Pulley Tensioning Systems & Rope
Welcome to the world of pulley tensioning systems. Pulley tensioning systems (also referred to as pulley rig or just “rig” for short) have become the most widespread way to tension longer and tighter slacklines. These systems come in all shapes and sizes. They range from extremely bomber systems for high-tension longlines and tricklines, to lightweight rigs for travel. A variety of manufacturers offer pulleys and pulley systems, so it’s important to understand some basics before both investing in pulleys and rigging amazing slacklines. Understanding the different components and their limitations is essential to building a safe pulley and effective system. This information can be used to build your own system and/or upgrade your rig it as you go.
We’ll start with the basics of rope construction. Typically, ropes used in tensioning systems have two essential components; the sheath and the core. The core takes the most significant load in the rope and it’s wrapped inside the sheath. You should only see the core from either end of the rope. If you see the core in the middle of your rope, that’s a problem. We call that a core shot. The purpose of the sheath is to protect and contain the core. At this point, the rope has been so severely damaged that the internal fibers of the rope are exposed. Time to retire the rope!
You need sufficiently strong rope that is compatible with the other components in your system to reeve your pulleys with. Reeving is threading the rope through the pulley system to make the system operational. It is preferable to use static or low stretch rope. Using dynamic rope (used for lead rock climbing) is inefficient because when loaded, the rope stretches; essentially losing efficiency across the entire system. When possible, it’s nice to use thinner ropes because they are less bulky and run through the system more efficiently.
The downside to smaller ropes is that the mechanical components (ie. the rope brake) generally have an acceptable minimum diameter. So you want to make sure the rope you have will work with the devices in the system. Smaller ropes can also be a little rougher to handle at the final stages of tensioning. Pulling hard on a small diameter rope starts to hurt over time. A way around that is to use an ascender like a rope grab to increase your pulling abilities or use a large steel ring as a handle with the rope connected via a releasable knot.
Common pulley system configurations result in a 4:1 or 5:1 system of mechanical advantage. Because the rope runs through the pulley system, the breaking strength does not have to be very high compared to the webbing. Typically, it’s ok to get by with a 1/3rd of what the webbing is rated to when using the 5:1 configuration. Again, this allows for the use of rope with very thin diameter but remember to pay attention to the MBS (stated by the manufacturer).