As promised, in this article we will look at anchor and belay systems in relation to mechanical advantage systems (MAS). We'll also explore how to simply figure out what a mechanical advantage system's value is by simply looking at it.
First, let's briefly review the three different types of MAS.
The first being the simple MAS. In these systems, all the moving pulleys move towards the anchor at the same speed. An example would be a simple 4:1 and a 4:1 block & tackle system.
The second type of MAS was a compound MAS. It is created by adding one simple system to another simple system on the input side. An example of a compound MAS would be a simple 4:1 MAS This is simply a 2:1 pulling another 2:1. If you recall from the last article, in compound MAS when we add one system to another we multiply the two system values together to obtain the new MA value. In the case of the example above 2:1 x 2:1 = 4:1.
The last system we'll review is the complex MAS. A complex MAS is created by combining a simple MAS with a compound MAS. Just like the compound MAS, the system value is calculated by multiplying the two system values together. Remember, regardless of which system you are building, you must pay close attention to the first number in the MAS (5:1). This will play a role in determining how much rope you will need to build your system.
In a 5:1 MAS for every five feet of rope pulled your load will move one foot. If you have a victim that needs to be hauled 20 feet you'll need 100 feet of rope just to achieve your 20-foot raise. In actuality, your rope length will be over one hundred feet leaving excess rope for hauling. So, with that said take the time to properly size-up your rescue and prevent yourself and your team from being left short when it counts.
Determining The System's Value
Now that we've had that brief review, let's dive into how to figure out what a system's value is. For us to be able to figure out a system value using the math mentioned above, we need to have the ability to pick a system apart and figure out what systems are actually there. How is this done? By simply looking at the system and visually organizing it. You'll find when working with ropes that the more organized you keep your systems the easier your rigging operation will go. That's something we'll talk about again later.
For now let's apply it to the topic of MAS. When calculating the value of a system you need to "read" the system, or in other words see what systems are there. First, start with the pulleys. Remember anchored pulleys provide no mechanical advantage, while traveling pulleys do.
Let's look at Figure 1, it depicts a 3:1 MAS. Simply follow the rope through the pulleys. You should find a 2:1 and a 1:1. (2+1+3) Although a 1:1 provides no mechanical advantage it's still a simple system.
Let's take a complex 6:1 MAS. Take a moment and really look at Figure 2. What do you see? Follow the rope starting at the load end. You can clearly see a 3:1 MAS being pulled by a 2:1 MAS. However due to the self-defecting middle leg of this system the MA in actuality is 5:1.
Let's look at Figures 3 and 4. Both are 4:1 MAS; just different versions. Even though the systems were constructed differently if you look you can still pick out the two 2:1 MAS pulling each other. Let's look at Figure 5; take a moment and break the system down. What do you see? You should see a 2:1 pulling a 3:1 equaling a 6:1 MAS.
If you find this to be a bit slow going no need to worry. This is a skill that takes time to perfect. With practice and a little patience you will be proficient in no time.
Now let's take a minute and focus on the 4:1 and 5:1 block and tackle system. The 4:1 block and tackle system can easily be converted to a 5:1 by simply flipping the system around. To better explain, take a look at Figure 6 you and you will see a 4:1 MAS. Notice in a 4:1 the knot faces the anchor. If you flip the system so the knot faces the load and add a directional pulley to the anchor so you can still haul in a downward motion you have a 5:1 MAS.