Hello Everyone,
I have looked through the forums and found very helpful information regarding my subject. However, I could not find the "Post Reply" button. So I am sorry that I have to make a new topic. Well anyways I have a little cart. Its an Anchor Blue Cologne box. Its 3.5 inches width and about 5 inches length. I am using Milk caps for wheels. The axles of the cart is inside straw. The cart itself is fairly light. The objective of this project is to build a device that will transport a 1kg mass a distance of 6 meters using the energy released as the mass falls a distance of no more than 10 cm. The mass must stay together for the trip. I have some questions however.
1. Is the cart suitable for the project? The wheels is not straight but the cart can go straight for a distance.
2. How should I make the pulley system?
3. How do I connect the string of the pulley system to the axles of the cart since the axles are inside the straw?
I have some more but I have to think about them. Please help.
Just based on my physics knowledge since your 1 kg mass falls a distance of 10 cm it loses a certain amount of gravitational potential energy equal to about .98 joules using Pe=mgh. You can probably the effects of air resistance as they will not be appreciable. you can find the formula and related info here :http://en.wikipedia.org/wiki/Potential_energy. I'm not sure about the details of your system but if i understand correctly part of this energy will go to moving your cart and object( since your conversion from PE to movement or Kinetic energy will not 100 % efficient you'll get to use even less than .98 J). What will ultimately determine how far your cart will go is how powerful the force of rolling friction is. This resistive force will do work on the cart until all of its KE is used up. To maximize the distance your cart goes you should try to decrease the roughness of your wheels which will reduce the force of friction and also maximize the amount of energy converted from gravitational energy to kinetic energy. I have also attached a link discussing rolling friction :http://en.wikipedia.org/wiki/Rolling_friction. Just as a side note 1 joule of energy is not that much ( I'm not sure if your current design will take 6 meters or any design, unless your coefficient of friction is really low.) Since W=Fd Dividing your less than .98 by 6 would mean that your friction force would have to be less than .I5 N. If your cart's mass is negligible then your normal force is simply the force of gravity on your 1kg block (9.8 Newtons) your coefficient of friction will have to be less than about .01. If you have any questions feel free to post a response or it might help if you ask your schools physics teacher. I hope I was of some help.
Thank you for your help. I am not too concerned about friction, however it is a factor I cannot ignore. You are right the some of the energy will be used to move the cart. My questions though are
1. How should I make the pulley system?
2. How do I connect the string of the pulley system to the axles of the cart since the axles are inside the straw?
Thank you again. Help with the questions please. Thanks in advance.
I am actually unfamiliar with your project idea and am not sure what "pulley system" you are referring to. If you could post a more a detailed description of your device I might be able to help you further. I was actually unable to find your previous post. It returned a couple of dead links when I searched. Searching "Joule" on this forum may return a couple of helpful results.
I was able to find the links yesterday, but for some reason, I cannot find them today. I will try to look for the topic itself. Let me try to explain the pulley system better.
I want something like this. Except the problem is that I dont know how to build something that can hold it up. I know I have to get a small pulley system. But what can I do to make it stand up? Like the picture I want something that makes it stand off to the side.
If you are interested in supporting the pulley system you might want to try to rigging the top of the pulley to some large stable object. For example supporting the system by tying several strings to the top board and then connecting those strings to a strong ceiling using suction cups might support the entire system if it the pulley is not too heavy. You could also try building a superior structure to hold the pulley system but this might be more difficult. Try a couple of different set-ups(vary the connections,etc.) for the first example and see how far you get.
I was thinking more about the second option you gave me. I really dont know what you mean about the first option. The first things first I have to get a pulley. Should I get a small one or a medium sized one? I am also a bit worried the my cart is going to break right when the mass falls from the 10 cm mark. What can I do to fix that?
First I will explain the pulley system (as I see it in my mind) to make sure I understand it correctly. You will have a top board which on the bottom is linked to the center of a rotating wheel with a string. Then this wheel will have another string looped over it to which you will attach the weight. With respect to the first solution I offered this top board will have strings connected to its top surface as well. these strings which are connected to the top surface will then have their other end connected to a ceiling or a pole,etc. that it is above the entire system.
