Pulley system progression – is it worth it in practice?
A pulley system progression is when you go from a small to a large mechanical advantage (MA). For example, from a 3:1s (s means simple) to 3:1sCD (CD means Change of Direction) to a 5:1s, to a 9:1c (c means compound).
Note: You need some prior knowledge of Pulley Systems to get the most of this article. You can download a free 10-page guide (Pulley Systems 101) here to get you started: overtheedge.training
- the load,
- the number of haulers,
- the distance to be hauled and,
- friction.
The questions – on Pulley System Progressions?
These are the two questions I had regarding Pulley System Progressions:
- When you go from one pulley system to the next in a progression, is it worth it in practice, i.e. increase MA/decrease the force for the haulers?
- Is there any pulley systems in the progression that stand out as more or less efficient than the others.
The theory behind Pulley Systems
We used the T (tension) method to work out the MA assuming 100% efficient pulleys.
Here is an example of how to do this for a simple 3:1. Use the same method for any of the previous pulley systems.
We also used the T method to work out the MA with 90%(0.9) efficient pulleys. Note: This 90% is very close to the Petzl Partner and Mini Pulleys manufacturers recorded efficiency at 91%.
Here is an example of how to do this for a simple 3:1, which now becomes a 2.71:1 with the additional friction created by the pulleys. Use the same method for any of the previous pulley systems.
The testing equipment
- Load cells: x2 – Rock Exotica/CMC Enforcers, accuracy: +/- 2%, records 0.02kN increments
- Rope: 9.5mm Kordas Fina semi-static Type A (EN1891), 4.1% elongation, Breaking 26.3kN
- Pulley system pulleys: Petzl Mini and Petzl Partner (91% efficiency)
- Redirect pulley: Rock Exotica Large Omni Block
- Rope grabs: Petzl Tibloc, Petzl Basic
- Load: rocks in an Aspiring 75litre PVC Bag
- Premade 5:1 pulley system
- One person applying force to the end of the pulley system – via a cowstail on a harness.
The setup
The method
- All load cells were reset and checked reading 0.00 before the testing began and in between each test.
- With the premade 5:1 pulley system, we tested the force being applied on the end of the pulley system after the redirect pulley (0.92kN)
- For the rest of the testing (as shown above), I walked back monitoring the load cell, reading both max and actual to make sure they were similar.
- The load was hauled off the the ground around 1m before stopping each test.
The results
The MA has been calculated using an excel spreadsheet and rounded to 2 decimal places at the end but not during the calculations.
- MA 100%: this means the Mechanical Advantage if there was no friction loss in each pulley – 100% efficient.
- MA 90%: this means the Mechanical Advantage if there was 10% friction loss in each pulley – 90% efficient.
- Haulers 100%: this means what the haulers have to pull if there was no friction loss in each pulley – 100% efficient.
- Haulers 90%: this means what the haulers have to pull if there was 10% friction loss in each pulley – 90% efficient.
- Actual: means the ‘Actual’ force pulled when tested. Max force recorded. As the force on the end of the pulley system was 0.92kN, I multiplied by 1.087 to convert to 1kN.
- Difference: means the difference between ‘Haulers 90%’ and ‘Actual.’
Conclusions – about Pulley System Progressions
Answering the two questions I had regarding Pulley System Progressions:
- When you go from one pulley system to the next in a progression, is it worth it in practice, i.e. increase MA/decrease the force for the haulers?
- Given the accuracy of the Enforcer and the test method, the ‘Actual’ results were close to the predicted 90% efficiency of the pulleys.
- The results showed that you do get additional benefit by undertaking a progression with efficient pulleys.
- As part of the system (the hauler), I could feel the amount of force change.
2. Is there any pulley systems in the progression that stand out as more or less efficient than the others.
- There is little difference between a 5:1cx and 5:1s despite one having 3 pulleys and the other 4.
- The 5:1s (4 pulleys) has both travelling pulleys attached to the same rope grab, and therefore the MA (2x 2:1s) is effectively acting directly on the load.
- The 5:1cx (3 pulleys) has one of the travelling pulleys MA (2:1s) going around a fixed pulley before acting on the load.
- Recommend you limit the use 3:1sCD as an option to pull in the opposite direction, e.g. downhill. In the test results, 0.48 (48%) of the load had to be applied by the haulers. Consider using the 5:1cx for pulling in the opposite direction.
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