In the world of rope rescue, we use mechanical advantage systems really for two reasons. The first being a force increasing tool and the second, a means of reducing the weight of a load during a lowering operation.
Mechanical advantage systems (MAS) are broken down into three categories:
- Simple MAS
- Compound MAS
- Complex MAS
Although the three types of systems will provide the same outcome, each is unique in assembly and force calculations. Allow me to explain.
First, we have the simple mechanical advantage system. In a simple system, all of the moving pulleys in the system move towards the anchor at the same rate of speed. Below are two examples of simple pulley systems (see Figures 1 and 2.)
Physics tells us that anytime we add a system to another system on the input or hauling side we multiply those numbers to get the new MA. If I had a 2:1 MAS and then added a 2:1 MAS on the input side I would then have a 4:1 MAS.
Now, we'll look at compound mechanical advantage systems. The compound mechanical advantage is the result of adding one simple system to another on the input side. The same example we just used regarding the 4:1 MAS can be used here.
The last type of MAS, we will explore is called a complex mechanical advantage system. Complex MAS are simply done by combining a compound and a simple system. The example, shown in Figure 3, shows a 3:1 MAS being pulled by a 2:1 MAS resulting in a 6:1 MAS. When all is said and done, simple MAS have a greater stroke than compound or complex MAS of the same rating.
Theoretical Mechanical Advantage
When reading textbooks or learning about MAS in a classroom setting, the force and load calculations are considered the ideal mechanical advantage. In reality, what we are actually working with is called the theoretical mechanical advantage (TMA).
It's theoretical because every time a bend is created in the rope or the rope passes through a pulley, a percentage of the overall power of the system is lost. A good rule of thumb is a 10 percent loss for every pulley the rope passes through and two to three percent loss for a bend created in the rope.
Use pulleys in your system that are at approximately three times the size of the rope's diameter. This will ensure the least amount of strength loss by the rope passing through the pulley. When all is said and done, what you're working with is referred to as the actual mechanical advantage (AMA). Without special tools, such as a dynometer, you won't be able to figure out the AMA. As long as you have a pretty good idea of what your TMA is, it's safe to say you're in good shape.
Creating a Mechanical Advantage System
How does a mechanical advantage system work? Well it's pretty simple. Let's take a 2:1 MAS. It's comprised of an anchor, rope, pulley and carabiner (see Figure 4). The pulley attached to the load is called a traveling pulley. This pulley creates your mechanical advantage. However, for it to work the other end of the system must be anchored. If you have a pulley attached to a non-moveable object, it is referred to as an anchored pulley. It serves only one purpose: a change of direction.
As you introduce force into the system (pulling), that force is focused on the traveling pulley. If the load the pulley is attached to weighs 100 pounds, the 2:1 MAS now makes that same load 50 pounds (2 ???? 100 pounds = 50 pounds). To break this down further each side of the pulley will see half the load, 50 pounds on one side 50 pounds on the other thus creating the 2:1 MAS.
The first of the two numbers signifies the mechanical advantage (for example: 3:1). It also tells you that for every three feet of rope we move the load will only move one foot. Now, here is a bit of information for you that will be of use later down the road when calculating the MAS you will need. A rescuer can pull with a force load of 50 pounds so one rescuer utilizing a 4:1 MAs can pull a 200 pound load with relative ease.