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29. The Long Radius Elbows.—The principles of pattern cutting that apply to long radius elbows such as used in conveyor systems are the same as in the preceding problems. There are, however, certain rules that apply to "Blow Pipe Elbows" that should be thoroughly understood. Figure 86 shows a partial elevation of a five-piece elbow. Since the pattern for all pieces can be laid out from the pattern of the first piece, it follows that all necessary information can be obtained from an elevation of the first two pieces of an elbow. A draftsman rarely draws more than two pieces of the elevation and divides the arc of the throat instead of the arc of the back. He uses the arc of the throat because it requires less room than the arc of the back and produces the same result.

Center Line Radius.—It has been determined by careful experiment that an elbow having a center line radius equal to twice the diameter of the pipe to which the elbow is to be joined, offers the least resistance to the flow of air, or other material, through the pipe. According to this rule an elbow for 12-inch pipe would have a throat radius of 18 in. and a center line radius of 24 in. A blow pipe elbow should never be "peened." All laps and edges should be closely riveted and soldered air-tight, the inside to be made as smooth as possible. All laps should be made in the direction of flow of air or other material through the pipe.

Laps for Riveting.—In the case of the "peened" elbow no allowance is made for joining the pieces. This alters the throat radius somewhat but this fact is generally neglected. In blow pipe systems the work must be exactly to measurements. Laps for riveting are, therefore, added as shown in Fig. 89. It should be noted that the rivet holes for the longitudinal seams are on the circumference lines of the pattern, while those for the transverse seams are in the center of the lap. The rivet holes are equally spaced and as the lap is ¾ in. wide, the centers of the holes are ⅜ in. in from the edge of the lap, and ⅜ in. in from the miter cut of the adjoining piece of the elbow. Laps are added to one miter cut only (of each piece) and start with the lap on the big end.

Thickness of Metal Used.—Another rule always to be observed is to make the elbow at least two gages heavier than the pipe to