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friction loss occurs when ropes come into contact with pulleys. If a carabiner is used in
place of a pulley, then even greater friction loss occurs.

To calculate the losses due to friction, one must know the efficiency of the pulleys being
used. Using the pulley efficiency information, friction loss through the system can be
calculated. Figure 183 shows the calculations for a pulley system with pulleys that have
an efficiency of 0.90.

Figure 183- Simple 3:1 or 2.7 AMA

Assuming that the rescuers pull on the pulley system with one unit of
tension, only 0.90T will be transferred past the first pulley. When that
0.9T reaches the second pulley, only 0.81T will be transferred onward
(0.9 X 0.9 =0.81) as the friction loss is compounded over two pulleys.
Follow this process all the way through the pulley system. When you
are finished, use the T-Method to determine the final AMA, which in
this situation is 2.71:1

If higher efficiency pulleys are used (e.g. 95%), the AMA is increased
to 2.85:1, which is closer to the TMA of 3:1. Also important to note, is
that if you are using pulleys of different efficiencies, less loss occurs if
the most efficient pulley is placed closest to the pullers. This is
because the loss at the first pulley is compounded throughout the
entire system.

PULLEY SYSTEM RIGGING:
Prusiks (referred to as “soft safeties”) are utilized as haul Figure 184- Mechanical
advantage pulley system
and ratchet rope connections since they can handle shock with fixed and moveable

forces without catastrophically failing a line. Prusik Hitches pulleys. From First Principles
of Physics. H. Carhart and H.
Chute. 1912.

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