Consider this what will jump and jiggle around less when you drop the weight?A smaller or bigger pulley?
Unfortunately I don't know of any real trick to prevent your cart from breaking except making it stronger. If you want the amount of energy to be the same you can't really use a smaller weight without increasing the distance it falls. I would suggest experimenting with your 1kg weight and dropping from short distances to see how your cart reacts.
I now see what you are trying to say. I think that can work. Its alot easier in my opinion. I will have to try it. I think that all this can work. Now I am a bit worried about my cart. I know I have to make it stronger but I dont know how. Anyone else know? Thank you phi-unit for the pulley system idea.
Unfortunately reinforcing cart will probably add weight to it making it not travel as far.Ideally you want a low-weight high strength reinforcing material if you are concerned your cart will break. No material immediately comes to my mind that will be easily accessible. Once again doing a couple of test runs might help. If you could post some links relating to your project so I know it in a more holistic sense I might be able to help u further. I really only know in detail the parts you have shared with me. In the meantime I will ask any of the other experts whether they have any ideas.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
1. larger wheels - if your axles are crooked or tolerances not very good, the less times the wheels turn to go the 6 meters the better. Vehicles with small wheels tend to go off course. It's usually worth the weight to do large wheels, especially if you're making this out
of hand tools and not a machine shop.
2. Axle size compared to wheel size. If you have a 10cm drop and need to travel 6 meters and all you are doing is turning the axle with a 10cm of string, the wheels need to be pretty big compared to the axle. Ideally, 10cm of axle revolutions will equal 6 meters of wheel revolutions, then the weight won't crash into your frame until you've already reached 6 meters.
Most vehicles of this type I've built or seen built were rubber-band powered or gravity powered (roll down a slope).
I can guarantee you from experience that you want to build the thing quickly and test it over and over and over till you are sure it will behave in a predictable way. I saw a lot of pretty good designs fail at test time because the builder only did a couple trials and got lucky on those tests and missed underlying problems.
good materials for reinforcing - popsicle sticks, strong tough plastic (like you might get in a bleach bottle or even sealed packaging), cardboard is remarkably strong for its weight. Go to a hardware store and you can likely find long, light aluminum slats or rods.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
For the pully.....I'd actually probably use something like a big water bottle, maybe 1 liter. Attach it to the frame, chop it off at 10cm height and use it to support the whole pully infrastructure. Something round and made of plastic, already sturdy but easy to cut up with a knife or even scissors. The cylinder would be very stable and very strong and also very light.
This would be a lot easier than building a frame to hold the pully, and it has the advantage that the weight can't swing around much as your cart moves, but it does mean that your weight is going to be over the center of mass of your cart, not offset.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
You need enough weight over the axle that will be turned by the weight to not cause the wheels to "spin". The traction the wheels have is based on their material, the floor material and the weight over the axle. You can avoid wheel spin by widening the axle relative to the wheels too (gear ratios again).
If I was designing this I'd want the 10cm drop to be as much of the 6 meter distance as I could (it's got power for most of the trip) rather than relying on inertia to send it to the finish line, but I don't have a good sense for how well that would work. You need to get something basically functional as soon as possible so you can experiment and if your design allows rapidly changing wheel and axle sizes to experiment you will have an easier time. The 18th is not very far away.
Hi, i'm currently working on the same project, and i would like to know what type of string should be use to wrap around the axle and attached to the weight.
Thank you very much for the advice. Based on what you said, I have to start from scratch. Now I am coming up with the design of my cart. Here it is:
Wheels- CDs
Body-Undecided. Here are my options though: Wood (I dont know how big though), or Video Casette. I don't know which to choose. Any Suggestions?
Questions: 1. How do I make the axles? I need to know how to connect the CDs on each end and how to connect to the cart.
2. How do I wrap around the axles? A bit confused. Alittle clarification.
Clarification: 1. Can you clarify the pulley thing?
