Segmented bend and elbow development
Changing direction in plate: free, 90° and 180° lobster back bends, the two-gore elbow, the reducing bend, rectangular and flat-oval duct bends, and the pipe offset. Every gore is a cylinder cut at an angle, and the wave formula is the same for all.
Segmented Bend
The segmented bend, or lobster back: lengths of tube cut at an angle and welded up to approximate a radius. More gores, smoother bend, more welding.
90° Segmented Bend
The segmented bend with its angle fixed at 90 degrees. Enter diameter, radius and gore count and get the flat pattern free, with the formula and the ordinate table.
180° Segmented Bend
The 180 degree segmented bend — the full return. The tube leaves in the direction opposite to the one it entered, around the radius you give.
Elbow
The two-gore bend: two half shells cut at an angle and welded to each other. The cheapest direction change in plate, with one circumferential weld.
Pipe Offset (Portal)
The U-shaped offset: two elbows joined by a straight run, solved together so the cuts match. It comes out as four flat patterns, one per segment.
Rectangular Duct Bend
The mitred bend in rectangular section. Same calculation as the round one, but the cut is constant on two faces and varies linearly on the other two.
Tapered Segmented Bend
The mitred bend that also reduces, along the same arc. The gores are all different — s + 1 patterns instead of the two a constant bend takes.
Oval Segmented Bend
The segmented bend for flat-oval duct — the section used where the headroom will not take the equivalent round. Two patterns, not one.
Which bend to use
- A 90° change of direction, the most common of all: 90° Segmented Bend, with as many gores as the shop wants.
- Any angle, from 15° to 180°: Segmented Bend.
- A 180° return, the pipe coming back parallel: 180° Segmented Bend.
- Two gores only, the cheapest bend: Elbow. For where pressure drop does not matter.
- A pipe offset, two bends and a straight to get round an obstacle: Pipe Offset (Portal).
- A bend that also reduces the diameter: Tapered Segmented Bend.
- Rectangular or flat-oval duct: Rectangular Duct Bend and Oval Segmented Bend.
What they all share
A segmented bend is a sequence of cylinders cut at an angle: two half gores at the ends, cut at one end only, and full gores in between, cut at both. The full gores share the bend angle equally, each half gore counts as half a gore, and the cut at each end is half the gore angle. The wave of each gore on the opened plate is the same formula as the mitre-cut shell.
What is asked for is the outside diameter, the bend radius on the axis, the angle, the number of gores and the thickness. The result comes gore by gore, with the ordinates for marking out, the PDF template at 1:1, the DXF and the 3D view of the assembled bend.
Frequently asked questions
- How many gores should a 90° bend have?
- It depends on the radius and on what flows through it. Three to five gores are usual in duct and stack work; more gores bring the bend closer to a bent pipe and reduce the pressure drop, at the cost of more welding. The 90° Segmented Bend calculates with the number you choose.
- How do I work out the cut angle of each gore?
- The full gores share the bend angle equally and each half gore counts as half a gore; the cut at each end is half the gore angle, and the wave on the opened plate comes from that angle and the diameter. Each bend page shows the angle and the ordinates of every gore.
- Segmented bend or bent pipe?
- A bent pipe needs a bending machine and pipe of the right diameter; a segmented bend is made from plate, in any diameter, with rolls and welding. At large diameters, in duct and stack work, the segmented bend is the rule.