Mechanical Advantage Progression

Written By: Lance Piatt

Thank you to Peak Rescue Institute for the graphics

In this section, students will learn five different MA systems: 1:1, 2:1, 3:1, 4:1, and 5:1. This range of MA systems is sufficient to address the majority of rescue hauls encountered in normal field operations. As with most other aspects of rope rescue, consistent practice building and operating MA systems will ensure that the rescuer is prepared to apply these concepts and skills in an emergency.

Figure 9-1 illustrates the 1:1 and 2:1 MA systems. A 1:1 haul system is included here primarily to help demonstrate the theory and progression of MA systems. A 1:1 haul system provides essentially no mechanical advantage. Since the change of direction pulley is attached to the anchor it generates force on the anchor rather than the load. Removing the pulley and applying the same force to haul the rope directly uphill will result in roughly the same force to the load.

There are practical applications for the 1:1 system pictured. Adding a progress capture device will render the system self-tending, meaning the rope can be released and the load will maintain its position.

The 2:1 is the simplest of the MA systems presented here. The haul pulley may be attached directly to the load or it may be attached with a prusik at any point on a separate rope supporting the load. Notice the haul pulley in the 2:1 system splits the load between the anchor and the haul team.

To illustrate, if the load weighs 100 pounds, the anchor will support 50 pounds and the haul team will support the other 50 pounds. Slightly more than 50 pounds of input from the haul team will raise the load.

The next illustration (Fig. 9-2) shows 3:1 and 5:1 MA systems. This 3:1 MA system, commonly known as a Z-rig, is one of the most frequently used haul systems in rope rescue. The 3:1 is a progression of the 1:1 discussed previously; the increased MA results from routing the haul end of the rope from the change of direction pulley through an additional haul pulley attached with a prusik back to the load side of the rope.

The 5:1 MA in the same illustration is a progression of the 3:1 and is built with one additional haul pulley attached with a prusik. This system incorporates haul pulleys moving toward one another, which is one of the reasons it is considered a complex MA system.

Resetting the complex 5:1 will require moving the haul pulleys away from one another, one toward the anchor and one toward the load. Also, this system requires that haulers pull away from the anchor and toward the load, which may or may not be an advantage depending on the application.

Figure 9-3 shows an alternate 5:1 MA system. The simple 5:1 requires an additional change of direction pulley, and both haul pulleys move in the same direction at the same speed, which simplifies resetting the system. In addition, the haulers will pull away from the load and toward the anchor with this system, which again may or may not be an advantage depending on the application.

The final haul system taught in the Technician Course is a 4:1 simple MA, commonly known as a block and tackle or a set of fours. This system is generally built and left pre-rigged using purpose-made tandem pulleys at both ends. At least one of the tandem pulleys will have a Becket attachment used to fasten the bitter end of the rope.

Depending on its orientation, the simple 4:1 can actually be either 4:1 or 5:1.

If the block and tackle pictured below were flipped end for the end it would be a 5:1 MA (Fig. 9-4).

Peace your Days

Lance

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