The T-Method is used to calculate the forces in a system. Only moving pulleys create mechanical advantage.
Step 1: Start at the input end of the haul line, the end where the haul team will be pulling on the rope. The unit of tension at this end will be 1.
Step 2: This unit of 1 follows along the rope until the first pulley is reached. If 1 unit enters the pulley, 1 unit must exit. Like the pulley in the example, the addition of both units (in and out) will produce a force of 2 units at the top of the pulley, which in this example is the Prusik Hitch connection to the load line.
Step 3: Continuing out of the pulley, the rope next enters and leaves a fixed pulley. Since this pulley does not move, no units of force are added to the mechanical advantage. Therefore the 1 unit comes out of the pulley and moves down to where the Prusik Hitch is attached.
Step 4: The moving pulley is where the mechanical advantage is gained. The 1 unit coming out of the change-of-direction pulley and the 2 units at the Prusik Hitch add together, yielding a total of 3 units of tension applied to the load. When comparing the 3 units of tension at the output end to the 1 unit of tension at the input end, a 3:1 M/A is produced.
Source: This article was adapted from The CMC Rope Rescue Field Guide, Revised Fourth Edition, by CMC Rescue, Inc., of Santa Barbara, California. Copyright © 2013. All Rights Reserved. Excerpted with Expressed Written Permission.
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