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
Any kind of string that will hold the weight reliably and that is handy enough to tie off should work ok. You don't want thick twine and if you use something like fishing line it might not hold the weight or might be too stiff to work with.
The main properties of string to worry about are strength, flexibility and "stretch"...if they have a lot of spring in them the weight will bounce up and down after it is dropped.
But almost anything should work. Ordinary kite string, for example.
Thank you very much for the advice. Based on what you said, I have to start from scratch. Now I am coming up with the design of my cart. Here it is:
Wheels- CDs
Body-Undecided. Here are my options though: Wood (I dont know how big though), or Video Casette. I don't know which to choose. Any Suggestions?
Questions: 1. How do I make the axles? I need to know how to connect the CDs on each end and how to connect to the cart.
2. How do I wrap around the axles? A bit confused. Alittle clarification.
Clarification: 1. Can you clarify the pulley thing?
Thanks in advance.
I am sorry. I dont like doing this. But can you answer my question please? Thank you very much and sorry for this repetitive post.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
Arg. I just posted a big long reply and it timed me out. Let me try to summarize.
0. I like the VCR cassette idea for a body. It's pre-built and very sturdy and easy to drill into.
1. CD wheels. Too narrow and slippery if you use just one to grip the ground well or to attach to anything. If you glue 2 or more together and do something like wrap a rubberband or electric tape around the rim, it might grip well enough.
2. Wheels to axle. Same problem. More CD's make it easier to grip, but CD's have this big hole in the middle. I read somewhere about somebody using wine-bottle corks to grip the inside of CD's...it's about the same size and cork can be compressed to fit and it then expands and grips...the same thing that makes it good for wine bottles would make it good as a tool for gripping the axle. After stuffing a cork into the CD assembly, you could drill out the middle a bit smaller than the axle and the axle would probably be gripped well enough.
3. Axle....consider those large, round pencils that grade school children use, or that are sometimes seen as souveniers. They're about the right shape and size and very sturdy. Larger diameter is good for gripping the cork well but won't be too big for a VCR cassette.
4. Pully assembly. Take a soda or water bottle, plastic, probably 1 liter or less. Cut the top and bottom off (knife or even scissors) so what you have is a cylinder. punch a ballpoint pen or similar round, smooth object through the top, with enough height so that the weight
can fall the right distance if the top of the weight is raised to the ballpoint pen position.
5. Attach the cylinder to the VCR cassette, with the pen positioned directly over the axle. You can probably just duct-tape the thing on, but if it isn't sturdy enough that way, you can take a larger bottle, make vertical cuts in the cylinder to the height you want it to really be, and fold the bottom out as flaps that can be used to screw the assembly to the VCR cassette or provide more surface area for the tape.
6. put the weight inside the cylinder and thread your string over the pen and down to the axle (you may need to drill a hole in the cassette for this purpose. There needs to be enough string so that when the weight is at rest, the vehicle can roll freely. This might be done by tying it onto the axle loosely, attaching it firmly (drill hole in axle, thread the string through the hole) but with plenty of extra string or not attaching it with anything studier than a bit of tape. You will have to experiment.
7. prime it for racing by rolling it backwards until the weight is at the proper height. Let go. Ideally the axle should turn, the wheels turn and it goes forward.
Troubleshooting #7
If it goes forward but stops too soon, try lubricating the axle. Graphite from a pencil or grease can help.
If the wheels spin without the car going forward, you don't have enough friction. The easy fix is more weight over the axle, but you may need to glue on more CD's to the power wheel to thicken the wheel and get enough surface area to get some traction.
If the CD wheels spin around the cork, you need either more CD'sor a bigger cork or both. (more CD's give more surface area to grip, a larger cork will squeeze harder against hte CD's.
If the axle spins inside the cork, you can try drilling a smaller hole in the cork or alternately drilling a hole through the cork and the axle and sticking a small nail or perhaps paperclip through the hole.
Thank you bradleyshanrock-solberg for the detailed instructions,
I am still confused about something though:
how is the cylinder placed on top of the vcr? Is it like that?
Are there 2 sets of wheels, or one?
