In the second part of his series on using mechanical advantage, Dave Vickers discusses how to build upon a Portawrap lowering system to more safely dismantle a tree
There is some mystique surrounding setting up and using ropes and pulleys to create mechanical advantage. Last month we saw that using a moving pulley provides additional ‘assistance’ when pulling loads. This month, we’ll be building on this to present a real world tree dismantling scenario.
There are times when we want to lift the section being cut; this can be safer as the lifting action can be achieved from the ground, allowing the climber to complete the cuts then move away from the cut section. The issue is that unless you are using a system such as GRCS (Good Rigging Control System) or similar, you’ll have no way of lifting the timber. Flying capstans and bollards are designed to add friction into the system to control lowering, they are not designed to lift loads.
We can overcome this issue by using a further system that gives us the ability to lift the section. This ‘bolt-on’ system can make use of mechanical advantage to help us lift the load, so let’s take a look at how to achieve it.
We initially set up the normal lowering system using a Portawrap device as shown above right. The lowering rope is tip-tied to the section to be lifted – this gives us the leverage needed. We now use a second line as our lifting line, connecting in two pulleys. The first is fi xed to the top of the Portawrap and the second is essentially attached to the lowering line.
This second pulley is moveable, providing mechanical advantage. The main photo on the left shows one method of installation using the lifting line attached to the lowering line via a friction hitch (Blake’s Hitch in this case), with the moveable pulley attached to the lifting line via an alpine butterfly knot.
The system as shown provides a mechanical advantage of 4:1, so for 1000N (equivalent to 100kg) of force we exert, we can lift a 400kg section. The photo to the left shows one member of the ground crew on the lifting line and the other on the lowering rope, with the climber completing the cuts. Once the climber has moved away to a safe point in the tree, the section can be lifted and the slack in the lowering line taken up. Once the branch has snapped off , the lifting system can be removed and the section lowered as usual.
Next month, in the final article we’ll take the basics of this system and apply it to assist with felling.
Drivelink Training provides City & Guilds NPTC forestry and arboriculture short courses; run by David Vickers, a City & Guilds NPTC approved trainer and assessor, qualifi ed 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.
