Mechanical Advantage

Carefulness costs you nothing. Carelessness may cost you your life.

Mechanical advantage (MA) can turn a strong man into a superman. Better yet, you can control the force, getting just as much as you need. This is not the case when pulling with a skid steer or truck. Skids and trucks can quickly generate forces that break trees and gear. Human power, amplified by MA can provide the control needed to do your job safely.

Pulleys with multiple sheaves (wheels) will allow the production of more force. The bell-shaped Prusik minding pulley is the workhorse for lifting, hauling, and tensioning heavy loads. When combined with a Prusik knot they allow the rope to pass when it is pulled and to grab when it is released.

Most people can pull their own body weight. A 200 pound worker named Joe can pull 200 pounds. With pulleys, he can increase that.

To determine the amount of force applied in a simple system, count the ropes at the moving pulley.

In this figure, the worker has a 3:1 MA. There are 3 ropes pulling at the moving pulley. The fourth rope (in the hand) is a redirect because it is not attached to the moving pulley. This system would allow Joe to pull about 600 pounds.

Add one pulley and create 5:1 MA. There are 5 ropes on the moving pulley. Now Joe is pulling roughly 1000 pounds.

What would Joe be pulling with this rigging?

You got it! 800 pounds.

This is a short and very helpful video from ISA about MA.

Phillip Kelley
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