing and only about 2 N/mm² for soft soldering — a factor of 50 difference. The reason lies in the filler metal and processing temperature: soft solders below about 450°C, such as tin-lead or lead-free tin alloys, have markedly lower inherent strength than braze fillers above about 450°C, such as silver- or copper-based alloys, which also often form a stronger metallurgical bond with the base metal. Load-bearing mechanical engineering joints therefore practically require brazing; soft soldering is used mainly for electrical contacts and lightly loaded connections.
Typical applications
The calculation is used for pre-sizing soldered and brazed lap joints, for example pipe socket connections or electronic assemblies, and as a first decision basis for whether soft solder or braze is even viable for the given load.
Assumptions, limits and common mistakes
The model gives only the nominal, evenly distributed shear stress without an application factor for shock loading and without a safety margin; a complete verification per Roloff/Matek additionally multiplies load by an application factor KA and requires a safety factor of S=2-3 against joint shear strength. Temperature resistance, corrosion in the solder gap, and strength at operating temperature are also not included.
Common mistake: A common mistake is mixing soft-solder and braze characteristic values, even though allowable stress differs by an order of magnitude between them. It is also easy to skip checking operating temperature, even though soft solders in particular can lose considerable strength at elevated operating temperature.
Frequently asked questions
What's the difference between soft soldering and brazing?
Soft soldering works below about 450°C with markedly lower joint strength, brazing above 450°C with roughly fifty times the allowable shear stress; see the section above.
What allowable stress should I use for a load-bearing joint?
For load-bearing mechanical joints, typically a braze guideline value of about 100 N/mm² for structural steel, always referenced to the specific filler alloy and a safety factor of S=2-3 against joint shear strength.
Why isn't an application factor included?
This calculator represents the pure nominal stress; a complete verification per Roloff/Matek additionally multiplies load by an application factor KA for shock loading.
How do I find the overlap length for an allowable stress?
Choose lu as the target, enter the allowable stress as τ, and provide force and joint width; the result is the approximately required length.
Does operating temperature affect solder joint strength?
Yes, soft solders in particular lose considerable strength at elevated operating temperature; this calculator represents only the room-temperature nominal stress.