Mechanical Advantage

The discussion about raising explained that "mechanical advantage" is a measurement of how much your rope and pulley system leverages the force than you put into it. This page explains a few terms relating to mechanical advantage. The following pages explain how to calculate mechanical advantage.

A system's mechanical advantage is expressed as a ratio using a colon. For example, a 2:1 or 3:1 system (these are pronounced as a “2 to 1" and “3 to 1"). The first number represents the force on the load and the second number is the force that the rescuers are pulling on the rope. For example, when using a 3:1 system, for every three pounds of load, the rescuers will be holding one pound. This means that the rescuers will only need to pull 100 pounds to raise a 300-pound load.

The following links explain how to rig various mechanical advantage systems:

Simple, Compound, and Complex

Mechanical advantage systems can be either simple, compound, or complex.

  • In a simple system, one rope is routed between pulleys on the anchor and load, and all of the pulleys that move (i.e., the "traveling pulleys") do so at the same speed and in the same direction as the load.
  • In a compound system, one simple system pulls on another simple system and some of the traveling pulleys move at a different speed than the load.
  • A complex system is a mechanical system that doesn't meet the definitions of simple or compound systems. Complex systems usually have traveling pulleys that move in the opposite direction of the load. These systems are less commonly used by rescuers.

Simple 4:1

Compound 4:1

Complex 5:1

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