Using mechanical advantage… to our advantage – David Vickers

by | Apr 15, 2015 | Features, Technical

“Give me a lever and a place to stand and I will move the earth.” Dave Vickers of Drivelink Training explains how Archimedes’ boast is relevant to arborists

Using ropes and pulleys is common in the arboriculture and forestry industry, whether for rigging, vehicle debogging or assisting with a fell.

There appears to be some mystique about using ropes and pulleys to create mechanical advantage and during rigging refresher courses I often find that people hadn’t realised they could set-up a system that would allow them to lift branches despite using a portawrap/flying capstan device; or that they could use a similar set-up when felling to gain greater control over the tree being felled.

In this short series, we’ll be going through mechanical advantage, how to set-up rope and pulley systems to create it, how we can implement it in real world scenarios and how to ensure safety when using it. Hopefully this series will be of most use to those who wish to undertake the Emergency Treeworks, or Assisted Felling courses/assessments, but I also hope it is of wider use to the industry.

What is Mechanical Advantage?
When we hold a rope to pull something along, we bear the full weight of that load, plus any friction. We can use pulleys to help us, however. How we set these up is of key importance as they have different roles to play, affecting the system as a whole. Take an example where we want to lift a branch that has a mass of 100kg. Our climber rigs a pulley to a fixed top anchor point, so one end of the rope is attached to the branch and the other to my 75kg mass. As soon as the climber severs the branch, the 100kg load comes on to the line, lifting me into the air. All I do is reduce the impact when the branch hits the ground.

The fixed top pulley offered no mechanical advantage and served as nothing more than a fairlead for the rope. If we had set-up a 2:1 system, however, my 75Kg could have lifted that 100Kg load and theoretically I would have balanced a load of 150Kg. But as we only used one fixed pulley there was no mechanical advantage.

Lesson #1: fixed position pulleys offer no mechanical advantage, simply helping us to pull loads using a more ergonomic position.

Mechanical advantage can be defined as the ratio of output force to the input force applied to a system. With our fixed position pulley, one unit of input force gave one unit of output force, so my mass of 75kg (0.75kN) resulted in an output force of 0.75kN – the lifting action on the branch. As the branch was applying 1.0kN downward force, this meant that I was going to be lifted up. In the next instalment, we’ll begin to take a look at how we can set-up a system to create mechanical advantage.

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.

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