# Homework Help: Design of a Simple Weight Transportation System

1. Nov 18, 2015

### Cal-cool-us

Hey guys, hope this is in the right place first off, but anyway, for one of our engineering projects we are required to design a "rover" that can transport a maximum of 40 pounds (the increments in weight increases by 10lbs per run starting at 10lbs and ending at 40) a minimum distance of ~20 feet. The constraints are that it can use no electric power of any form or any type of fuel, compressed gas, combustion, etc. It must also fit a profile of 2'x2'x3' and the total cost cannot exceed 80\$.

The solution I have in mind currently is that I would simply take the concept of how one of those rubber band cars work and try to apply it in a scaled up version. Essentially I was looking at 300lb resistance bands that are wrapped around an axle to create tension in the band and when the tension is released the axle rotates thus causing the wheels to move.

The main problem I am facing currently with this design is firstly, to withstand a perpendicular force of 300lbs I would need a very thick axle which would have a high circumference and so I would need more elasticity in the bands to be able to wrap around the axle enough times to cause enough rotations of the wheels to travel the distance that I need. Would my concept work in practice? Or is there a better way to go about this that I am not thinking of? I appreciate any and all help that can be provided. Thanks guys!

Last edited by a moderator: Nov 18, 2015
2. Nov 18, 2015

### Staff: Mentor

Welcome to the PF.

Can you post a sketch of your concept? I'm having trouble visualizing it.

Also, can you just use the potential energy of the weight to power the rover? Once on the vehicle, the weight slowly falls through your max allowed height and you convert that motion into horizontal motion via gears of some sort...

3. Nov 18, 2015

### Cal-cool-us

Thank you for the welcome! My drawing skills are pretty horrendous...the extent of them amounts to little more than stick figured I'm afraid. I can try to help you visualize it by explaining better perhaps. Think of a rubber band powered car. The rubber band is usually anchored at one end of the car and is stretched towards, say, the rear axle. The axle is then twisted to wrap the rubber band around the axle and once let go, the rubber band tries to release its tension which involves unraveling itself from the axle and causing the wheels to turn. If you skip to around 10:30 of this video, you can see a better visualization of how it works in principle:

Now, obviously to get the kind of force I need for this project, I need to scale it up, though this is where I am unsure if a 300lb resistance band would do the trick. I could always add a second band, but at that point I am worried the force on the axle would be too great. Do you think I would be able to get the type of results I need from this method? As for what you are proposing, using the potential energy seems like a very good idea, though with the lighter weights such as 10 and 20 lbs I am not sure it would have enough energy to move the rover 20-30 feet especially depending on how much the rover itself will weigh and considering the maximum height I could achieve is 3 feet (probably going to use wood for the rover since it is much easier for me to work with). I am also admittedly bad at actually designing and building things for the most part due to my lack of experience, so I am unsure if gearing it is within my limited skillset, though if you could explain to me a general idea of how to go about it, that would be of great help as well. Thank you for the input so far, though, it is greatly appreciated!

4. Nov 19, 2015

### CWatters

Using the PE of the load means that the heavier the load the more energy you have available to move it. Nice.

What is the surface like? If it's smooth and you use hard tyres you won't need much power to move it.

Is it a timed run?

5. Nov 19, 2015

### Cal-cool-us

The surface is said to be "...mostly smooth and paved though there will most likely be minor height variations simulating possible terrain on Mars..." which is a bit gimmicky but I digress, the run is timed but time matters only in determining the overall winner of the competition. Winning would be nice and all but I have been so bogged down with other work I don't have much time left to get my prototype tested and all the bugs worked out (One day I will learn to stop procrastinating!). I like the idea of utilizing the actual weight of the payload to propel the vehicle, but I am just unsure how to implement it in practice. With only 3 feet of height to work with my total potential energy (especially with the lighter loads) I worry would not be sufficient without extensive gearing and figuring out proper gear ratios. My concern is increased even more considering I have NO idea how I would even begin to implement gears into such a system since I have never really worked on/designed something that utilized them.