... device, ensure shutter totally open and close, windproof, rainproof and dustproof.5Newly punch forming fan blade, reasonable angel and reach the max. air flow, mini ... ,high resistant.The use rises the tightener pulley organization,the leather belt cannot the loose belt,fall the belt. Design by computer make fan blade forming ... . Adopt rivit which is use to plane,fastness never looseness. Exhaust Fan