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51. The Curved Elbow in Rectangular Pipe.—This type of fitting is extensively used in ducts for heating and ventilating systems because it offers a minimum of friction to the moving air. The elbow discussed here has an angle of 90°.

The Profile.—A plan or profile, Fig. 148, is drawn according to the dimensions given. The corners are lettered, and extension lines are carried upwards from points A and D to locate the elevation properly.

The Elevation.—After a base line is drawn, points 1 and 12 are located, and a distance of 3¾ in. (to scale) set off to the left of point 1 to serve as the center point of the elbow. The limits of the elbow are defined by erecting a perpendicular at this point. The arcs of the throat and the back are drawn. These arcs are divided into equal spaces and are numbered as shown.

The Pattern.—The pattern for the body, Fig. 150, is a copy of the elevation. To this is added a ¼-inch single edge on the throat and back. A 1½-inch edge for "shipping" the pipe is added as shown. Figure 151 shows the pattern of the throat piece which is a rectangle whose width is equal to line AB of plan, and whose length is equal to the stretchout of the spacing of the arc of the throat. Fig. 149. To this rectangle must be added 1½ in. for "shipping" and to each long side a -inch edge for a hammer lock. Figure 152 differs from Fig. 151 only with regard to its length, which is taken from the stretchout of the arc of the back.

The Hammer Lock.—The hammer lock is so called because it can be made up on the job, the only tool required being a hammer. Straight strips of metal are formed in the brake, to act as the sides of the fitting as shown in Fig. 153. The ¼-inch single edges of the body are worked up to a right angle and slipped into the slot of the hammer lock. The protruding edge is then closed down with a hammer as shown in Fig. 154. This gives the job an appearance of being double seamed, and requires much less time and effort than the double seamed job.

52. Related Mathematics on Curved Elbows.—Problem 27A.—How much would three curved elbows, Fig. 149, weigh, made from No. 28 galvanized iron (.7812 lb. per square foot), allowing 20 per cent for waste?