Vertical caving terminology and methods > SRT basic terms
A way of lifting heavy weights, or creating a lot of tensioning force on a rope, such as when rigging a Tyrolean traverse. The loaded rope passes through a pulley, or some other device that can capture the progress, such as an auto-locking belay device. If using a basic pulley, then a second device can be added that can capture the progress, such as a strong prusik loop (or an ascender, but this can really damage the rope due to the force on the cam teeth of a normal ascender). After passing through the pulley or belay device, it aims back in the opposite direction, towards the load. It then passes through a second pulley, and turns back towards the original direction, similar to a letter Z. The second pulley has an ascender clipped onto the loaded rope, facing towards the direction of the load. When the tail of the rope is pulled, it pulls on the pulley and ascender, with the pulley halving the effort needed. It also pulls on the loaded rope via the first pulley. While there are some losses in the system, this can effectively triple the pulling power on the loaded rope.
During tensioning, the pulling force on the ascender and each of the two pulleys is twice the pulling force applied to the tail of the rope. If the tension is then captured (such as a belay device instead of the first pulley, or an ascender at that pulley), then the device that captures the tension will be subjected to three times the original pulling force. If the weight of an 80 kg caver is used to tension the system, then each pulley and ascender has to be able to cope with 160 kg during tensioning, and the progress capturing device has to cope with 240 kg. That is before adding the force of a caver using the Tyrolean traverse (assuming this is being used to tension a Tyrolean traverse). Additional pulleys can be added at each end to increase the pulling power further, and it is very easy to exceed the working load limit of the various pieces of equipment. However, it should be noted that the losses in the pulleys can be quite significant (90% efficiency is good for a pulley, while a carabiner manages only about 35-45% efficiency), and the progress capturing device is likely to lose some of the progress as it clamps the rope, so a significant amount of the tension will in fact be lost, and the pulling force will never be as high as the calculations here would suggest. For example, if the system uses a Petzl GriGri instead of the first pulley, and a 90% efficiency pulley, then the expected efficiency is only a 2.15:1 mechanical advantage, instead of a 3:1, because a GriGri has about a 28% efficiency. With the Petzl Mobile pulley used in the Z-rig illustrations, Petzl claim 71%, but measurements gave just 67% (66.66% if you want to split hairs). The Petzl Rig's efficiency was measured as just 25% (actually 24.6%) with a 10 mm rope, giving a total mechanical advantage of 1.84:1. The Petzl Rig also loses about 1.2 cm of progress every time it captures, but the exact efficiency lost there will depend on the ratio of that to the pull distance. The "change of direction" pulley was old and stiff, and only gave 61% efficiency when pulled at 90°, this reduced the efficiency of the entire system down to just 1.13:1. It helps to have good pulleys, and an efficient progress capture device! For the best efficiency, the most efficient pulleys should be as early on in the system as possible, closest to where the pulling power is put into the system. Therefore it was a very bad choice to use the stiff pulley as the "change of direction" pulley.
It is possible to put the progress capturing device at the end of the system where you pull, which reduces the amount of tension that the device has to hold to double the pulling force on the tail of the rope; useful if the tension in the loaded rope will be higher than the device is capable of. However, when using that approach, every component in the system has to remain active for the entire time that the tension is needed, and cannot be repurposed for anything else for as long as the tension needs to be maintained. That means that every component needs to be able to cope with the initial tension, as well as whatever new tension will be distributed through the system when a load is hung on the system afterwards (very important for Tyrolean traverses). The position of the pulleys and ascenders cannot be reset to add more tension, and although there are ways to do it using something called a piggyback, that again relies on having a progress capture device that can hold the full load, so it defeats the purpose of moving the progress capture device to the end of the system. If there was not enough rope in the pulley system to achieve the desired tension, then it all has to be taken apart, the main rope has to be shortened, the pulley rope has to be lengthened, and then everything has to be set up again. As a result, this is not normally used in caves where space is limited, and equipment is a precious resource. This approach is commonly used with tensioning systems for slacklines, where there is more space to work with. During tensioning, the progress capture device and each pulley should experience a force of double whatever force is used to tension the system (assuming the tensioning rope is pulled in a direction towards the load). After tensioning, the progress capture device experiences the force that was used to tension the system, while each pulley still experiences double that. If the load on the rope is changed (eg. by a caver hanging on the Tyrolean traverse), the force on each device is calculated as the force on the loaded rope, multiplied by the number of loaded rope strands that the device has emerging from it (2 for each pulley, 1 for the progress capture device), divided by the total number of rope strands connected to the loaded end of the pulley system (3 in this case, since it is a 3:1 system). It is also worth noting that efficiency is a bigger problem with that approach. With a Petzl GriGri and 90% efficiency pulleys, it is only a 0.76:1 mechanical advantage (meaning it actually loses strength rather than gaining it), because the least efficient pulley - the GriGri - is the first pulley that takes the pulling power in the system. In the illustration, the efficiencies of the actual devices in use were so poor that it only gave a 0.54:1 mechanical advantage; roughly half the efficiency that you would get from just pulling on the loaded rope!
Pulleys can be used in a variety of ways to increase the mechanical advantage further, or multiply the advantage. Multiple pulleys in a single housing are known as a block, and when these are used with more complex arrangements to gain mechanical advantage, the setup is called a block and tackle. These methods are not covered here. Find a good tutorial on pulleys and mechanical advantage.
A significant consideration is how to release the tension in the system, when it is no longer needed. In cases where the load will be removed from the rope, this is not a problem. However, when used for a Tyrolean traverse, the tension remains in the system, and the progress capturing device needs to be able to release the tension while under load. For this reason, it is common to use an auto-locking belay device, a Stop, or a prusik loop as the progress capturing device, even though these may either lose some of the progress when capturing it, or may introduce significant inefficiencies into the system. These devices can allow tension to be released by pulling handles, or pushing on a friction hitch. If using a friction hitch, it needs to be one that can easily release while under tension, such as the autoblock knot, but this then comes with the risk of accidentally releasing it when it was not supposed to be. It is also frustrating to use, as it needs to be manually moved into position, to capture the tension in the first place. Using a friction hitch puts fingers in a risky position where they could get caught between a lot of cords and pulleys that are under extremely high tension, and needs to be done with great care. If using an ascender or a progress capture pulley (both of which can damage the rope, as described above), it is normally not possible to release the tension using the device itself. If even more tension can be added to the system, then it might be possible to disengage the progress capture device while the tail is being pulled, but this might not be possible, or it might be considered too risky to have fingers near the system while tension is being added. To solve this, the rope or devices may be connected to their anchors using a slipped knot. This, however, is much more risky, since the tail could be pulled by accident, releasing the load. To avoid that, the loop of the slipped knot can be tied in a stopper knot until it needs to be untied. Another alternative is to connect the progress capturing device to a descender, which is attached to a short rope. The short rope is connected to the anchor, and the descender is locked off. It can then be unlocked when needed, and allowed to release the tension.
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