Seams, bottom and rim

Four part cones, cut and formed, are still four pieces of sheet. The trough becomes a vessel at its joints: four vertical seams where the walls meet, a bottom joint all round the oval, and a rolled rim that stiffens the top and finishes its edge. Each joint must be watertight, carry its share of the load, and be made without forcing the oval out of shape. And because the outline’s curvature changes so much between ends and sides, the same joint behaves differently at the ends than along the sides.

This chapter makes the running case’s four seams, flanges the bottom plate up into the wall and works out how much its flange must shrink at the ends and the sides, rolls the rim, sets out the sealing and the leak test, and ends with a bottom flange that was turned up without notches at the ends.

The vertical seams

Chapter 5 placed the seams on the four shared generators, the straight lines from the rim’s tangent points to the bottom’s, each 456.2 mm long. Along each, an end piece meets a side piece, and the two cones share a tangent plane, so the joint is smooth and the two edges lie flat against each other.

This is an illustration. The shop laps each seam by 20 mm, with the lap carried on the side piece, and rivets it with 4.8 mm sealed blind rivets at about 50 mm pitch, ten rivets per seam, forty for the trough. Sealant is applied between the laps before riveting. Rivets with sealed bodies are used so that water cannot weep through the rivet’s own hole, and the specification for the trough’s use decides whether the rivets and sealant must be suitable for drinking water.

The seams could be welded instead, and on stainless steel troughs they usually are. On galvanised sheet, welding burns off the zinc beside the weld, which must then be restored with a zinc-rich coating, and welding galvanised steel produces zinc oxide fume, a hazard that Australian work health and safety guidance requires to be controlled with ventilation and protective equipment. The running case’s riveted laps avoid both.

The bottom plate

The bottom is a flat oval plate. Its mean-line outline at the bottom of the wall is the bottom oval, 1,650 × 750 mm with an end radius of 225 mm and a side radius of 1,500 mm. The plate is given a 25 mm flange all round, turned up to lie against the inside of the wall, where it is riveted through the wall and sealed. Turning the flange up inside the wall keeps the joint’s raw edge inside and lets water pressure press the wall onto the flange.

Before forming, the plate’s outline is the bottom oval offset outward by the flange’s 25 mm: struck from the same four centres, with an end radius of 250 mm and a side radius of 1,525 mm, and a girth, by the offset rule, of 3,912.5 + 2π × 25 = 4,069.6 mm. It is 1,700 × 800 mm overall.

Flanges that shrink

Turning up a flange on a convex edge makes it shrink. Before forming, the flange’s outer edge lies on the plate’s outline at the end radius plus 25 mm. After forming, it stands up against the wall, which leans outward at 9.46°, so its top edge lies only 25 × sin 9.46° = 4.1 mm outside the bottom outline. The edge must shrink from its original radius to its new one.

• At the ends, the edge shrinks from 225 + 25 = 250 mm radius to 225 + 4.1 = 229.1 mm: a compression of 8.4%.
• Along the sides, it shrinks from 1,525 mm to 1,504.1 mm: only 1.4%.

A 1.4% shrink is taken up by a flange in 1.6 mm sheet with a little care. An 8.4% shrink is not: the sheet has nowhere to go but into wrinkles, which buckle the flange away from the wall and leave gaps that no sealant fills reliably. The end flanges must be notched.

Figure 1Shrink of a flange turned up on a curved edge

shrink = [(r + w) − (r + w × sin α)] ÷ (r + w)

r

Radius of the bottom outline, 225 mm at the ends, 1,500 mm at the sides

w

Flange width, 25 mm

α

Wall lean, 9.46°, so sin α = 0.1644

1. Ends: (250 − 229.1) ÷ 250 = 8.4% shrink
2. Sides: (1,525 − 1,504.1) ÷ 1,525 = 1.4% shrink
3. Each end flange is 540.4 mm along its free edge, so about 45 mm must be removed or gathered

Notching the end flanges

Each end flange’s free edge is 250 × 2.1617 = 540.4 mm long before forming, and 8.4% of it, about 45 mm, must disappear when the flange is turned up. The shop removes it with V-notches cut into the flange before forming. This is an illustration: nine notches, each 5 mm wide at the flange’s edge tapering to nothing at the bend line, spaced evenly round each end arc, remove 45 mm and let the flange’s tabs close up against each other as the flange is turned.

The notches are sealed along with the joint, and the closed-up tabs are riveted to the wall like the rest of the flange. Along the sides, no notches are needed; the 1.4% shrink is worked out with a few blows of a hammer on a dolly as the flange is turned.

Figure 2The bottom flange at the ends and the sides

Edge	Radius before and after forming	Shrink	Treatment
End arcs	250 to 229.1 mm	8.4%	nine 5 mm V-notches per end, sealed
Side arcs	1,525 to 1,504.1 mm	1.4%	worked by hammer and dolly

Riveting and sealing the bottom

This is an illustration. The flange is riveted through the wall at about 75 mm pitch round the oval, about 52 rivets, with sealant between flange and wall. The rivets go near the top of the flange, about 15 mm above the bottom plate, so that the sealant bead below them is under compression from the water inside.

The bottom plate itself spans 1,650 × 750 mm unsupported except at its edge. Under 400 mm of water, the pressure on it is about 3.9 kPa, enough to make a 1.6 mm plate sag noticeably if the trough stands on a frame that supports only its edge. Troughs that stand on the ground are supported over their whole bottom; troughs on stands need the stand designed to support the bottom, not just the rim. That is a design matter for the trough’s specifier.

