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December 21, 2010 10:45 AM   Subscribe

A 600-pound, 24-foot-long retractable movie screen is suspended from two chains attached to an overhead pipe. I want to get it down. Come inside for the details.

The chains attach one foot from either end of the screen. I want to lower one end of the screen at a time by attaching a rope to that end of the screen, throwing the rope over the overhead pipe, and slowly letting out the rope until that end is on the ground. Then I'll repeat. How much weight is that rope going to have to hold at any given time? Is it safe to assume 300 pounds at first, and then steadily less? I should have paid more attention in my math classes. (This is sort of academic, as we're going to use a rope that can support the entire weight, but I'm curious about how this works.)
posted by goatdog to Science & Nature (17 answers total) 1 user marked this as a favorite
 
You are going to be putting extra stress on the other attachment. Was this engineered to begin with, or just hung up there? How is the pipe attached to the building structure?

The safest thing would be to rent a scissor lift and go up there and detach the chains and let it ride the lift down (if it does not crush the rails).

What you are doing MAY work, but I would want to have some people there, albeit way out of the way.
posted by Danf at 10:50 AM on December 21, 2010


IANAEngineer, but I wouldn't expect the load on the rope to change as a given end is lowered.

Hopefully you'll keep out from under this thing, and use something other than your body weight to control the rate of descent. If it starts to get away from you, for even a moment, you are unlikely to regain control until the thing hits the ground.
posted by jon1270 at 10:57 AM on December 21, 2010


The load will always be 300 pounds per side. Lowering one side doesn't change that until it touches the floor. The one possible exception is if the one foot section past the remaining chain touches the pipe or ceiling - then you've got a third point of contact.
posted by plinth at 10:57 AM on December 21, 2010


The overhead pipe is attached to a steel I-beam via metal brackets every three or four feet; these appear to be welded onto the I-beams. I think the entire apparatus (pipe, chains) was put there specifically to hold the weight of the screen. The screen hangs about six feet below the pipe, so it won't come into contact with the pipe at any point. I'll have at least 500 pounds of humanity on the other end of the rope, which will be at least 20 feet away from the screen's downward path. There's unfortunately no way to get a scissor lift into the space, and no budget for one anyway.
posted by goatdog at 11:12 AM on December 21, 2010


Why screw with a working geometry? Lower both sides at once.
posted by DU at 11:26 AM on December 21, 2010


It seems like that would double the number of people involved and also require two groups of people to more or less work in concert, which would increase the chances of a screwup.
posted by goatdog at 11:34 AM on December 21, 2010


Could I talk you into getting a pair of chainfalls for this operation? You should be able to rent them if you don't want to buy.

This will let you lower the screen to the ground in a controlled manner - at no time will you need to rely on human power to support the full weight of the screen.

You would want to attach both chainfalls to the support pipe, and then onto the screen. Lift slightly and remove the original support chains, and then lower the screen down evenly.
posted by davey_darling at 11:56 AM on December 21, 2010 [4 favorites]


500lbs of people, but they will not be tied to the rope. They will be relying on arm and finger strength. You can't co-ordinate that. You just can't. You will need to consult and probably hire a piano mover.

Doing this without a block and tackle and winch designed to lower heavy weight from considerable height is suicidal. A rope tossed over a pipe is not a proper tool for moving heavy equipment from high places - someone will get hurt, and seriously, if you half-ass it. This is not an askme kind of question, this is a professional mover kind of question.

If it's not in the budget to do it right, don't fucking do it.
posted by Slap*Happy at 11:59 AM on December 21, 2010 [4 favorites]


Came in here to urgently recommend some sort of rigging, and am glad to see that others have also chimed in already. Remember, if something goes wrong, someone could get permanently injured from this sort of thing. You need the mechanical advantage that block & tackle and winch when dealing with a load like this. Old stagecraft prof made us assume weight of moving object x 10 to account for dynamic loads. I would make sure all of the elements of your rigging system are certified for at LEAST 6000 lbs. to be safe.

