Thanks to Peak Rescue Institute Full Video at the bottom!
In general, the Ideal Mechanical Advantage (IMA) is the ratio between the distance the load moves and distances the haul team moves. In a 2:1 system the load will move 1′ to every 2′ of haul. However, this does not mean that lifting the load is twice as easy.
The Practical Mechanical Advantage (PMA), or simply put, the efficiency of the system, is the actual physical advantage the haul team ends up with. In short, based on the size of the haul team, try to build the MA system as small as possible. More pulleys create more friction, resulting in efficiency loss.
Consider the hauling field; that is to say, configure the MA system in a way that maximizes that amount of ground area the haul team can operate. This will also minimize the number of re-sets of the haul system. Build the MA system clean. Avoid crossed or twisted lines, as this will add unwanted friction in the system.
Ideally, when you make contact with a patient and you package a patient, you want to be able to use gravity to your benefit and just lower the patient. And that’s generally a relatively lower risk operation than raising a patient. But sometimes due to access issues or other issues, you may need to actually raise a patient back up toward where the team’s working above.
When it’s time to raise a patient, you’re going to need to build a haul system. When we’re talking about haul systems, we’re talking about using pulleys and carabiners and rope grab devices to basically create systems that magnify the effort that we’re putting into a rope system. I can take a rope that’s attached to a rescue load and just pull directly on that. I may be able to move the load that way. That would be ideal, a perfect feel for the load and it’s the easiest way. It would be the easiest way to move a load. So whatever amount of force that I can put into this system in terms of pulling on the load is the amount of force that’s imparted to the load. Often, however, with the size, the weight of the loads that we’re working with in the rescue world, a straight pull on the rope isn’t going to be enough.
Another way that I might give myself a little bit of an advantage over just a straight pull on the rope is to put the rope through a pulley attached to an anchor. This pulley serves as what we call a change of direction. A reason I might do this is to put myself in a more advantageous position. For instance, I’m pulling downhill and using gravity and my weight to help pull the load up the hill.
The amount of tension that I put into the pulley is transmitted through the pulley and down the rope to the load. If I can hang enough weight on this change of direction pulley, I can actually move the load uphill. So basically, the amount of tension that I put in is the same amount of tension that is transmitted to the load through the pulley.
One of the simplest mechanical advantage systems we can build is a two to one mechanical advantage system. And that’s essentially just attaching a pulley to the load, bringing the bitter end of the rope back up to the anchor and then pulling on the load… And this gets into the basic concept of haul systems and mechanical advantage systems where we use pulleys.
Pulleys have a doubling effect. What that means is if I put an amount of force into one side of a pulley, it has to balance around that pulley. So when I pull with the amount of force that I’m able to pull with, that same amount of force is transmitted around the pulley to the other side of the pulley. It’s supported by the anchor and the load feels double the force that I’m putting in.
So if I find a two to one haul system’s not sufficient, I can start building up more and more mechanical advantage. The next obvious step would be to move to a three to one. In this case, the rescue rope is actually attached directly to the load, comes up through a change of direction pulley, and then goes down through what we call a haul pulley.
A haul pulley is a pulley that’s in the system that’s moving and performing work. So what we have here is a three to one haul system. And once again, back to our concept of pulleys transmitting and doubling forces, let’s talk about that in terms of this system. I put a certain amount of force. I’m able to impart a certain amount of force into this system.
That amount of force is transmitted around the pulley and out the other side. So the amount of force that I’m putting in is the same amount of force that’s on this line. And we find that it creates a doubling of that force that transmits down to this rope grabber (prusik). That force continues up to my change of direction pulley, goes through it, transmits out the other side, and it comes down. And we can actually add that amount of force to the rope grab device. So I’ve got force plus force, two times that force. That force continues around and actually adds to the doubled force here. So I have three times the amount of force that I put into this system.
All right. Moving back to our first example, the first system that we showed, which was basically just a straight pull on a rope attached to a load. Basically, the force that’s imparted into one end of the rope is transmitted down the rope to the load. So if I have a load that weighs 100 pounds, if I’m able to pull slightly more than 100 pounds, I’ll be able to lift that. I’ll be able to lift that load just pulling straight on the rope. We have a system that we can use to help analyze this. And we can actually analyze a haul system or a mechanical advantage system, any system, and we can determine what its mechanical advantage is. And when we’re talking about this mechanical advantage, we’re talking about theoretical mechanical advantage. So that eliminates real world friction, all things like the friction that’s introduced by the inefficiency of a pulley, all of that. This is mechanical advantage in the perfect world.
But instead of talking about whatever amount of force or amount of pounds I’m able to generate hauling on a rope, let’s just assign a variable to that. And we’ll just say it’s one unit and we’ll call it one unit of tension. So that’s one T. So if I put one unit of tension … And my one unit of tension as a hauler, I may be able to pull on a rope … I may be able to put 60 pounds of tension in a rope by pulling on it in any kind of sustained manner. But that doesn’t matter. We’re just going to call it one unit of tension. That one unit of tension transmits down the rope to one unit of tension applied to the load. And again, if I can generate enough tension that is slightly more than the mass of the load or the weight of the load, then I can move that load.
Now, moving over to our second example of putting a change of direction pulley into our system. Again, I may do that so that I can use gravity and I can use a slope to help me pull against the load. I’m always going to start my one unit of tension, my haul force where I grab onto the rope, where the rope enters the system. And I’m going to follow it up. I’m going to follow it continuously through the system. So on one side of the pulley, I have one T. That one T is transmitted around the pulley and it comes out the other side. It continues down the line to the load. So pulling through a change of direction pulley attached to an anchor, I’m generating still, just like pulling it straight in line, I’m generating one unit of tension on my load.
The next system we just showed was a two to one haul system. Now we’re actually starting to get into using the nature of a pulley to help generate additional force for us to help us move a load. Let’s apply our T method here. One T as always. That’s what we start with on the haul strand of rope, transmitted down to our pulley. One T goes around our pulley, one T. Because we have one T on one side of the pulley, one unit of tension on one side of the pulley, one unit of tension on the other side of the pulley, that actually translates into two units of tension that’s felt by our load. That one unit of tension continues up the rope. And it ends at the anchor as one T.
Now, next let’s move on to our three to one haul system. One T transmitted down the line to our first pulley. One T comes out, one T. One T goes in, one T comes out. So we can say that two T is transmitted down through the prusik rope grab device. And we’ll just leave that there for now. We don’t want to break our stream in following the rope through the system. We have one T coming out of the pulley. One T comes up to this change of direction pulley on our anchor. One T comes out, continues down. We run into our two T here where we add it together. And we have three T or a three to one haul system.
Peace on your Days!