[IMAGE]
Also, does the cylinder work as the pulley? Because instructions for me is the weight can drop no more than 10 cm. If it does, how do I wrap the strings? You lost me there
Last edited by dang.kristie on Wed Jan 09, 2008 5:26 pm, edited 1 time in total.
I cannot thank you so much for the very detailed instructions on this. I feel it a lot more easier now that you explained it a lot better. Thank you very very much for the advice on this.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
I wish I knew how you got that diagram on, it would be easier if I could draw. It is very hard to translate something visualized in your head to the same image in somebody else's head just with words.
You'll want four wheels, but the front wheels don't do anything but roll. I've seen tripod designs too (one wheel in front) but it's easier to keep it going straight with four large wheels.
In your diagram, the cylinder is off by ninety degrees. It should not be lying sideways on the car, but standing upright. You could also use something like a soup can, with the top and bottom removed with a can opener. That would be stiffer and stronger but might have sharp edges on the inside and wouldn't give the "flap" option for a more sturdy attachment. It might also be harder to drill or punch a hole through.
Likewise the "pen", that you drape the string and weight over should be parallel with the axle, above it. The way you drew it, it is perpendicular to the axle.
The overall positioning is about right. You want most of the weight over the rear axle, the axle that you're using as your power source. The front wheel and front part of the cassette is to keep it stable and help steer. I suspect it will be easily heavy enough to be stable with two wheels and an axle and the lever arm of the cassette, but if for some reason your cart does "wheelies" (front wheels lift up when you release the weight) you can just add a little extra weight in front to keep it all stable.
This whole assembly for the weight...there are a lot of ways to do it. I thought it would be simpler to use a frame already constructed (soda bottle cylinder) than to make it out of parts. Also you seem to be already heading down that route, eg using the VCR cassette rather than trying to build your own frame from scratch. It's a similar idea. Look around for something about the right shape and figure out a way to attach it that is easy but sturdy enough. At the scale you're working with (1kg weight, 10cm drop) glues might not be enough but wrapping it in enough duct tape or any kind of mechanical join (screws, bolts) should be strong enough.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
I cannot thank you so much for the very detailed instructions on this. I feel it a lot more easier now that you explained it a lot better. Thank you very very much for the advice on this.
You're welcome. Just be aware that what I visualized in my head and tried to communicate probably won't work exactly like how I imagined. The real world always introduces extra challenges. I think the basic approach is sound, but I've never actually tried to power a vehicle by dropping a kg weight on it! It should work more or less like I described, but real world engineering is different from theory. Just as an example, what if dropping the weight causes the axle/wheel attachment to buckle? You'll have to figure out how to make that section stronger, wheels wider perhaps...something. If the weight just hangs there and the cart doesn't move, you'll need to
figure out a way to grease the axle better...etc.
You will need to put it together and experiment a bit to find the right way. I can't stress enough how important it is to just try it out and play with it. If you can get anything that rolls at all, you can then fine tune it to get the best score.
If my experience with similar projects is any judge though, a lot of students will never get a working cart. The difference between those who succeed and those who fail is usually how early they tried to get a basic car built. You need time to test it, to fix problems as they come up.
Making pictures are actually really easy, I just use the paint programs and they give you shapes to work with
[IMAGE]
So would it be kind of like that? I get that, but where does the weight come into the design? There still needs to be a pulley, unless thats what the bottle and pen is for..but where does it drop to land on the cassette, and how to do you attach it to the axle and pen?
Last edited by dang.kristie on Wed Jan 09, 2008 5:26 pm, edited 1 time in total.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
ok, that's a pretty good image, although the "pen" should be higher up, not halfway down, and the perspective is tricky...it should be parallel to the axle.
The pen replacing a pully. While a pully is better from a friction standpoint, it is extremely complex. Adding a pully if you don't happen to have one lying around is similar to running the axle through a bearing sleeve instead of just drilling a hole through the cassette tape and letting the axle touch the plastic. While your friction is better, it costs a lot in complexity. Even if you have a pully you still have to figure out how to attach it. A pen you can just poke through the plastic (better to drill it if you can, for an even hole). Fallback plan would be a metal can, as the weight on the pen might pull down and tear or deform the plastic. If you get a clean hole drilled though, plastic is pretty sturdy on these scales.
