How does this differ from the general segmented bend?

Not at all in the calculation: it is the same part with the angle pinned to 90 degrees. It exists separately to avoid mistyping the angle, which is the field that causes the most rework.

Can I use only two gores?

You can, and the result is the simple two-piece elbow. Above that the bend gets smoother and pressure loss drops.

Which number is the shop's 3 gores plus 2 half gores here?

Gores = 4. Here gores is the number of angular segments, each of 90°/gores; the plates are 2 half ends and gores − 1 full gores. With 4, each segment turns 22.5° and the half-mitre is 11.25°.

How high is the cut in the example?

For 800 diameter, 10 mm plate, radius 1200 and 3 gores: 215.7 mm at the throat and 427.4 mm at the heel of the end gore. The middle gore is double: 855 mm at the heel.

90° Segmented Bend development

What it is

The segmented bend with its angle fixed at 90 degrees — the mitred elbow of ductwork, the most common case in practice, with the angle field locked so it cannot be mistyped. Everything else works as the free segmented bend: you choose diameter, radius and gore count.

Where it is used

Direction changes in exhaust and ventilation ducting, large-diameter elbows fabricated from plate.

Measurements the calculation needs

H Bend radius (to the tube axis)
D Outside diameter of the tube
ANG Angle of the bend (fixed at 90°)
S Number of gores
DV Divisions
THK Plate thickness

Calculate with your own measurements

No account needed: enter the measurements in millimetres and get the figures and the drawing. The scaled PDF template, the DXF for the cutting table and the 3D model take a free account.

How it develops: the formulas

Half-mitre: β = 45° / gores 3 gores: 15°; 4 gores: 11.25°; 5 gores: 9°. Each middle gore turns 2β and the end gores turn β.
Cut height at each element: y(φ) = (R + r × cos φ) × tan β r = (D − t)/2 and R the axis radius. For D = 800, 10 mm plate, R = 1200 and 3 gores: throat 215.7 mm and heel 427.4 mm on the end gore; the middle gore is double, 855 mm.
Plate length: L = π × (D − t) 2481.9 mm in the example.
Shop gores: 3 gores plus 2 half gores = gores 4 Here gores is the number of angular segments, each turning 90°/gores. The plates are 2 half ends and (gores − 1) full gores.

Worked example

The values the form comes pre-filled with, run through the tool itself:

Bend radius (to the tube axis)	1200 mm
Outside diameter of the tube	800 mm
Angle of the bend (fixed at 90°)	90°
Number of gores	3
Divisions	16
Plate thickness	10 mm
The flat pattern fits a plate of	2482 × 855 mm
Flat patterns produced	2

Result of the example

Plate 1	2482 × 427 mm, cut 2 times
Plate 2	2482 × 855 mm, cut 2 times

90° Segmented Bend flat pattern: drawing of the template with the worked example dimensions — Cut lines in purple, welds in green and reference lines in orange. Plate 2482 × 855 mm.

Assembly notes

Weld the sheets with the longitudinal seams 180° apart, not lined up. Lined up, the piece assembles into a different shape — and the longitudinal seam crosses the circumferential one.

How to mark it out on the plate

Choose the gore count and work out β = 45°/gores; for the classic 3 gores plus 2 half gores, enter 4.
Mark the rectangle of L by (R + r) × tan β, divide it into dv elements and mark y(φ) on each; join with a flexible rule.
Mirror the wave for the middle gore (double height) and cut 2 ends and (gores − 1) middles.
Roll, assemble with the seams 180° apart and weld the circumferential joints.

Common mistakes and tolerances

Counting the half gores as gores: The bend with 3 gores and 2 half gores has 4 angular segments of 22.5°. Entering gores = 3 gives 3 segments of 30° and a coarser bend.
Radius under half the diameter: R < D/2 makes the gores cross at the throat; the calculation refuses it. For 90° duct bends, R of 1 to 1.5 × D is usual.
Seams in line: Turn each gore half a turn from its neighbour. In line, the longitudinal weld runs into the circumferential one and the bend assembles crooked.

Guides about this part

Lobster back bend (segmented 90° elbow): gore angles, cut heights and how many plates

Frequently asked questions

How does this differ from the general segmented bend?: Not at all in the calculation: it is the same part with the angle pinned to 90 degrees. It exists separately to avoid mistyping the angle, which is the field that causes the most rework.
Can I use only two gores?: You can, and the result is the simple two-piece elbow. Above that the bend gets smoother and pressure loss drops.
Which number is the shop's 3 gores plus 2 half gores here?: Gores = 4. Here gores is the number of angular segments, each of 90°/gores; the plates are 2 half ends and gores − 1 full gores. With 4, each segment turns 22.5° and the half-mitre is 11.25°.
How high is the cut in the example?: For 800 diameter, 10 mm plate, radius 1200 and 3 gores: 215.7 mm at the throat and 427.4 mm at the heel of the end gore. The middle gore is double: 855 mm at the heel.

90° Segmented Bend dimension drawing: H (Bend radius (to the tube axis)), D (Outside diameter of the tube), ANG (Angle of the bend (fixed at 90°)), S (Number of gores), DV (Divisions), THK (Plate thickness), marked on the part

Calculate a 90° Segmented Bend

You can work out the figures right here, with no account. PDF, DXF, 3D and history take an account; the free tier covers 20 calculations a month.