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Old Dec 5th 2012, 09:51 AM   #1
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Problems with pulleys

My brother has a 4'x16' model train layout in his garage. He has constructed a counter-balanced pulley system to raise the layout to the ceiling. However, something is not right--it simply doesn't work.

I'll upload a 3D PDF of the system, in the hopes that all those people with WAY more understanding of the physics behind this stuff can give us some ideas as to where we're going wrong.

Addl info: We've verified that the cables circle the pulleys in the directions shown in the PDF. The large pulleys are approx. 6" diameter; the small pulleys are approx. 1.5" diameter.

Thanks, in advance, for any help you can offer.
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Old Dec 5th 2012, 10:07 AM   #2
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Please clarify some things. I'm assuming that the pulley system shown is attached to the ceiling, the "40's" on the drawing represent the weight of the train set, and the "120" is a counter-balance weight - correct? This should work if all the pulleys are equal diameter, but you say they're not; I can't tell from the figure which pulleys are "large" and which are "small," so please clarify that. Finally, you say the system doesn't work - but in what way doesn't it work? Does the train set fall to the ground? Smash into the ceiling? Does it take more than 120 pounds to keep it steady?
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Old Dec 5th 2012, 10:50 AM   #3
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Chip - First off, thanks for your reply. That was quick!

In answer to your questions:

1. All of the pulleys immediately below the ceiling surface are "large." The six pulleys down at the 40# loads are "small."

2. My brother started with the train layout resting on sawhorses. He then poured 120# of lead shot into the counterbalance container. Then, he pulled down gently on the counterbalance...nothing moved. So, he pulled harder. Nothing. So, he poured in more lead shot, until he had, like, 160#. Nothing. So, he stood on the counterbalance. Nope. So, he called me to see if I could make any guesses why it's not working.

I asked him to verify that, without any load applied, all of the pulleys and axles turned easily. He said they do.

I asked him to verify that the cables circled the pulleys in the directions I've shown on my drawing. He said they do.

Now, I will say that, since he lives in another state, I have to rely on his answers being correct. That said, I'm at least hoping the gurus of physics can verify that my drawing doesn't have any obvious flaws.

Thanks, again, for your help.
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Old Dec 5th 2012, 12:00 PM   #4
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It seems to me that this should work just fine. The size of the little green pulleys is immaterial. But since it doesn't work let me ask a few more questions:

1. Are you sure that the total weight of the train set (including the little pulleys) is 240#?
2. Are the large pulleys all firmly secured to the axles that they are mounted on? How so?
3. I know you said he verified that the ropes are oriented as shown in the figure, but just to be sure have him take a couple of photos and email them to you. The fact that he gets no movement makes me think that some of them may be reversed.

I would suggest trying the system with a much lighter load to trouble shoot it. For example start with, say, a 5 pound load on just one of the little green pulleys and see if the system works (i.e. the pulleys turn and the load rises and falls as expected). Then put 5 pounds on a second little green pulley, etc, working up to all six having 5 pounds each. Verify that with 30 pounds spread over the 6 points it takes 15 pounds to raise it, as expected. Report back as to what he finds.

Last edited by ChipB; Dec 5th 2012 at 12:13 PM.
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Old Dec 5th 2012, 12:50 PM   #5
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I'll let you know what we find out. Thanks, again, for your help.
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Old Dec 5th 2012, 02:07 PM   #6
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Glad to help - I'm really interested to see what the problem is. One additional thought: there is a net force acting on the middle axle of about 120 pound to the side plus 80 pounds vertical - I wonder if perhaps the bearings at each end of the axle can't handle it, and so it's binding? Another reason to try it with a lighter load first.
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Old Dec 5th 2012, 02:18 PM   #7
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I called my brother, and suggested he:

- Detach all 6 of the support cables from the table.

- Hang a 5# wt on ONE of the support cables

- Put 5# in the counterweight and see if it works.

I figure that, if it just happens that he's mistaken about one or more of the cables running the wrong way, this will uncover the error.

Again, thanks, and I'll let you know what we find.
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