Managing Friction: Anchor Considerations in Rope Rigging Systems

Written By: Lance Piatt

We’re back at the Rescue Response Gear Rigging Lab. Today, during this presentation we’re going to talk about anchors. Now, this is going to be a broad-based presentation. We’re really kind of still in the awareness stages of this. We’re going to have future presentations that go into the specifics of the physics, and really go into greater detail of individual things that we’ll be talking about today.



So if we look around the rigging lab, we have an A-frame over here, we have a gym pull-in back over here, and we have this funny-looking log that’s between the truss system. And I’m holding on to this funny weight and this rope here. What is this all about? Well, it’s all about an introduction to the physics of anchors. And we’re going to talk about that now.

I have a fish scale. Just a cheap 50 pound capacity fish scale. And I’m interested in seeing what kind of effects the edge has on our load. So we’re controlling a 10 pound load. When I go down, we’ll see what the effects are and we’ll see what it is as I haul it back up again. So friction works for us, or it can work against us. One way or the other we’re having to deal with it. We’re either using it for it to our advantage or trying to overcome it.

So let’s lower our load down past this aggressive angle iron edge. 10 pound load, and as we look at the scale, we can see that it’s reading right around three pounds. Pretty good. Not much stress on this anchor on the lowering side. Now if we bring it back up, what happens? Well, I think, you know what the answer’s going to be. Our little 10 pound load is now more than doubling its load. It’s hitting 22-23 pounds coming back up. So we’ve more than doubled the load coming over this aggressive edge. The moral to this little story is we have to manage that, especially on the up. When things fail, it’s usually during hauling systems. That’s usually when we have failures. So it’s important to have a good high directional anchor with a pulley on it, or even a low directional anchor, but something that’s going to get rid of that aggressiveness at the edge. Especially during hauling systems.

What we’ve done now is added some pulleys and a rigging plate to our load cell. At first, I’m going to pull against this directional pulley, just a one-to-one. And let’s see what kind of effect we’re going to get. Remember, when we were hauling the rope back up, our 10 pound weight going over this edge was roughly 22 to 24 pounds, a magnification of little over two times. What do you think it’s going to be on a fish load cell or fish dyno when I pull on the one-to-one? Well, let’s do it. 42, 45, 46 pounds going against that one-to-one. Does that make sense then? This leg plus this leg have to equal the total at the pulley. So we have 22 pounds on this side, 22 pounds that we’re holding, discounting any friction within the pulley. 22 plus 22 is going to equal the 44.

Let’s go now to our three to one and see what kind of effects we have on our anchoring system. Here’s our Z rig. Anybody want any guesses on what the reading is going to be? 22 pound load with the friction included in it. Divide that by three. I’m pulling one third and two thirds are going to the anchor. It is reading right around 12 pounds. Does that make sense? And again, we have some friction that’s coming into play on the pulley, but roughly two thirds of the load is on this anchor. So we made out pretty well by putting the three to one in. The anchor made out pretty well by putting the three to one in. What we’re going to do next is add a change of direction to our little rigging plate here. So we will have a three to one CD. Let’s see what the effects on our anchor is with that.

Now we have the three to one with the change of direction. So how many legs of rope are going to this anchor? Well, we can count them. One, two, three, four. Remember, it’s a three to one. So one third of that load is on each one of these legs. There’s four thirds, or one and a third times the load. Let’s see what our hit should be. 30 pounds.


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Peace on your Days



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