Machinery's Handbook, 31st Edition
1446
Sheet Metal Joints
Hem:
The hem edge (Fig. 47 ) is a flat, finished edge. As with the flange, this hem must be designated by the drafter. For example, draw ing should show: 3 ∕ 4 inch (19 mm) hem out. The drive slip is one of the simplest transverse joints. It is appli cable where pressure is less than 2 inches (50.8 mm) w.g. This is a slide-type connection generally used on small ducts in combina tion with “S” slips, but should not be used above 2 inches w.g. This slip is also a slide type connection. It is made by elongating the flat drive slip and fastening standing portions 2 inches (50.8 mm) from each end. The design is applicable for any length in 2 inches w.g, 36 inches (914 mm) for 3 inches w.g., and 30 inches (762 mm) at 4 inches w.g. service.
Fig. 47. Hem
Flat Drive Slip:
Fig. 48. Drive Slip
Standing Drive Slip:
H
Fig. 49. Standing Drive Slip Flat Drive Slip Reinforced:
This reinforcement on the flat drive slip is made by adding a transverse angle section after a fixed interval.
Fig. 50. Drive Slip Reinforced Double “S” Slip Reinforced:
The double “S” slip is used to eliminate the problem of notching and bending, especially for large ducts. Use 24 gage sheet for 30 inches (762 mm) width or less, and 22 gage sheet over 30 inches-width. (22 gage = 0.76 mm, 24 gage= 0.60 mm) Normally the “S” slip is used for small ducts. However, it is also useful if the connection of a large duct is tight to a beam, column or other object, and an “S” slip is substituted for the shop standard slip. Service above 2 inches w.g. is not applicable. Gage shall not be less than 24, and shall not be less than the duct gage. When it is applied on all four edges, fasten within 2 inches of the corners and at 12-inch (305 mm) maximum intervals.
Fig. 51. Double “S” Slip
Flat “S” Slip:
Fig. 52. Plain “S” Slip
Hemmed “S” Slip:
This modified “S” slip is made by adding hem and an angle for reinforcing. The hem edge is a flat and finished edge. Hemmed “S” slip is mostly applied with angle. The drive is generally 16 gage (0.76 mm), forming a 1 inch height slip pocket and screws at the end. Notching and bending operations on “S” slip joints can be cumbersome and costly, especially for large sizes. Tie each section of the duct within 2 inches (50 mm) from the corner at maximum 6-inch (152.4 mm) intervals. Other Types of Duct Connections The clinch-bar slip and flange (Fig. 54) uses the principle of the standing seam, but with a duct lap in the direction of airflow. These slips are generally assembled as a framed unit with full corners either riveted or spot-welded, which adds to the duct cross section rigidity. Reinforcement may be accomplished by spot welding the flat-bar to the flange of the large end. Accessibility to all four sides of the duct is required because the flange of the slip must be folded over the flange on the large end after the ducts are connected.
Fig. 53. Hemmed “S” Slip
Clinch-bar Slip and Flange:
Fig. 54. Clinch-bar Slip and Flange
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