Enter the distance from the outer cylinder to the beginning of the knuckle for an expansion joint with an inside knuckle. Enter the distance from the outer cylinder to the intersection of the expansion joint web and the outer diameter for joints with a square outside corner. This value is shown as fb in the following illustration (see Expansion Joint Geometry).
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In both cases this distance is frequently zero.
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An expansion joint with an outside radius but no outside cylinder, this distance is the distance from the end of the knuckle to the symmetrical centerline of the joint.