Straight duct: blanks, seams and cross-breaking

Straight duct is most of any run by length, and its blanks are the pattern every other fitting is measured against. Getting the straight blank right means adding the seam allowances at the right edges, the transverse joint allowances at the right ends, and stiffening the panels that need it. None of it is difficult, but each item is a number that differs from shop to shop and from one seam type to another, and each is easy to apply to the wrong edge.

This chapter makes the running case’s straight lengths: 600 × 400 mm inside, from two L-shaped pieces cut across 1,200 mm wide coil, with Pittsburgh-lock corner seams and slip-on transverse joints. It works out the blank, the effective length of each section, the cross-break on the wide panels and the mass, and ends with two L-pieces that would not close because both had their pockets at the same corner.

The net L-piece

Chapter 1 found that each L-shaped piece, 600 mm plus 400 mm inside with one fold, has a net length of 1,000.21 mm when folded to a 0.8 mm inside radius with a K-factor of 0.44. That is the length around the duct from one corner seam to the other, before any seam allowance.

The piece’s other dimension, along the duct, is set by the coil. The shop’s coil is 1,200 mm wide, so each L-piece is cut 1,200 mm along the duct, and the transverse joint allowances come out of that width rather than being added to it.

The Pittsburgh lock

A Pittsburgh lock is a corner seam formed on a roll former. One edge is rolled into a pocket, a folded channel with a slot along it; the other edge is given a single short flange. At assembly, the flange is pushed into the pocket and the pocket’s outer leg is hammered over to lock it. The seam is strong, airtight with sealant, and fast to make, which is why it is the usual longitudinal seam on rectangular duct.

Its allowances depend entirely on the roll former’s rolls. The machine’s maker states how much sheet the pocket consumes and how long the flange must be for that machine, and a trial piece confirms it. This is an illustration. The shop’s roll former needs 25 mm on the pocket edge and 8 mm on the flange edge. Each L-piece has one pocket edge and one flange edge, so its blank is 1,000.21 + 25 + 8 = 1,033.2 mm around the duct.

Figure 1Blank for an L-shaped piece of rectangular duct

blank = width + depth + n × (BA − 2R) + pocket allowance + flange allowance

width, depth

Inside dimensions of the two faces, 600 and 400 mm

n

Folds in the piece, 1 for an L

BA − 2R

Extra length per fold, 0.21 mm here

Pocket, flange

Roll former's allowances, 25 and 8 mm

1. Net = 600 + 400 + 0.21 = 1,000.21 mm
2. Blank = 1,000.21 + 25 + 8 = 1,033.2 mm around the duct, 1,200 mm along it
3. Both L-pieces are identical; each carries one pocket edge and one flange edge

Transverse joints and effective length

The ends of each section join the next section through a transverse joint. The running case uses a simple slip-on joint: on the two wide faces, an S-shaped cleat slips over the flat edges of both sections; on the two narrow faces, the sheet’s edges are folded back 12 mm and a drive cleat, a channel slid on from the side, locks them together. This is an illustration of one common arrangement; Chapter 9 compares others.

The drive edges need sheet: 12 mm folded back at each end of the 400 mm faces. The S-cleat faces need none, because the cleat slips over the flat edge. So the L-piece’s two faces have different end allowances, and the corner where they meet must be notched at each end, removing a 12 mm square from the 600 mm face’s corner so that the 400 mm face’s fold can be made without the 600 mm face getting in the way.

Because the allowances come out of the coil’s 1,200 mm width, each section’s effective length, from joint to joint, is 1,200 − 2 × 12 = 1,176 mm. That is the number used for counting sections. The running case’s main straight run is 9,400 mm long between fittings, which needs 9,400 ÷ 1,176 = 7.99, so 8 sections, giving 9,408 mm, the extra 8 mm taken up at a slip joint.

