BIZOMAT MAKEOS · Fabrication
Lap Joint Overlap (Brazed, Soldered or Bonded)
How much overlap does this brazed, soldered or bonded lap joint need?
Overlap length for a lap or sleeve joint so the joint’s shear capacity matches or exceeds the strength of the thinner member, and the capacity at a chosen overlap.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thickness of the thinner member | mm | Thickness of the thinner part (for tubes, the wall). |
| Joint width (or circumference for tubes) | mm | Width of the joint, or circumference for a sleeve joint. |
| Strength of the member | MPa | Strength of the member (yield or tensile, matching your design basis). |
| Shear strength of the joint material | MPa | Shear strength of the braze alloy, solder or adhesive from its data sheet (reduce for temperature and ageing). |
| Joint strength ÷ member strength wanted | How much stronger than the member the joint should be. | |
| Overlap used | mm | Overlap length you plan to use. |
| Applied load (0 = not set) | N | Service load on the joint, if known. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Thickness of the thinner member
- What it is
- The thickness of the thinner of the two overlapping parts; for a sleeve joint on tubes, the tube wall thickness.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The overlap needed rises in proportion to the thickness.
- Get it right
- Use the thinner part, since it limits the strength the joint must match.
What this means: Joint width (or circumference for tubes)
- What it is
- The width of the overlap measured across the load direction; for a tube sleeve joint, the circumference of the joint.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Both member and joint capacity rise in proportion; the overlap needed does not change.
- Get it right
- For a tube, enter the circumference (π times diameter), not the diameter.
What this means: Strength of the member
- What it is
- The strength of the thinner member that the joint must develop, either yield or tensile to match your design basis.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 210 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The overlap needed rises in proportion to member strength.
- Get it right
- Use the strength of the material after brazing or soldering heat, which can be lower.
What this means: Shear strength of the joint material
- What it is
- The shear strength of the braze alloy, solder or adhesive layer, from its data sheet for the service conditions.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 5000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The overlap needed falls in inverse proportion to joint shear strength.
- Get it right
- Reduce the data sheet value for service temperature and ageing as the supplier advises.
What this means: Joint strength ÷ member strength wanted
- What it is
- How many times stronger than the member you want the joint to be, so failure happens in the member rather than the joint.
- Units and format
- Enter a plain number with no unit. The form accepts 0.1 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.25 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The overlap needed rises in proportion to the factor.
- Get it right
- This is a ratio, not a percentage.
What this means: Overlap used
- What it is
- The overlap length you plan to use or have already, checked against the required overlap and used for the joint capacity.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Joint capacity rises in proportion to overlap length.
What this means: Applied load (0 = not set)
- What it is
- The service load the joint must carry, if known; zero skips the load check.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter newtons, not kilonewtons. Zero means not set.
Method and formulas
- Member capacity = w t σ; joint capacity = w L τ.
- Required overlap L = factor × t σ ÷ τ.
- Brazed joints commonly use 3 to 6 × t overlap; longer overlaps add little because stress concentrates at the ends.
- Example: 1.5 mm sheet at 210 MPa with a 70 MPa braze needs 5.625 mm (3.75 × t).
Limitations
- Uniform shear assumed; real lap joints peak at the ends, especially bonded joints.
- Joint clearance, fill and fillet quality strongly affect strength.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Design brazed tube and sheet joints
- Choose overlap for bonded joints
- Check an existing overlap
Who it is for
- Fabrication supervisors
- Manufacturing engineers
- Estimators
What you get
- Overlap needed
- Overlap in member thicknesses
- Joint and member capacity at the chosen overlap
Interpreting the result
- For adhesives, a longer overlap gives diminishing returns; increase width instead.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1423–1430. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Lap joints and shear lag
In Adhesive Bonding by the Numbers
What it covers: How shear stress is really spread along a bonded overlap, the Volkersen shear-lag model, why longer overlaps stop adding strength, and choosing a design shear stress to size an overlap.
BIZOMAT 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 Lap Joint Overlap (Brazed, Soldered or Bonded) calculator