Mountain Guiding; Mountain Safety
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August 2002 - Tech Tip Supplement

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Mechanical Advantage Basics - Simple Pulley Systems

Important: This is not meant to replace hands-on instruction, nor to replace the need for hands-on practice. Also, this was a creative effort done in a non-mountain environment to demonstrate a particular point and does not reflect proper belaying, tying in, etc. In particular, the angles of the rope have been kept wide in an effort to make it as simple as possible to see the system. (And it is still complicated on some of them.)

Using the anchors on the office porch I've constructed various basic pulley systems, and they are set up . The goal is to review or introduce some of the most fundamental principles behind the commonly used mechanical advantage systems. Sometimes compound systems are used, which are combinations of basic systems like the ones here. Complex systems are not based on simple building blocks at all but are memorized ("tricks of the trade") systems.

One common mistake is to confuse a 2:1 system with a change of direction. In a Change of Direction the force is doubled on the anchor, not the load. Another observation you may make is that the 2:1 and 4:1 systems have the end anchored at the anchor point and not the load, and the rope is then looped to the load. The 3:1 and the 5:1 systems have the end anchored on the load (as would the trivial case of a 1:1 or direct pull).

As a final comment, the mechanical advantages mentioned here and used in common conversation are "theoretical" ones. Actual advantages are lower due to friction in the pulleys and elsewhere in the system. So a change of direction may be helpful or necessary but it will reduce your advantage a bit. Also, the more complicated the system is the more sources of friction it will have. Ultimately a point of diminishing returns is reached. Friction will depend on many things, including the type of pulley used (bushing vs. bearing, diameter, etc).

Once these basic systems are understood they can be combined to create compound systems with more confidence. To learn more about all of this you can take the second half-day of the self-rescue course.

		Change of Direction
		3-1 (Z) System
		3-1 (Z) System
		5-1 Simple System
		2-1 (C) System
		4-1 System

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