Figure 2What makes up an L-piece blank around the duct

1. 600 face600 mm · 58%
2. Fold correction0.21 mm · 0%
3. 400 face400 mm · 39%
4. Pittsburgh pocket edge25 mm · 2%
5. Pittsburgh flange edge8 mm · 1%

Cross-breaking the wide panels

A 600 mm wide flat panel, 1,176 mm long, in 0.8 mm sheet, is flexible enough to drum when the fan starts. The running case’s specification, an illustration, requires panels wider than 450 mm to be cross-broken, so the two 600 mm faces are cross-broken and the 400 mm faces are not.

Cross-breaking folds the panel very slightly along both diagonals, so that its centre stands proud of its edges and the panel becomes a very flat four-sided pyramid. A flat sheet can flip between bowing in and bowing out; a pyramid cannot, and its four triangular facets are each stiffer than the whole flat panel.

The numbers are small. This is an illustration. Raising the centre of a 600 × 1,176 mm panel by 6 mm makes the facets over the short edges slope at only 1.15° and those over the long edges at 0.58°, and the fold along each diagonal is about 1.3°. The sheet along each half-diagonal, 660.1 mm long, stretches by less than 0.03 mm: cross-breaking works by folding, not by stretching, which is why a brake can make it in two light passes.

The cross-break is made after the Pittsburgh edges are rolled and before the L is folded, on the flat 600 mm face only. Its direction, whether the centre rises outward or inward, is set by the specification for the duct’s pressure, and the shop marks it on the blank with an arrow.

Figure 3The cross-break on a 600 mm panel

6 mmRise at the panel's centreset on the brake

1.3°Fold along each diagonaltwo light passes

under 0.03 mmStretch along each half-diagonalthe panel is folded, not stretched

1,320.2 mmLength of each diagonalthe brake's fold line

Mass and handling

A pair of L-pieces, each 1,033.2 × 1,200 mm of 0.8 mm steel, weighs 2 × 1.0332 × 1.2 × 0.0008 × 7,850 = 15.6 kg, before the cleats. That is the mass of one 1,176 mm section, about 13.2 kg per metre of duct once the allowances are included, against 12.6 kg per metre for the bare 2,000 mm perimeter that Chapter 1 gave.

The 0.7 kg per metre difference is the price of seams and joints. On the 9,408 mm straight run, it is about 6 kg of steel, small in itself but a useful reminder that a take-off made from the duct’s perimeter alone underestimates the sheet by the seam and joint allowances, around 5% here.

Sealing the seams

A Pittsburgh lock is mechanically sound but not airtight on its own. Air leaks along the seam’s folds, and in a long run the leakage adds up to a noticeable share of the fan’s flow. The specification for the job sets the duct’s leakage class or sealing requirement, and the shop meets it with sealant in the pocket before the flange is inserted, or along the outside of the closed seam, or both.

The order matters again. Sealant run into the pocket before assembly is squeezed along the seam as the lock closes and fills the folds from inside. Sealant applied after closing covers only the outside line. The running case’s specification, an illustration, calls for sealant in the pocket for every longitudinal seam and at every transverse joint, so the shop’s sequence adds a step between rolling and assembly: a bead of duct sealant along each pocket, applied with a gun, immediately before the L-pieces are joined.

Sealant also affects the allowances slightly. A thick bead in the pocket can stop the flange from seating fully, and the duct comes out a millimetre or so larger across the seam diagonal. The trial piece that confirms the roll former’s allowances should be sealed exactly as production will be.

Checking a finished section

Three quick measurements on each assembled section catch most faults.

1. Inside dimensions at both ends: 600 × 400 mm, within the job’s tolerance.
2. End diagonals: across each end, corner to corner inside, both diagonals should be √(600² + 400²) = 721.1 mm. Unequal diagonals mean the section is out of square, usually because one fold was not at 90° or a lock was not fully closed. An out-of-square section will not slip squarely onto its neighbour.
3. Twist: with one end square on a flat bench, the other end’s corners should all touch. A twisted section, usually from a Pittsburgh lock closed unevenly along its length, rocks on two corners.

The diagonal check is the important one, because it shows what dimensions alone cannot. A section measuring 600 × 400 mm across both pairs of faces can still be a parallelogram with diagonals of 715 and 727 mm.

