Building redirect systems
A good understanding of the principles of force and leverage presented above can be used to design complex tree access and rescue systems .
The images below compare two different rope setups that could be installed to lift an injured climber up from a branch on which they have fallen .
In this image , a pulley has been installed on a branch end directly above the casualty . A haul system has been anchored to the base of the tree to lift the casualty .
It is easy to see that a large magnitude of force will be applied to the branch as the casualty is lifted .
The direction of the force is also weak , as force is being applied at an angle almost perpendicular to the branch . Branch failure is likely .
Timber strength and lever arm are discussed later in this chapter .
In the second example , the lifting line has been redirected multiple times through the crown of the tree . At each point , a perfectly-efficient pulley has been installed ( please see the following section on friction ).
At each point , the magnitude of the force has been significantly reduced .
More importantly , the direction of the force is now acting as compression in line with each branch angle .
Shear forces and bending forces have been reduced or almost eliminated .
This approach is much less likely to cause structural failure of the tree .
Note that this example is provided to illustrate how redirect systems can be used to reduce and optimise the forces applied to tree structure ; it would not be necessary to create such a complex system in most trees .
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