Figure [ 2 ] - Elbow Joints
The 90° bend above consists of two half patterns and two full patterns. When constructing a segmental bend we start and finish with a half segment. See [ Figure 2 ]. The above bend has three full segments. We arrive at this by adding the two halves to the two full bends. To find the angle for each segment divide the angle of the bend 90° by three. This gives us 30°. So each full segment has a 30° angle and a ½ segment has a 15° angle. For construction we make an angle of 15° first or equating at C and draw the line from left to right. Next segment will be 15° + 30°. We then get the second segment by constructing an angle of 45°. The segment after that will be 45° + 30° = 75° leaving us with a 15° segment finishing the bend.
When constructing a segment bend it is important that whatever is flowing through it is allowed to travel smoothly. The radii/sweep of the bend is most important to ensure the success of this. The engineer will have designed this into the drawing. This is normally referred to as part of the specification. A segment is sometimes referred to as a gore and a segmental bend is referred to as a lobster back bend. A segmental bend may be composed of right cylinders. If this is the case the first and last segments are ½ segments. Segmental bends are also constructed from oblique cylinders. In this case all segments are full segments and the development of the pattern is different. Below are views of the two types of segmental bends.