The Anchor Point

Ideas that hold up under load

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Hey Reader

Both are 5:1 mechanical advantage pulley systems. The one with three pulleys should beat the one with four. That's the logic, but what does the theory and testing say?

If you've moved past the standard 3:1 and started building 5:1 pulley systems, you've probably met both versions. The 5:1 simple (5:1s) is the one most of us learn first: built off the anchor with four pulleys. The 5:1 complex (5:1cx) is the one you build with a second rope grab, and haul back in the opposite direction, using only three pulleys.

Fewer pulleys means fewer friction points, so the complex system should be the more efficient hauling. That's the assumption you would make.

Let's have a look at the numbers.

This is easiest to see with the Tension(T) method, the standard way to work out mechanical advantage. Start with 1T (one unit of tension) where the haul team is pulling (indicated by the hand on the diagrams). Trace that tension through the system pulley by pulley. Where the tension joins at a rope grab, add the T's together.

Once you account for friction at 90% pulley efficiency, the 5:1 simple comes out at 4.1:1. The 5:1 complex comes out at 4.15:1. Barely any daylight between them, despite running one less pulley in the 5:1cx. Note: Practical testing with a load cell backed up this theory finding.

Here's why.

In the 5:1s, both travelling pulleys attach to the same rope grab and pull directly on the load. In the 5:1cx, one of travelling pulleys has to go around a fixed pulley first to get back to the load, and that redirect has its own friction cost. It eats almost exactly the saving you'd expect from dropping a pulley. Three pulleys doesn't get you a more efficient system just because it looks leaner on the ground.

Which means the choice between 5:1 simple and 5:1 complex isn't really about raw efficiency. It comes down to:

• the direction you want to pull; towards the edge or away from the edge,
• are you're carrying extra equipment either a pulley or a rope grab,
• the skill to rig it correctly, and
• you accept the drawbacks of this complex system are worth it; more load on the anchor and more resets as the rope grabs are coming together reducing the effective haul field

I've written up the theory and full load cell testing across the whole progression, from 3:1s up through the 11:1cx, with the numbers for each system side by side, in the link below.

If you've got a system you want load-cell tested, or a comparison like this one you're curious about, reply and let me know. That's usually where the next one of these posts comes from.

Grant

👉 Pulley Progrogression Efficiency Testing