How to lay out a lobster back bend
To lay out a lobster back bend, find the cut angle: the bend angle divided by twice the number of joints. A full gore measures twice the bend radius times the tangent of that angle along its center, longer at the heel and shorter at the throat. Lay out one gore by parallel line, then cut every gore from it.
Drawn by Stretchout from the worked example below. Parallel line development.
What you need
- Paper or light sheet for the wrap template
- A tape and a straightedge
- Dividers for the spacing
- A flexible batten
- Soapstone or a scribe
Worked example
We’ll lay out a 90° lobster back on 10" pipe (10.750" OD), 15" bend radius, 5 gores (3 full and 2 half), laid out on the OD as a wrap template. The numbers:
- Cut at every joint
- 11.25° from square
- Gores
- 3 full + 2 half
- Full gore at the heel
- 8 1/8"
- Full gore at the throat
- 3 13/16"
- Stretch-out (around the pipe)
- 33 3/4"
- Straight pipe for all gores
- 23 7/8"
Step by step
Step 1
Draw a baseline 33 3/4" long: the stretch-out. Square a line up at each end.
Step 2
Divide it into spaces 2 1/8" apart (16 around the whole pipe) and square a line up from each mark. Number them from the left.
Step 3
Mark each line up from the baseline for the cut:
Line Bottom cut Top cut 1 0" 8 1/8" 2 1/16" 8" 3 5/16" 7 13/16" 4 11/16" 7 7/16" 5 1 1/16" 7 1/16" 6 1 1/2" 6 5/8" 7 1 13/16" 6 1/4" 8 2 1/16" 6 1/16" 9 2 1/8" 5 15/16" 10 2 1/16" 6 1/16" 11 1 13/16" 6 1/4" 12 1 1/2" 6 5/8" 13 1 1/16" 7 1/16" 14 11/16" 7 7/16" 15 5/16" 7 13/16" 16 1/16" 8" Step 4
Join the marks with a smooth curve (a flexible batten helps) and cut on it.
Watch it laid out
Every step of the worked example. Play, or step through with Next and Back (or the arrow keys).
1.Draw a baseline 33 3/4" long: the stretch-out. Square a line up at each end.
The formulas
With n gores counting the two half gores at the ends, there are n − 1 joints. Each joint turns the bend angle ÷ (n − 1), so each cut is half that from square: α = bend ÷ (2(n − 1)). For a 90° bend in 5 gores, α = 11.25°.
A full gore along its centerline is 2·R·tan α, with R the bend radius. At the heel (outside) it’s 2·(R + r)·tan α and at the throat (inside) 2·(R − r)·tan α, with r the pipe’s radius. A half gore is half of that, plus any straight tangent.
Every full gore is the same, so lay out one, cut it, and use it as the template for the rest. Turn each piece over end for end when you cut them from straight pipe and there’s no waste between gores.
Questions
What’s the difference between a lobster back bend and a mitered elbow?
Same idea: a bend made from straight pieces cut at an angle and welded. Lobster back (or gored, or segmented) usually means several gores; a mitered elbow can be just two pieces.
How many gores should I use?
More gores make a smoother bend and a lower pressure drop, but more welds. Three to five for a 90° is common in the shop; bulk handling and ducts often go higher.
Where is the long point?
At the heel, the outside of the bend. Line 1 of the template is the heel; the throat is halfway around.
Do I lay out on the OD?
For a wrap template on pipe, yes: it wraps the outside. For a gore rolled from plate, lay out on the mean line. The calculator does both.
Can I cut all the gores from one straight length?
Yes. Cut a full gore, flip the remaining pipe end for end, and the next cut matches. The calculator gives the straight length you need.
Your sizes
Below is the gored elbow calculator, opened on the worked example. Put in your own sizes and it redoes every number and every step for your fitting. It’s also at /stretchout/shapes/elbow.