The Forces of Evil

Part One: Leverage on Lateral Branches

The forces imposed on a tree during a rigging operation can be enormous and these forces can work for you or against you. Rigging load forces can be a very complex thing and not many people fully understand how they work. Rigging principles such as rigging angles, fall factors, branch levers, mechanical advantage or disadvantage all play a part. I will attempt to explain some of these rigging principles and highlight the importance of them for everyday use during tree removal and pruning.

We know every tree is different in its crown shape, structural design and associated defects and for this reason, when rigging, we can only work with the trees design and do what the trees design will let us do. No one rigging design will work for every tree job we need to vary the rigging design but the rigging principles still apply, they are a constant.

When rigging we must ask ourselves what effect will the cutting of this branch have on the rest of the tree based on how I have rigged the lowering system. The effect is what counts here.

The effect can be critical to safety when most of the rigging load affect may be above the climber’s head. Tops of trees have been snapped off due to the effect of rigging loads. Also deadwood can be broken off overhead due to shock loading the tree.

The effect of rigging loads on the tree is expressed as force, this force can bend, flex and strain the supporting tree parts in many directions and this force can also occur suddenly creating a shock loading. The most common force is a downward loading and this loading can also be the most dangerous. In some cases it is possible to reverse the loading on the supporting tree parts, that is, a downward load of a hanging branch creates an upward force on the supporting branches.

Figure one
The load can create a reverse force on the supporting branches eg load is downward but force on the pivot point of the support branches is upward.

Improving the strength of a trees structure by rigging is a real art. Mastering this art will make the impossible job seem possible or the difficult job seem easy. To rig a tree with a major defect so that the defect is stronger under load is the critical thing. Angles of rope to the supporting branches plays a large part and so does leverage.

Like a lot of things, leverage can work for you or against you and it is how you rig the job that will govern which way it goes. Leverage comes into play every time you tie a branch off to be lowered. The position of the lowering rope on the branch to be removed will determine how much leverage is created and whether it will work for you or against you.

Figure two
Leverage at play on a lateral branch being removed.

In the diagram (figure two) the position of the lowering rope on the branch means that as the branch is cut at the branch to trunk junction, the load on the lowering rope is multiplied due to leverage. Before the cut end of the branch breaks clear, it will hold on and act as a hinge, as the foliage end of the branch drops, the rope stretches and the leverage creates extreme force on the rope and the supporting structure. This force will continue to grow until the cut breaks clear and the branch is hanging vertically.

If the lowering rope is lose or is lowered as the branch drops, the leverage factor will be much less if not minimal. If at times the lowering rope needs to be tensioned or held tight as the branch drops, the leverage factor will be great.

The leverage factor can create large forces that can be hazardous for the climber for the following reasons:

1. The force on the rope may cause it to fail.
2. The force on the supporting branches may cause them to fail.
3. The upward force on the cut end of the branch may strike the climber.

The leverage factor plays a critical part when climbing arborists are breaking lowering ropes, pulleys, karabiners or the supporting branch or branches. The leverage factor can place unnecessary force where it is not needed. Learn to use the leverage factor to your advantage, not disadvantage.

The next issue will discuss reverse forces and how they can benefit you.

Next: Identified Flying Objects