It is known that in ventilation systems having a plurality of air intake valves it is possible to arrange for a selective opening of valve plates in these valves between closed and fully open position, such that the opening is adjusted to the requirements. The selective opening is based on the valve plates being drawn to open through a direct pull and through a 2:1 pulley block pull, respectively. According to the invention it is possible to arrange for other transfer ratios, viz. in that a part of the opening of selected valves is effected with a transfer ratio of 2:1, while later during the opening this ratio is changed into 1:1 by way of an abutment (28) on the pulling wire (10) engaging the wire eye (12) on the valve plate (4) which, otherwise, holds the wire in a sliding manner. Hereby the total opening characteristic of the valves can be determined in a widely optimized manner in the single stable.
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