Greenhouses, their construction and equipment . lowered by meansof hinged braces orarms operated fromthe shafting. Thereare three generaltypes. The elbow arm ismost commonly usedbut has the disadvan-tage that a long lever-age is required, in order to open the venti-lators to the full width, which puts a consid-erable strain on the shaft. The double acting arm overcomes this dif-ficulty to some extent as it is possible to se-cure a wider opening with a shorter leverage,.but it is necessary to rotate the shaftthrough an extra half turn. On long runsthese arms are now being extensively used inpla
Greenhouses, their construction and equipment . lowered by meansof hinged braces orarms operated fromthe shafting. Thereare three generaltypes. The elbow arm ismost commonly usedbut has the disadvan-tage that a long lever-age is required, in order to open the venti-lators to the full width, which puts a consid-erable strain on the shaft. The double acting arm overcomes this dif-ficulty to some extent as it is possible to se-cure a wider opening with a shorter leverage,.but it is necessary to rotate the shaftthrough an extra half turn. On long runsthese arms are now being extensively used inplace of the common elbow arm. The extending arm is used in low houseis,or for side ventilators, or in other placeswhere an elbow or double acting arm wouldextend into the house so far as to be in the VENTILATING 13^ way. It folds together when the sash isclosed and occupies little space, but it ex-tends automatically when the shaft is is especially convenient under certain con-ditions, but it lacks the strength necessaryfor long Fig. 78.—Compact machine for operating side ventilators In all systems the arms are clamped se-curely and rigidly to the shafting, and asnear as possible to the hangers so as not tospring the shafting when heavily are spaced about 3 feet apart along thesash. If continuous sash are not used thearms should be distributed as follows: Forsash up to 4 feet long, one arm; from 4 to 7 138 GREENHOUSES
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishernewyorkorangejuddc
