Pulley friction

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Pulley friction

Postby bobby49 » May 16, 2020 4:32 pm

We know that larger pulleys that are made with ball bearings tend to have less friction. We know that smaller pulleys that are made with bushings (or nothing) tend to have more friction.

If I have two pulleys, is there an easy way to compare or measure the friction?
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Re: Pulley friction

Postby NZcaver » May 16, 2020 5:24 pm

If your pulleys are rated climbing/rescue/caving type pulleys, under specs they may list efficiency (i.e. lack of internal friction). Petzl does this. Some small pulleys with bearings are surprisingly smooth/efficient, like 91%.
https://www.petzl.com/US/en/Professional/Pulleys
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Re: Pulley friction

Postby bobby49 » May 16, 2020 7:55 pm

I have a variety of pulleys, and I need some test method or way of comparing pulleys.
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Re: Pulley friction

Postby NZcaver » May 17, 2020 7:30 am

bobby49 wrote:I have a variety of pulleys, and I need some test method or way of comparing pulleys.

Are they *rated* pulleys for life safety use, or other utility pulleys?

You could come up with a 1:1 or 2:1 rigging system with a rope and load, some form of winch with controlled speed/torque, and a dynamometer. But that's a lot of work.
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Re: Pulley friction

Postby bobby49 » May 17, 2020 12:20 pm

I have a variety of pulleys, rated and unrated.

It's hard to find a good dynamometer these days.
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Re: Pulley friction

Postby bobby49 » May 18, 2020 1:17 pm

Here is my thinking. Some pulleys are efficient and have nearly zero friction. Others have a lot of friction. Suppose that I have an inclined plane with a heavy weighted block on it. Then a rope is attached which goes over a pulley on top, and then the rope goes vertically down to a smaller weight. If the pulley is perfect, then the heavy block will tend to slide down. If the pulley has a lot of friction, then it will tend to hold the block without movement.
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Re: Pulley friction

Postby bobby49 » May 25, 2020 2:01 am

An old expert corrected some of my thinking. I was thinking of friction only inside the pulley, but there is also friction within the rope. The larger the diameter of the sheave, the less rope friction. Without too much expense, the force on the rope can be measured with a spring scale.
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