Z-shaped. rope-and-pulley rigging system that achieves a 3 to 1 mechanical advantage.
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A Z-Drag or Z-Rig is an arrangement of lines and pulleys commonly used in rescue situations. The basic arrangement provides a theoretical mechanical advantage of three. The name comes from the fact that the arrangement of lines is roughly Z shaped. Besides the mechanical advantage to pulling, it also uses only part of the total length of the rope for the block and tackle arrangement.
The typical configuration (see diagram) uses two single pulleys and two Prusik knot loops or other suitable friction hitches. These Prusiks provide fixed attach-points on the rope that can be moved when slightly loosened. The first Prusik knot provides the mechanical advantage. The second Prusik knot can be used to hold the position of the rope and is sometimes referred to as a 'progress capture device' or ratchet. It is also advisable to attach a towel or soft object (such as a life vest) to the end of the line near the connection to the object being pulled since the line is under high tension and could break free and present a dangerous flying hazard.
The Z-Drag is considered an important tool in mountain rescue because of its simplicity and is used for nearly all lifting systems.
It also serves as an excellent method for tightening the rope to be crossed in a Tyrolean traverse, where the other end is also fixed to a stable object.