| 摘要 |
PROBLEM TO BE SOLVED: To reduce collision noise during the contact of butterflies with the peripheral edge of an opening while controlling an increase in cost. SOLUTION: A duct body 1 has ribs 14 with sectionally circular surface extending from the opening face of an opening 11 toward butterflies 2, gradually higher as closer to central portions, the butterflies 2 first contacting the ribs 14 and then contacting the peripheral edge 12 of the opening 11 to close the opening 11. Because collision energy is dispersed during the contact of the butterflies with the peripheral edge 12 of the opening 11, collision noise is reduced.
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