Machinery's Handbook, 31st Edition
1444 Joining and Edging weld design. Seam types must be selected based on material and pressure. A duct section between adjacent hangers must be able to carry its own weight and to resist the external loads for which it is constructed. Reinforcing members must be able to resist the external deflection of the sheet and their own deflection. There is a relationship between duct width, reinforcement spacing, reinforcement size, pressure, and sheet thickness. For constant pressure and constant duct size, the thicker sheet allows more distance between reinforcements. The higher the pressure the shorter the spacing between reinforcements. Joints and intermediate reinforcements are labor- intensive and may be more costly than the savings gained by a reduction in wall thickness. Thicker duct wall and stronger joints are more cost-effective than using more reinforce ment. The following material illustrates various joint designs, used both in duct work and other sheet metal assemblies. Sheet Metal Joints Plain Lap and Flush Lap:
The plain lap (Fig. 35) and flush lap (Fig. 36) are both used for various materials such as galvanized or black iron, copper, stainless steel, aluminum, or other metals, and may be soldered, and/or riveted, as well as spot, tack, or solid-welded. Lap dimensions vary with the particular application, and since it is the duty of the drafter to specify straight joints in lengths that use full-sheet sizes, transverse lap dimensions must be known.
Fig. 35. Plain Lap
Fig. 36. Flush Lap Raw and Flange Corner:
The raw and flange corner (Fig. 37) is generally spot-welded, but may be riveted or soldered. For heavy gages it is tack-welded or solid-welded.
Fig. 37. Raw and Flange Corner Flange and Flange Corner:
The flange and flange corner (Fig. 38 ) is a refinement of the raw and flange corner. It is particularly useful for heavy-gage duct sections that require flush outside corners and must be field-erected.
Fig. 38. Flange and Flange Corner
Standing Seam:
The standing seam (Fig. 39) is often used for large plenums, or casings. Before the drafter is able to lay out a casing drawing, the seam allowance measurements are needed, so that panel sizes can be detailed for economical use of standard sheets. Considering velocity levels, standing seams are considered for duct interiors: 1-inch (25.4 mm) seam is normally applied for duct widths up to 42-inch (1067 mm) and 1 1 ∕ 2 -inch (38 mm) for bigger ducts.
Fig. 39. Standing Seam
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