Marking and sequence

The shop’s sequence for a straight section is:

1. Cut the L-piece blank, 1,033.2 × 1,200 mm, from the coil.
2. Notch the 600 mm face’s corners at both ends, 12 × 12 mm, where the drive edges will fold.
3. Mark the pocket edge “P” and the flange edge “F”, and the cross-break direction on the 600 mm face.
4. Roll the Pittsburgh pocket on the P edge and the flange on the F edge.
5. Cross-break the 600 mm face.
6. Fold the 12 mm drive edges at both ends of the 400 mm face.
7. Fold the L along the corner line, 600.0 mm from the pocket’s root line on the inside.
8. Assemble two L-pieces by pushing each F flange into the other’s P pocket, and close the locks.

The order matters. Cross-breaking a panel that has already been folded into an L is awkward, and rolling a Pittsburgh pocket on an edge that has already been notched can tear the notch’s corner.

The error: two pockets at one corner

This is an illustration. A new hand cut and rolled a batch of L-pieces for the run, all correctly sized, but rolled the pocket on the 600 mm face’s edge on every piece, without the P and F marks. That made every L-piece identical with its pocket on the wide face.

Two identical L-pieces, turned to form a rectangle, meet at two corners. At one corner, the wide face of the first piece meets the narrow face of the second; at the other, the narrow face of the first meets the wide face of the second. With pockets only on wide faces, one corner had a pocket meeting a flange, which locked, and the other had a flange meeting a pocket the other way round, which also locked, as intended. But the hand had rolled the second batch with pockets on the narrow faces, to “balance” the work, and mixed the two batches. Pairs drawn from different batches put two pockets at one corner and two flanges at the other. Neither corner could close.

Put simply, two L-pieces close only if, at each corner, one brings a pocket and the other a flange. That is guaranteed when every piece has its pocket on the same face, wide or narrow; it fails as soon as pieces with pockets on different faces are paired. Nothing about the pieces’ size or shape showed the difference, because both batches were 1,033.2 × 1,200 mm and folded identically. Only the edge profile differed, and it is hard to see on a stack of folded L-pieces.

The fault was found at the first assembly, and the shop recovered most of the pieces by pairing them correctly by batch. The lesson was in the marking. An L-piece made from a symmetrical pattern is only interchangeable if its pocket and flange are always on the same face. The shop’s rule now is that the pocket always goes on the wide face, every blank is marked P and F before rolling, and pieces from different batches are not mixed without checking.

Figure 4Before rolling and folding straight duct

• Do: Pocket on the same face of every L-piecewide face, by shop rule
• Do: Blank marked P and F before rollingthe roll former is fed the right edge
• Do: Drive edges on the narrow faces, notched corners12 × 12 mm notches
• Do: Cross-break direction marked on the wide faceas the specification sets
• Do: Effective length used for counting sections1,176 mm
• Avoid: Batches with pockets on different faces mixedtwo pockets at one corner

Exercise: the branch duct

The 300 × 200 mm branch duct is made from two L-pieces in the same 0.8 mm sheet, with the same roll former and the same 1,200 mm coil and drive-cleat ends. Find each L-piece’s blank, the effective section length, and the number of sections for a 4,500 mm straight run. Do its panels need cross-breaking under the running case’s specification?

Worked answer.

• Net L-piece: 300 + 200 + 0.21 = 500.21 mm; blank = 500.21 + 25 + 8 = 533.2 mm around the duct, 1,200 mm along it.
• Effective length: 1,200 − 2 × 12 = 1,176 mm, the same as the main, because the joint allowances do not depend on the duct’s size.
• Sections: 4,500 ÷ 1,176 = 3.83, so 4 sections, giving 4,704 mm, which is 204 mm too long. The last section is cut short: its blank is 4,500 − 3 × 1,176 = 972 mm plus 24 mm of joint allowance, 996 mm along the duct.
• Cross-breaking: neither face is wider than 450 mm, so no cross-breaking is required on the branch.

The branch’s Pittsburgh allowances are the same 33 mm as the main’s, so they are a larger share of its blank: 6.2% against 3.2%. Small duct carries proportionally more seam.