Introduction to MA systems

 

First off, thanks to some Very Bright People who helped with these posts. High fives to Barry O’Mahony, Bryan Hall (with Rose City Ropes) Deling Ren, and Derek Castonguay. Thanks, friends!


I'm busy and I have a short attention span. What's the takeaway?

  • Start with learning 2:1 and a 3:1 until you can build them with your eyes closed. Every other fancy system is really just a combination of these.

  • Real world mechanical advantage will always be less than theoretical mechanical advantage. Sometimes a lot less - that “3:1” is probably more like at 2:1.

  • Use pulleys instead of carabiners when possible.

  • Redirecting the pull adds friction. Don't do it unless you need to.

  • A pulley on the anchor only serves to change the direction of pull. A pulley on the load or the load strand creates mechanical advantage.

  • The efficiency of ratchets is worse than you may think.

  • Redirecting the pull through the anchor magnifies your pulling effort on that redirect point.

  • Mechanical advantage systems can increase forces on the anchor.

  • Static ropes are more efficient than dynamic ropes.

  • A pulley with a larger wheel is slightly more efficient than a pulley with a smaller wheel.

  • Don't use more mechanical advantage than you need to get the job done.

  • Alpine climbers and big wall climbers have different needs and different systems

  • These blog posts are mostly arranged with easier topics at the top and more advanced topics closer to the bottom.


Got some time and a longer attention span and really want to learn this stuff?

Good, let's get started.

I'll always remember my first encounter with pulleys. In the rural area where I grew up, we had a neighbor named Ray, who was always messing around with cars. One day my dad said, “Let's go visit Ray tomorrow morning, I think he has something special to show us.”

The next day we walk over to Ray's house. He’s elbow deep in an old Chevy, that’s parked under an oak tree with a stout branch. I see some ropes going back-and-forth between the tree branch and the car, but that doesn't mean anything to me. After a few minutes. Ray hands me a rope end and says, “OK, start pulling.” I pull hand over hand on the rope . . . and watch in absolute amazement as my 8 year old arms lift the entire engine out of the car! The magic of pulleys had me feeling, for a brief moment, like a superhero.


Mechanical advantage (mostly referred to from here on as MA) which rather magically magnifies your pulling power, is one of the wonders of human ingenuity. Something as simple as running a rope back and forth between an anchor point and a load can somehow give you the power to lift a load many times your own body weight.

It's thought that pulleys were a key component of building the Egyptian pyramids, and since then have been used throughout history in all manner of clever applications, climbing included.

 
crane with pulleys.jpg

This series of posts takes a deep dive into MA, how it works, and how it can be applied to climbing situations. The goal here is for anyone reading this, especially non-engineering people, is to start simple, get a bit more esoteric, and by the end have a complete and solid understanding of MA pulley systems and how they apply to climbing. If you make through all these posts, you’ll be an expert!

Knowledge of mechanical advantage systems is not helpful only for climbers. White water rafters and kayakers use similar rescue systems, and this series of posts will be helpful for them as well.

Note that this post is NOT a rehash of the specifics of how to perform alpine rescues or big wall hauling. Those topics are already well described in many other places on the web and in print.


Let's start with some definitions.

  • Mechanical advantage, or MA: This is the magnification of your pulling effort. It's expressed as a ratio, such as 2:1, 3:1, and is pronounced “2 to 1” and “3 to 1”. The first number is the force applied to the load, and the second number, which is always one, is the effort that you apply to the rope. For example, in a theoretical 2:1 system, if you pull with 100 pounds, you can lift a 200 pound load. A 3:1 system with a 100 pound pull let’s you lift 300 pounds. Magic! 

  • Theoretical (aka “ideal”) vs Real World (aka “actual”)MA: On paper, in a frictionless world with perfect pulleys and ropes that don't stretch, MA systems work as advertised. But in the real world, not so much. Mostly due to friction, real world, or actual MA will always be less than theoretical, or ideal MA. (Meaning, that theoretical 3:1 may be more like a 1.7:1, ouch!)

  • Efficiency: Closely related to MA is efficiency. Think of efficiency as: how much of your effort is actually getting through to the load to do some useful work? High efficiency is good, because it means you need to do less work. (And let's face it, most of us are lazy.) In the real world, haul system efficiency is affected by a wide range of things, such as stretch of the rope under load, the quality of your pulleys, rubbing and twisting of the rope, how often you need to reset your hauling system, and a few other quirky variables.

  • Friction: the main and messy variable that takes nice “theoretical” MA and turns it into “real world” MA. With a 2:1 system, in theory a 100 pound effort can lift a 200 pound load. But in the real world, friction will always mean you will be moving less than 200 pounds. Depending on your choice of gear, it can sadly be a LOT less. In climbing, friction mostly comes from the rope bending to go through a pulley or carabiner, or the rope rubbing on a rock ledge or a crevasse lip. Friction is not our friend, and we want to try to minimize it whenever possible.

  • Fixed pulley: a pulley/carabiner that’s attached to the anchor. This pulley does not move, and serves only to change the direction of pull. It does not create MA.

  • Moveable pulley: aka travelling or “tractor” pulley/carabiner. This is attached in some way to the load or the load strand. This pulley moves as you pull the rope. It changes the direction of pull AND creates MA. 

  • Progress capture: aka ratchet. If you pull on a load, a progress capture means the load won’t slip back once you stop pulling. A progress capture in climbing is typically either a simple prusik knot or a pulley that is combined with a rope grabbing mechanism. These pulleys, such as Petzl “Traxion” series, are expensive, but very handy. Progress capturing pulleys are optional for alpine climbing but mandatory for big wall hauling. More on ratchets in a later post.

 

Below are two examples of a progress capture - the humble prusik and the Petzl Mini Traxion (shown open to illustrate rope grabbing teeth.)

progress capture: Petzl pulley and sterling hollow block tied as 3 wrap prusik.

progress capture: Petzl pulley and sterling hollow block tied as 3 wrap prusik.

progress capture: petzl mini traxion

progress capture: petzl mini traxion