I have seen a rigging accidents and they are not pretty.
posted by smirkette at 12:26 PM on December 21, 2010 [2 favorites]


You didn't say how high this screen is off the ground or how big of a space you have to work in or if you can move some bigger equipment in the room. All important questions that could make it an easier task.

I would erect scaffolding to height and then support it on the scaffolding. Tie a block and tackle on each end from the supporting pipe. Each block and tackle would then be temporarily tied down while the scaffolding is removed. Then all you need is one person on each block and tackle to lower the screen. To be on the safe side you should have two people on each rope.
posted by JJ86 at 1:44 PM on December 21, 2010


The missing step is to remove the chains after the block and tackle are tied down^^.
posted by JJ86 at 1:45 PM on December 21, 2010


I am a structural engineer. Please don't go forward with your plan as it is, there will very likely be unfavorable consequences. Slap*Happy is absolutely right about this needing professional input from people experienced in moving heavy things on ropes, and a piano mover would be a great person to consult. Doing it without professional input is a Very Bad Idea. 500 pounds of men (which is what, 3 guys?) isn't even 20% of what I would choose, if I had to do it in a similar fashion, and I would never lower one end at a time when I have an option to lower it evenly.

plinth is inaccurate in the statement above: "The load will always be 300 pounds per side. Lowering one side doesn't change that until it touches the floor." The load does indeed change as one side is lowered. I could give you a physics explanation if you want, but in a more common-sense fashion: when you're carrying a couch up or down a flight of stairs with one man on each end of the couch, which side would you prefer to carry?
posted by hootenatty at 3:44 PM on December 21, 2010 [1 favorite]


In addition to the really good advice above re: rigging and safety margins, I'll add that you should never have any people where the object could land on them if it broke loose. That means thinking about how it could fall -- it could drop straight down, but it could also snap loose on one side first and swing out and down. So your safety zone is going to be quite large, and that really complicates your plan for swapping out the ropes mid-lower.

Please don't half-ass this. Ropes and gear are quite cheap, especially compared to even the most minimal of hospital bills.
posted by Forktine at 5:15 PM on December 21, 2010


hootenatty - I invite you to educate me. I thought about this picturing the force vectors in my head. I don't see the couch analogy as accurate - if the upstairs person stops lifting because they're canted over the stairs - sure the forces change, but in the chain case, there is an implicit horizontal component that comes into play and the fixed chain is no longer vertical, right, so the force projected along the chain can be thought of as a vertical component and a horizontal component, the H component increasing as the variable side is lowered, but the projected force should still be 300lbs.
posted by plinth at 5:28 PM on December 21, 2010


Plinth: when you get to a certain angle, one person is basically pushing the top of the couch sideways and the other is bearing almost all of the load.

Goatdog: I'm just here to add to the "do this right" pileon. Don't forget that you'll have to lift both ends of this up a few inches to detach the chain, and that it will need to be safely tethered when you do. 11mm mountain-climbing cord is rated about in the right area code for the kind of safety factor you want here, and 600 pounds doesn't have to fall very far to kill a guy.
posted by mhoye at 9:24 PM on December 21, 2010


11mm mountain-climbing cord

Probably not a good idea. Climbing rope is stretchy, because that elasticity helps when you have a fall; that stretchiness is not what you want in this kind of situation. You want static line, but with a huge safety margin to account for the dynamic loads. Remember, you aren't just allowing for 600 pounds of load just sitting there -- you are allowing for the dynamic loads for when those 600 pounds swing sideways or suddenly drop a foot.
posted by Forktine at 5:39 AM on December 22, 2010


Wow, I'm glad some of the helpers didn't show up and we had to reschedule. We're going to look into chainfalls, with input from a professional installer who's dealt with this kind of thing before. Thanks, folks, for ensuring that nobody I know will die in the process.
posted by goatdog at 8:51 PM on December 22, 2010 [1 favorite]


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