You may be wondering, does it really make that much difference if I use a pulley or a carabiner?
Short answer, it can make a lot of difference. Use pulleys whenever possible. We had a look at this above with the Sticky diagrams, but it's important to get this, so let's have a quick review.
Say you need to lift 100 pound load, with a 1:1 system redirected through a high-quality pulley which is 80% efficient, which is pretty typical for a standard rescue pulley. Here, you need to apply 125 pounds of pulling force to move the load. (The math for this is 100 divided by 0.8).
However, let's redirect that same 100 pound load through a carabiner, which has an efficiency of roughly 50%. Here, you need 200 pounds of pulling force to move the load. (The math for this is 100 divided by 0.5).
So, use a pulley and pull with 125 pounds, or use a carabiner and pull with 200 pounds? Easy choice!
Here’s a table of pulley efficiencies. You may have seen this in another post, but it’s important, so I’ll include it here again. This is for a 1:1 haul through a redirect point. (If that last sentence made no sense to you, read this post first.)
From the chart below, we can see if you have an MA system of 3:1 and only use 50% efficient carabiners, your real world MA is going to be about 1.75:1, ouch! (Plus, you still have the dismal progress of a 3:1, with only 1 foot of lift for every 3 feet of pull, even though you're pulling harder than you should have to.)
In this case, you may be better off using a 2:1 with one good pulley than a 3:1 with carabiners! We can see from the chart that a 2:1 with 20% friction (i.e., a 80% efficient pulley) gives us an MA of 1.80:1. But, a 3:1 with carabiners gives us an MA of 1.75:1.
So, use real pulleys whenever possible.
If you have to use a carabiner, which kind is best?
I’ve heard over the years that generally, a carabiner with round metal stock is is going to be more efficient than the new style “I-beam” construction with a narrower cross section. But is it really? if so, how much?
Here’s a Camp Nano carabiner on the left, and an old school Petzl Attache carabiner on the right.
I couldn’t find any sort of formal testing online that showed this, so I decided to try a little observational study myself.
10 pound barbell weight
Digital spring scale (about $11, I used this one)
9 mm dynamic rope
Old style Petzl Attaché carabiner (rounded)
New style Camp Nano carabiner (I-beam)
brand new rescue pulley
I tied the barbell onto the end of the rope, ran the rope through the carabiner on a bolted anchor to get a 1:1 with a redirect, clove hitched another carabiner in the pull strand and clipped the spring pulley to the carabiner.
I tried to pull straight down in a slow steady haul, and noted the most common reading on the scale. Any force over 10 pounds shows the inefficiency of the system.
Force needed with rescue pulley (baseline): 13 lbs - 77% efficient
Force needed with rounded carabiner: 20 lbs - 50% efficient
Force needed with a “I-beam” carabiner: 23 lbs - 43% efficient
(Math: 10 / 13 = .77; 10 / 20 = .50, 10 /23 = .43)
So, the rounded Petzl carabiner gives a slightly easier haul. (Note, this result was spot on with the often stated 50% efficiency rating of carabiners.)
Would you notice that extra bit of inefficiency in the real world? I’m not sure. But, if you have the option to use a round stock carabiner over an I-beam type carabiner use the round stock. Every little bit helps, right?
(Also, just for fun, I rigged two identical Petzl Attache carabiner side by side. The force needed to lift these was just 21 pounds, basically the same as the single Petzl Attache carabiner. In this case, adding one more carabiner really did not change the friction one way or the other.)
(Now, something that definitely ventures into engineering-land that is beyond the scope of my expertise is something called “coefficient of friction”. From what little I have read on this, the coefficient of friction for steel is different than that of aluminum, so apparently a steel carabiner will offer less friction than an aluminum carabiner. I do not have a steel carabiner or else I would’ve tested this, it would have been interesting. When I get my hands on one I will add that data point to this little experiment.)