Pressed and lockseamed bend take-offs
The radius rule describes the bend geometry. The published l dimension describes the finished fitting used in the offset. Those values are related, but they are not safe to treat as interchangeable.
What bend take-off means
In a two-bend offset, the theoretical travel runs between the intersections of the straight duct centrelines. Each bend occupies some of that travel. The fitting drawing normally labels the distance from the centreline intersection to the joint plane as l.
With two equal bends, that becomes travel minus twice the one-bend take-off. This is why a difference of only 4 mm on one fitting changes the straight cut by 8 mm.
Centreline radius is not the finished dimension
Many standard circular bends are described using a centreline radius close to one duct diameter: rm ≈ 1 × D. Pure geometry would give the following tangent distance:
For a 315 mm, 45° bend using rm = 315 mm, the unrounded calculation is about 130.5 mm. A published fitting table may state 130 mm. The table value is the dimension to use for that identified fitting because it describes what the manufacturer supplies.
Pressed versus segmented or lockseamed
| Fitting family | Typical construction | What to verify |
|---|---|---|
| Pressed bend | Formed shells, normally seam-welded or joined as a compact fitting | The exact published take-off for that diameter and angle; small sizes can depart from a simple formula. |
| Segmented / lockseamed bend | Multiple formed segments joined into the bend | Published angle, diameter, centreline radius and whole-millimetre take-off. |
| Site-fabricated or different brand | Construction selected by another manufacturer or fabricator | Its own drawing or a controlled measurement—never a value borrowed only because the diameter matches. |
Why manufacturer identification matters
Published data from different manufacturers often agrees on common sizes, but it is not guaranteed to match. One catalogue can show a different small-bend take-off from another while both products are sold as the same nominal diameter and angle.
VentTools therefore uses a recorded manufacturer table only when the selected diameter and angle are present in that source. Any value generated from radius geometry is labelled as an estimate. A measured take-off remains the safest fallback when the delivered fitting is different or cannot be identified.
How to check a bend on site
- Confirm the marked diameter and angle.
- Identify the manufacturer or supplier where possible.
- Check the current product drawing for the l dimension.
- If it cannot be identified, establish the two straight joint axes and their theoretical intersection.
- Measure from that intersection along the centreline axis to the relevant joint plane.
- Use the same reference on both bends; do not measure around the curved outside skin.
For an important cut, a quick dry assembly or mark-up is cheaper than remaking a large centre piece.
Published sources used for comparison
- Lindab BU 45° pressed-bend data sheet
- Lindab BFU segmented / lockseamed bend data
- Ductmann spiral tube and fitting catalogue
Manufacturers may revise dimensions without notice. Follow the current document for the exact supplied product.
Use the offset calculator