Bellow And Expansion Joints
The Bellows is the expansion joint's flexible component. It must be strong enough circumferentially to sustain the pressure and flexible enough longitudinally to accept the deflections for which it was built, and it must be able to do so as many times as necessary with minimal resistance. This combination of strength and flexibility is a novel design problem that is not commonly found in other industrial equipment components.
Most engineered structures are built to resist deflection when subjected to external forces. Because the Bellows must accept repeated deflections, and deflections cause stresses, these stresses must be kept as low as possible so that the repeated deflections do not cause premature fatigue failures. Reducing the bending stress caused by a given deflection is as simple as decreasing the thickness of the bending member, which in the case of the Bellows is the convolution. However, in order to bear the pressure, the convolution, which is also a pressure vessel, must have a thickness that is more than or equal to the allowed stress levels of the materials at the design temperatures. The unique design difficulty confronted by the expansion joint designer is the conflicting demand for thickness for pressure and thinness for flexibility.
Bellows are not springs since the majority of their deflections produce bending stresses that exceed the yield strength of the material. Understanding how various materials operate and their capabilities in this "plastic" deformation region necessitates years of expertise and the development of design equations
The fact that Bellows commonly work "plastically" should not be cause for concern because most of the materials used to make Bellows have comparable extremely ductile properties. These materials' endurance limit, which can be loosely defined as the stress at which failure occurs after ten million cycles of repetitive stressing, is nearly the same as their yield stress, or the point at which irreversible deformation occurs. A Bellows which is required to survive 3000 cycles of a specific deflection and pressure, and which ultimately fails after 10,000 cycles, has undoubtedly exhibited more than sufficient performance. However, it has encountered, during each and every cycle, bending stresses well in excess of the endurance limit and thus the yield stress, and once deflected, would not have returned on their own to their original un-deflected length or form, as a spring is meant to do. In other words, they'd "taken a set."
The majority of Bellows fail due to circumferential cracking caused by cyclic bending forces or fatigue. Because the ideal design is a compromise, or balance, of pressure strength and flexibility concerns, it is reasonable to conclude that their designs had lower margins of safety for fatigue than they did for pressure strength. The engineers who designed these Bellows have years of experience, which ensures that the designs in this catalogue, as well as those available to meet customer demands, will have the performance reliability that results in trouble-free, safe operation.