A 1kg weight dropping is pretty powerful compared to the friction resistance. It should be sufficient to cause a string to slide over a smooth, round plastic surface like a pen and turn an axle (possibly greased or lubricated with graphite) against plastic. This is especially true because we don't really have to worry about long term wear and tear on the string or the axle or the cassette - aside from testing this will only have to run one or two times.
The weight goes inside the cylinder (I don't have a good sense of how big this weight is, width/length/height. this is one area where visualization can break down). It rises up as you wind the string, and drops down inside the cylinder as the cart goes forward. The string is tied to the weight, goes over the pen, comes down, goes through the cassette and is wound around the axle. (I'm not sure if it is good or not to attach the string to the axle, that's an area for experimentation. It worked when I did it with rubberband-powered cars but we also needed it to stop at a certain point, which is not an issue in this test...longer distance is better grade)
The connection between the weight and the axle therefore is the string. To work consistently the "pen" would need to be pretty flat (if it is tilted, the string will slide to one side, the weight rubbing against the wall of the cylender). Alternately you might groove the "pen" somehow so the string couldn't slide left or right even if the pen wasn't perfectly level (or wrap tape on either side of the string, getting a channel for it without letting it slide sideways) These little tricks are things you learn by doing. It might not be a problem at all or it could be a problem big enough to require more engineering.
Ok I think I understand most of it, but what do you mean when you say
It rises up as you wind the string, and drops down inside the cylinder as the cart goes forward. The string is tied to the weight, goes over the pen, comes down, goes through the cassette and is wound around the axle.
The weight goes through the cart? How is that possible when in the end, the requirement is that the weight stays with the cart? Do I have to cut a hole in the cassette?
Also, is the pen suppose to be drilled through the entire can or just one side? Thank you
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The weight drops onto the top of the cassette and stays there. This is especially likely as it is surrounded by a cylinder. The only hole drilled through the VCR cassette is for the string.
The pen goes all the way through the cylinder. There is no way it would support 1kg if it wasn't supported by both sides of the bottle/can. I think what may be wrong in your visualization is that you may be assuming the weight is outside the cylinder. The whole assembly is inside....string, most of the pen, weight.
It would work even better with a box-like shape instead of cylinder - more room inside and easier to keep stable when attaching to the cassette, but cans and bottles are common. I can't think offhand of a sturdy, hollow, light box-like thing of the right size that is just lying around the house.
If you guys had legos to work with all kinds of things are easier. The front wheels, for example. Lego wheels are great if they just need to spin around. There are lego attachments for round parts on axles too. 1kg is pretty heavy for legos but you could do a lot of the hard stuff with it. You'd likely want something else to support the weight, but maybe not if you made a big blocky cart. It would not be like those pictured in the link with narrow, high frame. The 1kg weight would tear that apart. But you could build a sturdy box-like container and drop it just like I've proposed here, with normal bricks.
Legos are very easy because they are designed to attach to each other, and you can build very sturdy structures if you attach a lot of holes to the pegs. A lot of what is hard in this project is the attachments.
Occupation: Software Engineer/QA Lead - Quality, Risk Assessment, Statistics, Problem Solving
Project Question: BS Caltech Engineering & Applied Science (Mechanical Engineering, Materials Science) Research in Traffic and Ceramic Composites 25 years doing IT, various roles, for multinational manufacturing company
The string should ideally be attached to the middle of the rear axle, and to only one axle. You drill the hole in cassette over the axle, maybe slightly offset behind it and thread the string through the hole (the string goes from the back of the axle to the back of the pen, preferably straight up),
The front wheels are only for stability. Don't attach any power source to that axle. The wheel/axle assembly can be entirely different if you want, as long as something up front supports the cart and turns, but unless there is too much friction from a rotating axle in front, it's probably easier to use the same approach on both axle/wheel assemblies.
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