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DESCRIBING PULLEY SYSTEMS

Through common terminology rescuers can accurately describe a specific pulley to be
employed in a rescue. Stating “build a 3:1 simple system” is more precise than saying
“z-rig” (common slang for this pulley system). A request for a “compound 6:1 with a
simple 2:1 acting upon a simple 3:1” provides clear direction to personnel assigned to
rigging on an incident.

A pulley creating a change of direction

provides no mechanical advantage and
is used to redirect the direction of pull on
a rope (Figure 179). This is employed in
situations where it may be advantageous
to have rescuers pull downhill or through Figure 179- Simple 1:1 with a change of direction.
a natural clearing.

CALCULATING MECHANICAL ADVANTAGE
Mechanical advantage in a pulley system is achieved by increasing the number of times
an initial input force applied upon the load. This is achieved in numerous rigging
possibilities with simple, compound or complex pulley systems. The input force is the
tension applied by pulling on the system and it is expressed as one unit of tension.
Understanding how this one unit is transferred through a pulley system permits
calculating the TMA, which is referred to as the “T-Method” (Tension Method).

By assigning one unit of tension (T) to where the pull is applied to the system, then

following the path of the rope through the pulley system to the load itself, the TMA can
be determined by keeping track of how that initial unit of tension is distributed
throughout the system. Simply compare the amount of tension that is applied to the
load with the input unit of tension.

Understand that wherever a junction occurs with the ropes of the pulley system, such as
one rope acting on another or one rope acts upon more than one rope, then the tension
on one side of the junction must be equal to the tension on the other side of the
junction. Additionally, on each side of the junction, the tension must be distributed
appropriately (not always equally) to each rope. As an example, if a rope having one
unit of tension makes a 180° change of direction through a pulley (considered a
junction), then whatever that pulley is connected to receives two units of tension (Figure

180.a). In other words, two ropes each having a tension of one (two total units of

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