School Project

[stopimin]

Can you guys help me please?
I need to be able to use a motor and a 9V battery to move up a string that is about 20 feet long at a 30 degree incline. i need ideas. please remember that the battery and motor must move with the load, and cannot be a pulley system with a motor at one end. To get full credit it must move to the top in less then 15 seconds. Thank you

 

[dknguyen]

So what? THe climber can't spool up the rope inside of it? Just make two pulley wheels that clamp on the rope where one or both of the wheels are motorized.

 

[stevez]

Are you supposed to provide a design/explanation or are you to actually build it? Reason I ask, if it's design only and you've not been given a budget then you have some freedom. If you actually have to build it then your design will be limited to what you can construct and afford.

You've got enough information for a start. If you are going to use a wheel with some means to assure traction with the string then the velocity is pretty well defined. 20 feet in 15 seconds - that's 1.33 ft/second. If you know the size of the traction wheel then you can establish the rpm.

Torque will be a challenge but I'd take a theoretical value and double or quadruple it. The challenge is that torque comes from the weight you lift - and you don't know the weight just yet. If you are required to use a 12 volt battery and only that then pick one. If "string" is what you are to use then I'd guess that they mean string and not rope - so whatever this is must be relatively light. You are "lifting" at a 30 degree angle - that's easier than straight up.

IM me if you like and I'll help you thru this. Keep in mind that I/others won't do the work for you - just help get you unstuck.

 

[Sceadwian]

A pair of spring loaded wheels spinning in opposite directions like a cable car tram. Stupid simple, very fast. Just use concave wheels to keep the rope in the center. Just a suggestion, spin the wheels on a power drill or dremel tool and use sand paper to carve the concave shape, you don't need much.

 

[stevez]

A rubber band or short length of inner tube on the periphery of one wheel may provide some give and some traction - the other wheel being concave as suggested.

I did give some thought to a trap that might have been set but my intuition and a quick calculation suggests otherwise at this point. A battery has some weight and has a finite limit to the amount of current/power it can deliver - therefore there is a boundary, in terms of upward velocity, for each battery. The trap would be to specify an upward velocity that it outside the bounds of any battery (to lift itself). My rough estimate may be incorrect so be careful. The basic definition for horsepower might provide a good start for this analysis.

 

[Triode]

If you can, id wrap the string around the wheel. I happen to work at a place that makes large cable driven machines, and the cable wrapped around the wheel two or three times on a pass through is a common mechanism.

 

[stopimin]

Thank you for your ideas.

A problem that i am concerned with facing with the 2 wheels is that if it gets off balance while moving it will fall of the string. If i wind it a couple times around a wheel, then the other wheel will have nothing on it and just cause it to be dead weight wont it? Also how do i stop the actual motor from spinning instead of the gear (the stick on some motors, sorry dont know name) at the end. Do i need to have more weight in the loadage?
thank you

 

[Triode]

as for one of your questions, if you wrap it around the wheel, you can wrap it around the other one too. Infact in some machines two rollers directly over eachother have a cable figure 8 around them, Its hard to explain without a sketch, but --8-- the rollers turn opposite ways like they were interlocking gears, the string comes into the middle, up around the top one, though the middle, down through the bottom one and through the middle again. They should be as close together as posible. The advantage of this system is that it eliminates a problem where the string tries to progress to one side due to its coiled shape on a turning drum, then hits the side, climbs over itself and gets caught up. With this method you can coil it one way on one wheel and the opposite way on the other, so it wont climb as much, also you only have to loop it once. We use steel drums and steel cables, but I dont see how it wouldent work with string and a plastic wheel with a rubber rim. A broken printer would be a good place to look for rubber rim wheels, gears, spacers and axels that you need.