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At the 3 GeV-RCS (Rapid Synchrotron) in J-PARC (Japan Accelerator Research Complex) project, pure Ti is adopted as a material for bellows because of its small residual radioactivity. The space available for the bellows is so narrow that the requirements are hard. The typical nominal length and inner diameter are 100 mm and 400 mm, respectively; nevertheless, the spring rate of about 100 N/mm is needed.
We must be prepared to run the risk of vacuum leak from the Ti bellows with the long weld line, because a small amount of oxygen causes the weld zone of Ti to be brittle. Therefore, we decided to make the hydro-formed bellows as flexible as possible.
The thin plates and a lot of convolutions are preferred in order to make a flexible bellows: From this viewpoint, the welded bellows has an ideal structure. To realize the hydro-formed bellows like the welded ones, it is essential to bend and press the Ti plates and to keep the 2 sides facing each other in close contact. There are 2 problems: 1) the necessary elongation for close contact bending is about 2 times larger than the permissive; 2) The spring-back effect for Ti is too large to keep the close contact. However, annealing the Ti plates after bending (pressing) decreases the number of lattice defect and disappears the spring-back effect. Therefore, the repetition of the combination of bending (pressing) with annealing enables us to realize the close contact of Ti plates.
We have confirmed the above effects of annealing experimentally, and then the production process of the bellows has been established. In the first place, we produced the experimental bellows with the inner diameter of 50 mm. It has the nominal length of 64 mm and 46 convolutions, and shows suitable performance: 1) the movable axial displacement and lateral deflection are +- 15 mm and 5 mm, respectively; 2) spring rates are 1.5 N/mm (axial) and 4.0 N/mm (lateral), respectively; 3) endurance test shows the fatigue life of over than 1 million cycles with the displacement of 20 mm. Thus we have succeeded in developing the hydro-formed bellows with the same flexibility as the welded ones.
Now we have completed the performance tests about the prototype of the large bellows, and mass production has been performed. |