To reduce the number of rings required by every bolt and simultaneously to reduce an excessive load for the bolt which is derived from difference in the amount of expansion between a flange and the bolt in the process of temperature up in a chemical plant by sharing the ring concerning regulation of a tightening force of the bolt by tightening a force detection seat.

When tightening a tightened object 5 with the bolt 6 and a nut 7, elastomers 2 on a sheet, such as disc springs are superimposed with a pluraltiy of sheets at a predetermined height and then the perimeter is supported from outside in tube-shaped fashion by the ring 4 and its upper and lower sides are sandwiched by washers 1, 3. The ring 4 a part of which is notched is formed of a flat spring. When attaching or detaching it, the part of the ring is opened by fingertip and is inserted around the outer periphery of the elastomer 2. As the nut 7 is tightened, gap G between the elastomer and the washer 1 amounts to zero. This serves as a regular tightening force. After that, the ring 4 is opened and then can be removed and can be commonly used. The elastomer 2 is enabled to absorb a difference in expansion amount between the flange and the bolt in process of temperature up by removing the ring.

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