| 摘要 |
PROBLEM TO BE SOLVED: To prevent excessive upward displacement of an upper plate for excessive load input without providing a hook member. SOLUTION: The upper plate 2 is supported on an inner peripheral part 11 of a lower plate 1 by interposing a ball 31A and rotates within a horizontal surface. A set plate 4 is connected to the inner peripheral surface 11 of the lower plate 1 and clamps an inner peripheral surface 21 of the upper plate 2 together with the inner peripheral surface 11 of the lower plate 1 by interposing a ball 31B. A rib groove 24 to annularly extend and to open upward is bent and molded, made to intrude and be positioned in the rib groove 24 by extending an outer peripheral end part 44 of the set plate 4 downward at an outside position of the ball 31A of the inner peripheral part 21 of the upper plate 2. A rib groove 43 to annularly extend and to open downward is bent and molded, made to intrude and be positioned in the rib groove 43 by extending an inner peripheral end part 25 of the upper plate 2 upward at an inside position of the ball 31B of the outer peripheral part 42 of the set plate 4.
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