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Some pulley efficiency measurements

Tuomas Pöysti 2020

This is how I measure pulley efficiency. The method has evolved over time, but all values given here are from the latest iteration. I have carried out several different sessions, typically going through all of the pulleys I currently owned, and depending on interest of that time, different kinds of line materials, carabiners or bollards, descenders and such.

I’ll write individual articles on line materials and descenders, let’s focus on actual pulleys here. Quite frankly, I find this a bit boring topic. As we know, pulley efficiency is practically a function of sheave diameter, unless of course some very bad bushings are used. My pulleys tend to have ball bearings, so there is little to learn by measuring them. What is actually interesting is how line material, deviation angle or load factors in, but they are topics of other articles.

In this case, an 11mm semistatic (Petzl Grillon rope) was used. Here it is, a table of efficiency values from some of my studies:

To make things at least a bit interesting, let’s try how these compare to the “theory” shaped in this article. According to it, pulley efficiency (the same rope was used) should be

e = 1/(1 + 6.2/D).

Here are the plots of measured and theoretical efficiencies:

Some notes. CT Roll’n’lock and Camp Tethys Pro drop below the “expectation” value, Pro Traxion and OmniBlock 2.0 do clearly better than expected.

Roll’N’lock has some kind of bushings, barely that, I think. It performs 6 percentage points worse than the “theory” value suggests. To me, this is the most interesting data point of this study. And this is how pulley efficiencies should be treated: in comparison to expectation value based on sheave diameter.

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