Lateral Deflection: Parallel Offset & Transverse Movement Explained
Lateral refers to the direction perpendicular to the centerline of the pipe expansion joint (in any plane). Other terms for this deflection are parallel offset and transverse.
This movement occurs with both ends of the pipe expansion joint remaining parallel to each other, with their centerlines being displaced or no longer coincident.
When analyzing the deflections in a piping system, it is not uncommon to find that different lateral deflections can occur in more than one plane.
Because an expansion joint can deflect in any radial direction, multiple lateral displacements must be combined into a single resultant value when selecting a bellows against its rated lateral deflection. See more information in our catalog.
Calculating Resultant Lateral Deflection
The magnitude of the resultant lateral deflection (D) is calculated using the formula:
D = √(x² + y²)
where x and y are the component lateral deflections in orthogonal planes (e.g., horizontal and vertical).
When structural components such as hinges are included, the direction of lateral movement must be carefully considered, since these components restrict motion to specific planes. It is also important to determine whether lateral deflections may occur independently during operation.
Excessive lateral deflection beyond rated limits can induce non-uniform stress in the bellows convolutions, leading to premature fatigue failure. Allowable lateral deflection ratings are established per EJMA standards and are closely tied to convolution geometry and expected thermal cycles. Expansion joints are typically specified with a lateral deflection range based on cycle life expectations and convolution geometry.
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