Twin elbow branch calculator
For two mirrored segmented elbows rising from the top of a main pipe and turning away from each other: enter the main pipe, the spacing and the elbow. You get the main with both holes, the elbow pieces and the branch piece.
Enter your measurements
Seam
4,303.3mm
Exact geometry
- Cut length
- In the app
- Cut curve length
- In the app
- Sheet size
- In the app
Every result is in Trueflat
The free web version shows the main result and the drawing. Trueflat gives you every mark and measurement, and exports a cut sheet. It is free to download.
How to lay it out
Lay out the main pipe
Draw the main pipe rectangle with the seam on the bottom and mark the two branch centres on the top line, one branch spacing apart and centred on the length.
Mark both holes
Mark the hole points around each centre by their distance along the pipe and their arc from the top line. The second hole is the mirror of the first.
Lay out the branch piece
Draw its stretchout and mark two curves on each line: the fish mouth at the bottom and the miter at the top.
Lay out the other elbow pieces
Lay out the remaining pieces as for a segmented elbow of the branch diameter.
Cut two sets
Cut every piece, the branch piece included, twice and turn the second set over for the mirrored elbow.
Good to know
- FabLayout runs the same code as Trueflat; the result is exact geometry.
- Source: No published source. Method: each straight line of the branch is intersected with the main surface and both parts are unrolled, developed on the sheet middle; checked by two independent implementations in Trueflat.
Method and formula
Two equal segmented elbows of the branch diameter stand on top of the main pipe, their axes the branch spacing apart and centred on its length. Each turns away from the other. The last piece of each elbow is the branch piece: it enters the main pipe square and on centre.
- Elbow: miter angle β = α ÷ (2(m − 1)) and piece lengths as on the lobster back elbow page.
- Branch piece, bottom: the fish mouth of a square pipe tee, √(R² − r² × sin² θ) above the main axis.
- Branch piece, top: the miter to the next piece, at Hb − r × tan β × cos θ, where Hb is the elbow corner height: from the main axis to the last miter corner on the elbow axis.
- Holes: the second hole is the mirror image of the first across the middle of the main pipe.
One elbow is shown, because the second is its mirror. The holes must stay at least 5 mm apart and at least 5 mm from the pipe ends. The pattern is exact. Enter measurements to the middle of the sheet. Which face of the branch seats on the main depends on how you fit and weld it, so this shape is developed on the sheet middle only.
Worked example
Main 300 mm, 800 mm long, branch spacing 400 mm; elbows 100 mm, bend radius 150 mm, 90°, 3 pieces, corner height 250 mm. β = 22.5°. The miter at the top of the branch piece is at 229.29 mm on the throat line and 270.71 mm on the back line. The shortest branch line is 79.29 mm. The holes are 300 mm apart and 150 mm from each end.
Questions
Why does the result show only one elbow?
The two elbows are mirror images. Cut each piece twice and turn the second set over; the two holes in the main are already both drawn.
What is the elbow corner height?
The height from the main pipe axis to the point where the branch piece's top miter crosses the elbow axis. It sets how high the elbows turn.
How is this different from the tee on an elbow?
Here the main is a straight pipe and the elbows form the branches. On the tee on an elbow page the elbow is the main part and a straight branch goes into one of its pieces.
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