Pulley Radius: how to factor this into calculations?

[LLogan]

Hi guys,

My team is thinking of using a winch for an elevator system. Naturally, we need a few pulleys. From what I learned in my physics courses, pulleys just redirect the direction of force/weight. However, we all know the AP curricula mostly assumes the "perfect physics world."

Theoretically, smaller pulleys have a lesser moment of inertia, but I am not sure if that even factors into our FIRST-scale applications. On the other hand, I have heard anecdotally that larger diameter pulleys are "better" (sorry, don't know the physics-related term I can give for this).

Please help enlighten some confusion on my team.

-Logan

[DonRotolo]

Pulley inertia can be ignored. Really small ones have to rotate faster, and friction increases a little. Big ones rotate slower, reducing friction effects.

In general, for most pulleys, you pick the size based on purely mechanical considerations: Does it fit the mechanism well. For an elevator, something in the 1 to 4 inch range (depending on the size of the elevator structure members) would probably be about right.

Unlike gears and sprockets, the size of pulleys used for an elevator system (those that are not being driven by a motor at least) doesn't matter much.

[theprgramerdude]

Would having a pulley with too small a diameter really matter in FRC-application type loads? I'm thinking of a scenario in which a high load on a tightly wrapped belt could cause it to suffer extreme fatigue and snap.

[LLogan]

Thank you very much for the input!

We are thinking of wrapping 1/8in steel cable around pulleys that are 1/2in in diameter. They need to be as small as possible to fit inside of our elevator assembly.

Our fear is that there is not enough "wrap" around the pulley, and pulling the wire around such a small diameter will be almost akin to pulling the wire across a sharp corner. I am not sure if I described this well enough....

[artdutra04]

Originally Posted by LLogan

Thank you very much for the input!

We are thinking of wrapping 1/8in steel cable around pulleys that are 1/2in in diameter. They need to be as small as possible to fit inside of our elevator assembly.

Our fear is that there is not enough "wrap" around the pulley, and pulling the wire around such a small diameter will be almost akin to pulling the wire across a sharp corner. I am not sure if I described this well enough....

What type of steel cable is it, and what is its minimum bend radius? The minimum bend radius will likely be your minimum pulley size.

[LLogan]

I don't know much about the cable other than the fact that it is 1/8inch in diameter and was taken from one of our mentors' planes.

[artdutra04]

Originally Posted by LLogan

I don't know much about the cable other than the fact that it is 1/8inch in diameter and was taken from one of our mentors' planes.

How many strands does it have? The more strands it has, the more flexible it'll be.

[Teched3]

Minimum bend radius is very important - the more strands, generally the smaller the acceptable radius. Have you investigated Spectra cord? Very flexible, light weight, and stronger than steel cable for a similar cross section. Teams have been using it for years.
Here's an EBay item number to 1/8" cord at a very good price---180614836330

[IKE]

If you must have that small of a pully, you might want to look into Spectra or other performance kite strings. You can get a roll of 450 feet for about $100. Really great stuff.