Basic mechanical advantage systems are a wonderful tool for the rigging arborist, and the applications are vast with some creativity. Creating pulling force in felling operations, pre-tensioning lines in different rigging scenarios (speed lines and lifting tip-tied limbs) and hauling applications such as in a rescue scenario are some ideas that quickly one to mind. Surely there are many more.

It is important to understand how quickly large forces can be generated with basic mechanical advantage systems,  where exactly in the system those forces are input and output, and how they affect the safe working load limit on the hardware selected for the system.

Continuing along with my studies of the The Art and Science of Practical Rigging, I came across this excerpt:

“A basic mechanical advantage system has three unknown forces: 1. An input line pull. 2 An output force at the moving block. 3. A reaction force at the anchor block” (88).

With this information, we can see that we need to be careful about several factors, which are: what we are using to create input force (human power?, machine power i.e. truck, mini, etc.), what the anchor block is attached to (tree, vehicle, floating anchor, etc.) and also the working load limit of the line that the moving block is attached to, not to mention the load limits of all of the hardware in the system.

When rigging with mechanical advantage, it is important that we define these unknowns and calculate roughly what the effect will be of the mechanical advantage on those variables or from those variables.

For instance, if you consider input force, one person pulling can be defined as somewhere around two hundred pounds, and the MA will multiply that force in some way. Six people pulling on the same system greatly affects the outcome, potentially for better, potentially for far worse depending on the combination of the other unknown factors within the system.

The input force that I focus on here may be the easiest of the variables to control, but it is also important to understand how much mechanical advantage we are creating with the hardware choices we make.

“The output force at the moving block is the line pull multiplied by the number of the parts of line at the moving block. Mechanical advantage is defined as the output divided by the input, or load at the moving block divided by the line pull” (88).

Can the anchor handle this load? Can the hardware handle this load? Can the tree part in which the working end of the rig line is anchored to handle this load?

Tree parts don’t have working load limits, obviously, and so experience, in this sense, becomes a fourth, and very important variable that now comes to fruition.

Therefore, we can come to a few conclusions here: pulling with diesel machines and trucks is not always a great idea, pulling with seven ground persons is not always a good idea, and adding every pulley  you have into a MA system is not always a good idea.

Understanding the effects of the basic MA system on hardware, tree parts and anchor choices is always a good idea.

This is really an issue of compatibility, balance, and working within the limits of our gear. We need to identify the unknown variables when working with basic MA systems and properly identify whether those variables are appropriate for the output that we realistically need. We need to think before we act when multiplying force.

Pulling harder is not always the best answer.

Mechanical advantage is not just simply an equation, or a joker’s riddle. The systems we build could potentially put coworkers, property and other valuable targets on site in very grave risk.

As rigging arborists, we must travel into this abyss of art and science and find define the unknowns, in order to eliminate risk and seek complete control of all moving parts.

That’s the real advantage.