The Z-pulley (fig. 13-12) magnifies the muscle power of small climbing parties by offering a 3-to-1 theoretical mechanical advantage through the use of two pulleys. It can be set up and operated with no help from the fallen climber, making it valuable in rescuing an unconscious person.
Fig. 13-12. Raising a climber with the Z-pulley
The Z-pulley system, which uses the accident rope, requires more equipment and is somewhat more complicated than the C-pulley.
You can get to work on the Z-pulley as soon as you have set up the anchor that is established after any crevasse fall. As usual, the accident rope will be secured to this anchor via a sling tied to the rope with a friction knot (prusik or Bachmann) or an ascender.
Take the loose end of the accident rope and lay out a long loop on the snow, in front of the anchor, so that the loop and the taut line leading to the fallen climber form a giant flat Z.
At the first bend in the Z (by the anchor), place a rescue pulley. Clip a locking carabiner through the pulley and attach the carabiner to the anchor with a short sling. (Some climbers prefer to establish a second anchor to secure this pulley.) The pulley will be located on the climbing rope, between the friction knot and the anchor.
At the second bend in the Z (closer to the crevasse lip), place a second pulley on the rope. Clip this "floating" pulley into a short sling that also is attached with a friction knot to the taut rope going to the fallen climber. You may have to see it to believe it, but you now have a Z-pulley setup, ready for use.
To work the system:
1. Pull hand over hand on the free end of the rope, hauling the climber upward.
2. Be sure the rope slides freely through the friction knot or ascender that is linked to the climbing rope near the anchor. This will be the friction brake that holds the accident rope whenever you reset the pulley. If it's a prusik knot, one rescuer needs to help keep it in the open, sliding position. If it's a Bachmann knot or mechanical ascender, it should be able to tend itself.
3. As the floating pulley is hauled in close to the other pulley, slowly relax your pull on the rope in order to let the friction brake take the weight of the climber. Don't let the two pulleys touch because that will uncio the Z con figuration and convert your system to a direct pull—the old brute-force method.
4. Reset the Z-pulley by loosening the friction knot that is attached to the floating pulley and sliding it way out toward the crevasse lip once again.
5. Haul again on the free end of the rope.
6. Keep repeating the process.
Remember that in any rescue system calling for pulleys, you can substitute carabiners if necessary. However, they create far more friction and the rope will be much harder to pull.
Continue reading here: Piggyback systems
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