Bellows Deflections — The Expansion Joints Blog
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Bellows Deflections

January 18th, 2011

In order to properly specify expansion joints for piping systems, the piping specialist and must be aware of the various types of deflections that can be taken by an expansion joint bellows. Piping flexibility programs can determine where stresses are excessive, and if an expansion joint is necessary or not. Expansion joint placement and proper selection still depends on understanding the different types of bellows deflections.

Axial Deflection (View Axial Movements)

Axial deflection refers to movement of the bellows along the longitudinal axis of the bellows. Compression is the axial deflection which will shorten the bellows length, while extension is the axial deflection which extends the expansion joint. Often confusion occurs because thermal expansion in the piping will cause the expansion joint to compress. The specification for an expansion joint should always state the movements as they affect the expansion joint, and not as they are produced by the system. Piping which is operating at temperatures lower than ambient, such as in cryogenic systems, will contract, causing the expansion joint to extend.

Axial Deflection

Lateral Deflection

Lateral DeflectionLateral deflection refers to the direction perpendicular to the centerline of the expansion joint (in any plane). The movement occurs when both of the ends of the expansion joint remain parallel to each other, while the centerlines are being displaced, or no longer coincide. 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.  Since an expansion joint is round, these various deflections must be resolved into a single resultant lateral deflection in order for the bellows to be properly selected in terms of the rated lateral deflection. The planes of the various deflections must also be clearly understood if the expansion joint is to contain structural components such as hinges, which may inhibit movements in certain directions, and if the individual deflections can occur separately during the life of the expansion joint.

View Lateral, Angular and Combined Movements

Angular Deflection

Angular DeflectionAngular deflection happens when an expansion joint experiences bending about its center, which is on the centerline and halfway between the ends of the bellows. It can occur in any plane that passes through the centerline. As in lateral movements, piping analyses may reveal angular deflections occurring in more than one plane. Angular deflections in two planes produce a single angular deflection ia a single resultant plane. As in lateral deflection, this plane must be understood in structural components, such as hinges.

Torsional Deflection

Torsional DeflectionThis is when the bellows is twisted on one end in the opposite direction of the other end. Expansion joints are not normally expected to absorb torsional deflection, since the bellows is essentially inflexible in this direction. If torsional loading is present, U.S. Bellows can provide anti-torsion features, which prevents these loads from damaging the bellows elements.

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