Counter Weight Flying: Difference between revisions

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Double Purchase sets are installed when the cradle cannot have the same travelling distance as the fly bar. With the cradle only travelling half the distance and the control line having to travel through an extra set of pulley blocks then the weight in the cradle must be double that of which is on the bar in order for it to be balanced. Although the travelling relationship between the bar and the cradle is doubled the control line still operates on a 1:1 ratio due to it's double drop above and below the cradle.
The reason for using a double purchase system is usually lack of vertical space. A Double Purchase system does not need as much height for the cradle, frame and runners as a single purchase system. In the Single Purchase set the cradle, frame and runners need to be the same as the full travel distance of the flying bar, in a Double Purchase system the cradle will only move half the travel distance of the flying bar.
 
The Double Purchase sets in the New Athenaeum are located above the dock doors into Room1. Since the runners could not come down to the floor level due, to the door, Double Purchase sets were installed.
 
In a Double Purchase set two lines support the load through a pulley on top of the cradle. Since each line has the full weight of the bar, double the weight must be added to the cradle to balance the set.  
 
The simplified diagram below will, hopefully, begin to illustrate the relationship of weight and travel in a double purchase set.  
 


[[Image:DP-Counterweight.jpg]]
[[Image:DP-Counterweight.jpg]]
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