In the final part of his series on creating mechanical advantage, Dave Vickers looks at using a rope-based system to help with felling trees
Last month we saw how to create a system to lift a section of timber when dismantling a tree, before lowering it to the ground.
In this final part, we’ll look at using a similar system to assist with felling trees, and whilst this can be achieved using a winch, we can also do this using a ropebased system with safety backup.
In the photo below we can see that a felling line has been attached high up on the stem of a tree using utility arb poles (see inset picture), and the other end is fed back to the anchor point on the ground – this can be the stem of another suitable tree.
Last month we used two double-block pulleys to provide a lifting system with a 4:1 mechanical advantage, and this time we’ll do the same thing to provide a pulling system with a 4:1 advantage. The fixed pulley block is attached to the ‘ground’ anchor using a round sling and the pulling line is fed from the tree to be felled through the fixed pulley and back up to the moveable pulley.
The moveable pulley provides the mechanical advantage and must be affixed to the pulling line coming from the tree, and although there are many ways of doing this, a friction hitch will work for this purpose (as shown in the photo to the right). With the rope fed through all the pulley blocks considerable force can be exerted to the tree stem to bring it over, but if the person pulling the tree over lets go, the tree could still topple backwards, trapping the saw, or worse.
So with this system we need to create a backup, so that even if the person pulling does let go, the tension on the tree can be maintained. This is achieved by using a friction hitch on the ground pulley. You will need to choose where to fit this carefully, as fitting it at the wrong point will mean that you cannot pull on the system.
Rather than make this system up from scratch, it is possible to purchase ready made systems, such as the Rock Exotica Aztec, to achieve the same result, giving a 5:1 or 4:1 mechanical advantage in a compact set up.
Whether you choose to build your own system or buy a ready made one, having an understanding of the applications where we can make use of mechanical advantage is of benefit. In the course of this mini-series we’ve looked at two practical applications – lifting timber during rigging operations, and for assisted felling – both of which used pulleys to multiply the input force and to provide ergonomic, safe working for operators.
Finally, when using these rope-based systems, do ensure that you are aware of the increased loading that the equipment will be subject to and always make use of safety backup systems.
Drivelink Training provides City & Guilds NPTC forestry and arboriculture short courses; run by David Vickers, a City & Guilds NPTC approved trainer and assessor, qualified teacher with a BA (Hons) Ed. and QTLS. Drivelink Training provides training from basic maintenance to dealing with windblown trees, from basic tree climbing to rigging / dismantling. Visit www.drivelinktraining.co.uk to find out more.