Rolled shell development
The rolled shell is the straight tube of plate, and the basis of nearly everything: stacks, ducts, silos and tank shells are stacked cylinders. Here are the plain shell, the mitre-cut and double-cut ones, and the shells with the nozzle or branch hole already marked on the plate.
Cylinder
The rolled shell is the straight tube: plate run through the rolls and closed with one longitudinal weld. Work out the plate free, on the neutral axis, no account needed.
Truncated Cylinder
A shell with one end cut on an inclined plane. The angle cut opens out as a sine wave, its height from the cut angle and its length from the circumference.
Double Cut Cylinder
A shell cut at an angle on both ends, each with its own angle — the intermediate gore that clears two offsets with a single length of tube.
Cylinder with Branch Hole
The shell course with the branch hole already in it — the missing half of every branch job. The only sheet here with an internal cut-out.
4-Way Cross Shell
The run pipe of a cross, with both holes. What it gives, and the single-hole shell cannot, is the relative position of the two holes on the flat sheet.
Cylinder with Rectangular Hole
The hole IS a rectangle, which is what fools everyone — but the side running across the shell does not measure y. This is the half people get wrong.
Cylinder with Conical Branch Hole
The round-hole shell will not do: a conical nozzle reaches the shell already flared, and opens a larger hole of a different shape.
Which shell to use
- Straight tube, stack, duct, tank shell: Cylinder. The plate is a rectangle of π × (D − t) by H.
- A shell meeting another out of square, or the gore of a bend: Truncated Cylinder. The cut becomes a wave on the opened plate.
- The middle gore of a bend, cut at an angle at both ends: Double Cut Cylinder.
- A shell that receives a tee or saddle branch: Cylinder with Branch Hole. The hole comes marked on the plate; the branch itself is in Pipe Branch (Tee).
- A run pipe with two opposite branches, the cross: 4-Way Cross Shell.
- A grille mouth or damper on a pipe: Cylinder with Rectangular Hole; the collar is in Rectangular Tap on Pipe.
- A shell that receives a conical nozzle, the hopper reinforcement: Cylinder with Conical Branch Hole.
What they all share
Every shell opens into a rectangle whose length is the perimeter on the neutral axis: π × (D − t), with D the outside diameter and t the thickness. That offset is what makes the seam close; without it there is π × t of plate too much, nearly 10 mm on 3 mm plate.
The dimensions asked for are shop dimensions: outside diameter, height and thickness, plus the angle or the hole where there is one. The result comes as a table of ordinates for marking out, a PDF template at 1:1 and a DXF with cut and marking lines on separate layers, plus a 3D view to check before cutting.
Frequently asked questions
- How do I work out the plate for a rolled shell?
- Length = π × (D − t) and height = H, with D the outside diameter and t the thickness. For D = 1000 mm in 3 mm plate: π × 997 = 3132.2 mm. The Cylinder page already takes the thickness off and shows the result.
- Why take the thickness off the diameter?
- Because the plate neither stretches nor shrinks on its mean surface, the neutral axis, at mid-thickness. Rolling to the outside diameter leaves plate over; to the inside, plate short. That is why the calculation uses D − t.
- Is the mitre-cut shell the same part as the gore of a bend?
- Yes. Every gore of a segmented bend is a cylinder cut at an angle at one or both ends, and the wave on the opened plate is the same. The 90° Segmented Bend delivers the gores ready made.