The rim

The rim is rolled outward over a 10 mm steel rod. Chapter 3 found the rod’s length, 4,420.2 mm, and Chapter 6 added a 27 mm band to every wall pattern for the wrap. The rod is bent to the trough’s two radii, laid on the walls’ top edges and the sheet rolled over it on a rim-rolling or wiring machine, or by hand with a hammer and a grooved dolly on work this size.

At each vertical seam, the rim band crosses the lap, and the sheet there is two layers thick. The shop notches the lap’s top corner by the rim band’s 27 mm before rolling, so that only one layer wraps the rod at the seam, and seals the notch. Without it, the rim at the seam would be wrapped by four layers in effect, and it would stand proud and be hard to close.

The rod’s joint goes at the middle of a side, where the rim is straightest, and its ends are butted and sleeved before the sheet is rolled over them.

The order of assembly

An oval trough goes together in an order that lets each joint be made while the shape can still be adjusted.

1. Form the four wall pieces and check each against its template’s curve at the rim and the bottom.
2. Clamp the walls together on a level floor, upside down on their rims, with the seams held by temporary fasteners through the rivet holes. Upside down, the rim edges rest on the floor and set the walls’ height together.
3. Check the rim’s shape with the end and side templates, and the oval’s length and width across the rim, adjusting the seams before any rivet is set.
4. Rivet and seal the four seams, working from rim to bottom on each.
5. Fit the bottom plate, already flanged and notched, into the inverted walls, check its fit all round, then rivet and seal it.
6. Turn the trough over and roll the rim over its rod, starting at the rod’s joint on a side and working round both ways to the far side.
7. Check the rim’s level at the seams and the middle of each side, and the oval’s dimensions, before the leak test.

Riveting the seams before the bottom goes in lets the walls settle into their true shape first; fitting the bottom to riveted walls then shows at once whether the bottom’s outline and the walls’ agree.

Cut edges in galvanised sheet

Every cut edge, rivet hole and notch exposes bare steel through the galvanised coating. On thin sheet, the zinc on either side protects a narrow cut edge sacrificially for a time, but the wetted inside of a water trough is a demanding place for an exposed edge, and the edges at the bottom joint and the notches are permanently wet. The shop seals every cut edge inside the trough with the same sealant used in the joints, and touches up any area where the coating was damaged in forming with a zinc-rich paint. The coating requirements for a particular use, including any for drinking water, are set by its specification.

Rivets and sealant per trough

The joints’ quantities go on the job’s bill of materials, counted rather than estimated. This is an illustration. Each trough takes 40 rivets in its four vertical seams and about 52 in its bottom joint, 92 in all, ordered with a small allowance for losses. The sealant runs along four seams of 456.2 mm, 1.82 m, round the bottom joint’s 3.91 m, and along the notches and cut edges, about 6.5 m of bead per trough. At the shop’s usual bead size, that is a known number of cartridges per trough, which the store checks before the batch starts.

Counting also catches a design that has more joints than it needs. A trough made in two halves, with seams only at the middle of each end, would need two seams instead of four, and those seams would fall on the tight end arcs, where they are hardest to make; the four-piece design puts its seams on the shared generators, where they are easiest. The count and the geometry point the same way.

Leak testing

Every trough is filled with water to the rim, left for a set time, an illustration being 24 hours, and inspected for weeps at the seams, the bottom joint and the rivets. A weep is marked, the trough drained and dried, and the joint resealed from inside before the test is repeated. The test also confirms the capacity, as Chapter 4 described, and shows whether the bottom sags on its support.

Figure 3Joint checks before the leak test

• Do: Vertical seams on the shared generators, lapped and sealed4 seams, 10 rivets each
• Do: Bottom flange notched at the ends before formingnine V-notches per end
• Do: Flange riveted near its top with sealant belowabout 52 rivets at 75 mm
• Do: Lap corners notched under the rim bandsingle layer round the rod at seams
• Do: Rod joint sleeved at the middle of a sidenot on an end arc
• Avoid: End flanges turned up without notcheswrinkled, gapped, leaking

The error: an end flange without notches

This is an illustration. On the first trough of a second batch, a new hand turned up the bottom plate’s flange all round without notching the ends, having seen the side flanges turn up easily. The end flanges wrinkled into four or five folds each, and the flange stood away from the wall by up to 3 mm between the folds.

The hand sealed and riveted it as well as they could, and the trough passed a quick visual check. The 24-hour leak test found it: both ends wept at three or four places along the bottom joint. The bottom plate was drilled out and replaced with a notched one, and the shop added a line to its work instruction for the bottom: “NOTCH END FLANGES: 9 × 5 mm V per end, before forming.”

The deeper lesson is the one the shrink calculation teaches. The same flange that forms easily along the gentle sides cannot form unaided round the tight ends, and on an oval both happen on one plate. A work instruction that treats the flange as one feature hides the difference; one that gives the ends and sides their own numbers shows it.

Exercise: a narrower flange

The shop considers a 20 mm bottom flange instead of 25 mm. Find the shrink at the ends and the sides, and how much flange length must be removed at each end. Would the end flanges still need notches?

Worked answer.

• Ends: the free edge moves from 225 + 20 = 245 mm radius to 225 + 20 × 0.1644 = 228.3 mm, a shrink of 6.8%.
• Sides: from 1,520 to 1,503.3 mm, a shrink of 1.1%.
• Each end flange’s free edge is 245 × 2.1617 = 529.6 mm long, and 6.8% of it, about 36 mm, must be removed.

At 6.8%, the end flanges would still wrinkle if turned up unaided, so they still need notches, perhaps seven of about 5 mm. The narrower flange reduces the problem but does not remove it, because it is the tight end radius, not the flange’s width, that makes the shrink large.