Bellows can reduce the reaction force at the supports, arising from the elongation of the piping systems. So, Bellows are used for the absorption of thermal expansion of piping in liquid and gas transportation systems. But, under high internalpressure, bellows elongate and make a large reaction force at the sipports.We quantitatively examined these reaction forces at the supports caused by thermal expansion and internal pressure. Its results made clear that the reaction force due to internal pressure was much larger than the one due to thermal expansion. Therefore,we developed a few kinds of pressure balanced expansion joints. The present paper reports on the structure and mechanism of these devices.
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