This is my first column for Fire Engineering, and in it I will review two examples of how firefighters can apply common sense to come up with solutions for even the most difficult technical rescue situations. The trick is to keep in mind the results your trying to achieve as well as the characteristics and limitations of the system(s) you’re working with.
Z-RIG SYSTEM: A BIRD’S-EYE VIEW
Let’s look at setting up a 3:1 (Z-Rig) Mechanical Advantage System (MAS) The hardware cache needed to build this system is as follows (after each piece is its use):
- Two pulleys (turn or redirect the rope, provide a mechanical advantage)
- Three carabiners (used to connect hardware to the rope or each other)
- Two sets of Prusiks (two long, two short; used to grab the rope and hold a load or a connection point to pull)
- One length of rope.
Look at photo 1 then draw it on a piece of paper it will be easier to follow along, especially if you don’t just happen to have some rope and hardware lying around. Notice that it takes the shape of a “Z,” thus the Z-Rig name. If you look at the rope or the “Z” created by the rope, you should notice two round bends in the rope. Each bend in the rope gets a pulley.
Connect the carabiners to the pulleys. Since you put those carabiners onto our pulleys, you need something to connect to the rope, which is where the prusiks come in. Connect the pulley closest to the load you wish to move via a triple-wrap prusik (photo 2). When a load is applied, the triple-wrap prusik pulls down on the rope with a torsional force, which makes the prusik grab the rope.
Tie the triple-wrap prusiks to that portion of the rope on the load you want to pull (known as the “load line”) and connect them to the carabiner on the pulley. Now connect the other pulley to our anchor. Mechanical advantage systems always have one end connected to the load and one to an anchor, since without the anchor the systems won’t work: It would be like playing tug of war with nobody on the other side providing resistance.
Visualize pulling or hauling on the rope. The load is moving closer to you and in turn, the pulleys are moving closer together right. At some point those pulleys are going to meet and go no further. If, after getting to this point, your load hasn’t reached its destination, return the pulleys to their original position.
Remember, if you let go of the rope, the load will drop. Use those same triple-wrapped prusiks to grab and hold the load. Look at the system you just built and find the load line. Keep in mind that a prusik works when it’s pulled on. Looking at the pulley on the anchor (anchored pulley) apply those prusiks on the side the rope first enters coming from the load.
Start at the load and follow the rope until you get to a pulley. Once you find it, you’ve reached your destination. Attach both prusiks. Photo 3 shows you the completed system (I’m not going to cover resetting the system in this article.)
This example above is designed to show you that, if you take a step back and consider the goals of the scenario you’re faced with, you can, with minimal guidance, easily set up an effective technical rescue application.
CRIB STACK CONSIDERATIONS
Another example: The basic crib stack is used all the time for a multitude of operations, its main purpose, however, is to catch and support a load. To successfully support a load, you want to transfer the supported load from point A to point B in a straight manner. To illustrate this principle, take a pencil and hold it on an angle vertically with one end touching the table. Now apply pressure with your hand on the top. What happened? The pencil slid to the side and collapsed to the table. Now hold the pencil straight and perform the same test. See the difference? Loads like to travel in straight lines. If they don’t, gravity takes them for a ride.
The crib stack is transferring the load just the same as the pencil did, but there are additional considerations. Notice that the 4×4 pieces of cribbing intersect (photo 5). Each intersection is called a contact point and provides the crib stack its strength. One contact point continues the path for the load to travel from point A to B. Contrast this with the stack in photo 6, in which the contact points don’t line up, meaning that the load will see voids or empty space on the way from point A to point B. The heavier the load, the more dangerous this situation gets.
In technical rescue, common sense can take you a long way. Add a solid knowledge base to that common sense and you can accomplish anything.
Mike Donahue 2015-