Bellows Expansion Joint Torsional Deflection / Combined Deflections/ Cyclic Deflections

Bellows Expansion Joint Torsional Deflection / Combined Deflections/ Cyclic Deflections

Torsional Deflection

        Torsion refers to twisting one end of the bellows with respect to the other end, about the bellows centerline. Expansion joints are not normally expected to accept torsional deflection,since the bellow is essentially inflexible in this direction. The piping designer should carefully provide the system with restraints so that the expansion joint is not expected to accept TORSIONAL DEFLECTION . This catalog contains the calculated torsional spring rate for each expansion joint so that piping designers and analysts can provide the proper inputs for piping flexibility computer programs. If it is not possible or economically practical to relieve the expansion joint of excessive torsional loading, then U.S. Bellows’ designers can provide anti-torsion features to the expansion joints which prevent these loads from damaging the bellows elements.

Combined Deflections

Very often an expansion joint must accept a combination of some or all of the above defections,most often because of space limitations in the system. In order to clearly understand whether the proper expansion joint is selected, the movements should, if possible, be broken down into the above various types and stated that way in the specification. Then U.S. Bellows’ expansion joint designer will confirm that the offered expansion joint can accept each of these types of movements,when combined.Most catalogs, including this one, give the movement capability of each expansion joint in an uncombined form,or “non-concurrent.” 

Cyclic Deflections And Cycle Life

Most deflections are repeated a number of times during the life of the piping system,since the defections usually are produced by changes in temperature which occur each time the system is started and stopped, and from predictable variations in the way the system is used. Repetitions can also occur as a result of repetitive mechanical movements and from vibrations. Each time a deflection occurs it is a CYCLY. The number of cycles is important to assure the proper design of the expansion joint,since each design has a finite, but predictable life. Vibrations which cause repetitive deflections can cause a premature failure of an expansion joint. Even though these deflections may be small in magnitude, they usually accumulate huge numbers of cycles in a short period of time.Since the bellows are metallic structures, they have specific and predictable resonant frequencies, like the pitch of a tuning fork. When driven by outside vibrations of the same frequencies(or harmonics of them), they can magnify the incoming deflections until they exceed the yield strength of the bellows material and induce early fatigue failure. When a piping system is known to have equipment which can produce vibrations, such as pumps, fans, and other motor or turbine driven devices, their rotational speeds or frequencies should be stated so that designers can assure that the proper expansion joint does not have a resonant or harmonic frequency that is close to those.