136 



GREENHOUSES 



Fig. 77. Device for 

 operating side ventilators 



Ventilator Arms. 



Ventilator sash are 

 most commonly raised 

 and lowered by means 

 of hinged braces or 

 arms operated from 

 the shafting. There 

 are three general 

 types. 



The elbow arm is 

 most commonly used 

 but 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 shaft 

 through an extra half turn. On long runs 

 these arms are now being extensively used in 

 place of the common elbow arm. 



The extending arm is used in low houses, 

 or for side ventilators, or in other places 

 where an elbow or double acting arm would 

 extend into the house so far as to be in the 



