| 摘要 |
PROBLEM TO BE SOLVED: To ensure a closed condition in which a door fully closes a discharge opening, by facilitating the lowering of the door down to the lower limit position at which the discharge opening is fully closed, even though foreign matter such as sand is accumulated in a sand pool. SOLUTION: A door 5 is provided at the lower end part 11A with a wedge part 11 incorporating a downward sharp end 11A, the wedge part 11 being extended over the entire widthwise length. In a condition in which the door 5 opens a discharge opening 3, even though foreign matter 12 such as sand is accumulated in a sand pool 10, the wedge part 11 incorporating a sharp end part 11A can enter the sand pool 10, wedging into the accumulated foreign matter 12 as the door 5 is lowered down to a lower limit position where the door 5 fully closes the discharge opening 3. Accordingly, the lowering of the door 5 down to the lower limit position can be facilitated, thereby it is possible to ensure a closed condition in which the discharge opening 3 is fully closed by the door 5.
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