How many gores does a 180 degree return need?

Since the angle is twice that of a 90 degree bend, the gore count usually doubles to keep the same finish. Five to seven is the usual range.

Where is the radius measured from?

From the centre of the bend to the tube axis, as in the other segmented bends.

180° Segmented Bend development

What it is

The 180 degree segmented bend — the full return. Same construction as the segmented bend with the angle locked: the tube leaves in the direction opposite to the one it entered, around the radius you give.

Where it is used

Returns in coils, plate-built heat exchangers, ducting that must double back within a short run.

Measurements the calculation needs

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

How it develops: the formulas

Half-mitre: β = 90° / gores 5 gores: 18°; 6 gores: 15°; 8 gores: 11.25°. The return needs twice the gores of a 90° bend for the same finish.
Cut height at each element: y(φ) = (R + r × cos φ) × tan β For D = 800, 10 mm plate, R = 1200 and 5 gores: throat 261.6 mm and heel 518.2 mm on the end gore; the middle gore is double.
Plate length: L = π × (D − t) 2481.9 mm in the example.

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	180°
Number of gores	5
Divisions	16
Plate thickness	10 mm
The flat pattern fits a plate of	2482 × 1036 mm
Flat patterns produced	2

Result of the example

Plate 1	2482 × 518 mm, cut 2 times
Plate 2	2482 × 1036 mm, cut 4 times

180° 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 × 1036 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.

Common mistakes and tolerances

Too few gores: With 3 gores the return has 30° half-mitres and each middle gore turns 60°: the bend comes out faceted and pressure loss rises. Five to seven is the usual range.
Radius under half the diameter: R < D/2 makes the gores cross at the throat; the calculation refuses it.

Guides about this part

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

Frequently asked questions

How many gores does a 180 degree return need?: Since the angle is twice that of a 90 degree bend, the gore count usually doubles to keep the same finish. Five to seven is the usual range.
Where is the radius measured from?: From the centre of the bend to the tube axis, as in the other segmented bends.

180° Segmented Bend dimension drawing: H (Bend radius (to the tube axis)), D (Outside diameter of the tube), ANG (Angle of the bend), S (Number of gores), DV (Divisions), THK (Plate thickness), marked on the part

Calculate a 180° Segmented Bend

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