The 3 kinds of pulley systems
I'm reading up on pulley systems, and I’m hearing about “simple”, “compound”, and “complex”. What do these terms mean, and which one should I use?
Now, this is heading into slightly more advanced territory, but if you’ve read this far you hopefully still have an interest in slightly esoteric things like this. :-) As to which one to use, there’s not a quick and easy answer.
The one potential issue with the compound and complex systems is that you usually have to reset the pulleys more often as they “collapse” (or, are pulled into each other) when you pull. If you have a large working area, like the top of a crevasse, this is probably not a big deal. If you have a tiny working area, such as a hanging rock belay, then it might be more of a problem.
If you want to geek out on this, look at the YouTube video links at the bottom of the page on compound pulleys and start playing around on your living room floor. That's really the single best way to learn this. You can do it with some parachute cord and a few carabiners, you don't need pulleys or a even a real climbing rope.
1 - Simple system
When you pull the rope, the pulley(s) move in the same direction and the same speed toward the anchor.
As the rope is pulled, the pulley moves toward the anchor at a constant speed. There are three strands of rope going to and from the load and load strand, so this means it's a 3:1 MA. This is also known as a “Z drag”, because the shape of the rope is a “Z”. (If you tilt your head to the left . . .)
In a simple pulley system, when the rope end terminates and is attached at the anchor, then the MA will result in an even number (e.g. 2:1, 4:1, 6:1, etc.).
When the rope end terminates and is attached at the load, then the resulting MA will be an odd number (e.g. 3:1, 5:1, etc.).
In the photo below, the rope end attaches to the load, so we have an odd MA number, 3:1.
A 3:1 simple system. The pulley moves at a constant speed toward the anchor.
2 - Compound system
When you pull the rope, the pulleys move in the same direction, but at different speeds toward the anchor.
This can be created by building a 3:1 Z drag, and then adding a 2:1 onto the strand you’re pulling. With a compound system, the mechanical advantage of each separate pulling system is multiplied.
Below, we see a 3:1 on the white rope, and a 2:1 on the black rope. Together, the two systems are multiplied to get a 6:1. Note that the white rope will move the load 1 foot for every 3 feet of rope you pull, while the black rope moves upwards 1 foot for every 2 feet of rope you pull. Therefore, the black rope will reach the anchor point before the white rope, meaning you need to reset the system more often.
Note - If you have 3:1 set up and and need more pull, making a compound 6:1, as we see below, is often a great idea. An example would be crevasse rescue on a two person team, when one person on top may have to do all the pulling. If you have a lot of friction from the rope running through the snow, and/or your partner in the crevasse is not able to assist you, the 3:1 is probably not going to work. Then, the 6:1 is going to be your best friend. Adding the 2:1 only requires one additional pulley and carabiner. Sweet!
Note: For a compound pulley system, you can add the very Crafty Rope Trick (CRT) of building a second anchor that’s farther away. This can allow you to completely collapse the 3 to 1 system before the 2 to 1 system collapses, which means you need to reset the system less often. Granted, this trick is probably more appropriate for professional riggers or maybe search and rescue teams, and not so much for climbers, but it’s still a pretty cool trick.
6:1 compound system, 2:1 on a 3:1.
Two different pulleys move at two different speeds in the same direction.
3 - Complex system
A complex pulley system is one that doesn't quite meet the definition of a simple or compound. A complex system has a pulley(s) that moves in the opposite direction of the load. Complex MA systems are okay, but a simple or compound system is usually a better choice, because they are generally easier to rig and require fewer resets.
Below we have a 3:1 simple system. With the addition of a friction knot (red) and carabiner, we now have a 2:1 pulling on the 3:1. Because this is a complex system, the two components are added together, giving a 5:1.
This is now a complex 5:1 system. When the rope is pulled, both pulleys move toward one another. When the pulleys touch (aka “collapse”), you need to reset the system. Probably not a problem if you have a large area to work in. But if you’re on a tiny rock ledge, you’ll only get a foot or so of lifting until the pulleys collapse, which is going to be a hassle.
Compare this with the compound 6:1 diagram just above. With the 6:1, you get a little more MA, plus avoiding the collapsing pulley problem, so that's why the complex system is usually not the top choice.
5:1 complex system.
A basic 3:1 with a red friction knot added, and the pull strand redirected through it.
Two different pulleys move toward each other at different speeds.
