Engineering Fair project

[CraigD]

Originally Posted by hongher2005

Alright, thanks.. also, i saw the spool, but i dont get how it works...

Pardon the crappy mouse-sketch, but hopefully even a badly drawn picture can communicate the idea better than words.

[freeztar]

It's too bad you can't cut the rubberband.

Does anyone know if there's a certain spool size that is the "sweet spot"? It seems that you'd want it as small and light as possible. Maybe a wine cork?

[Turtle]

I read the rules. Flywheels encouraged I hear. Substitute the stretched rubber band wound on a spool for the metal spring in this design.

http://links.jstor.org/sici?sici=004...3E2.0.CO%3B2-X

[hongher2005]

hmm.. Im starting to think legos are heavy.. what would happen if I could make the car out of like a plastic bottle or somethin..?

[freeztar]

Or balsa wood...

Just remember that the car must be able to carry 1lb of weight and it needs rails or some other form of containment to keep the baking soda box from falling off.

[CraigD]

A unusual feature of this contest is that it is to take place on “on a level, industrial type, carpeted floor”, contradicting the usual approach of minimizing rolling resistance by using very thin wheels. Another is that the car must carry a 1 lb (0.45 kg) load, contradicting the usual approach of making it very light. As with most contests of this sort, all cars have the same amount of available energy (possibly – see below), using the same type of fairly small rubber band.

Let’s look at the basic mechanics of this.

The amount of energy the car takes to move a distance over the carpet is given by the formula , where is the force required to move keep the car moving, and is the opposite force to the force of friction. As a general rule, rolling friction is proportional to speed, so the faster the car goes, the shorter the distance it will go.

So the design goal should be to have the car move as slowly as possible, without stalling (stopping) due to some small obstacle or imperfection in the carpeted floor or its wheels or bearings.

This force will come, of course, from the rubber band (or possibly some more complicated scheme where energy from the rubber band is stored in another form, such as a spinning flywheel, though since every transformation of energy in a real machine involved some loss due to friction, I’d be inclined to Keep It Simple). The rubber band, however, provides a varying amount of force at different stretched lengths, making things more complicated.

Good first steps in the design process are to measure the forces involved

For , this means building the car without the rubber band drive, putting a 1 pound mass on it, and pulling or pushing it across a carpeted floor as identical to the one to be used in the competition as possible with some sort of force-measuring device – a scale, in other words, the more sensitive, the better. Since most scales are made to measure vertical, not horizontal force, getting a working test rig may involve a bit of a construction itself.

For the rubber band’s force , you need to get a plot of it at different stretched lengths. You could use a scale for this, too, of reverse the process and hang weights of known mass from the band and measure how far it stretches.

Once you know the force of the rubber band and the force required by the car, you can calculate the necessary size of the transmission spool and the wheels . For the car to move, . How much greater? Make a guess, then test it.

Here’s the fun and complicated part: remember that, as the rubber band pulls the string to turn the spool, axle, and wheels, if gets shorter, and the force it exerts decreases. So, to get the most efficient transmission, needs to vary as the thread is pulled from it.

There are several ways to do this, some involving gears, some not. You could go with a fixed-ratio transmission, avoiding this complication, but if you do, you’re likely to lose to a more efficient design.

Another primary design goal is for the car to minimize , by making the axle bearings as well as you can (ball bearings, etc), large wheels, etc. A truly weird configuration might do well – “thinking outside the box”, combined with making and testing lots or prototype models could allow you to hone-in on a winning design.

Last, keep in mind a rubber bands can store energy other than by being stretched lengthwise – they can be twisted. Figuring out the best way to use the band will take imagination and experimentation. It’s possible, if the rules permit it, that conditioning the rubber band with pre-stretching, oils or balms can increase the energy it can store. Other tricks are likely possible, and within the rules.

I imagine the winning car will be an interesting, and possibly surprisingly weird machine. Best of luck to you, hongher – may the winning car be yours (or your team’s)!

[Turtle]

Originally Posted by CraigD

A unusual feature of this contest is that it is to take place on “on a level, industrial type, carpeted floor”, ...

Here’s the fun and complicated part: remember that, as the rubber band pulls the string to turn the spool, axle, and wheels, if gets shorter, and the force it exerts decreases. So, to get the most efficient transmission, needs to vary as the thread is pulled from it.

There are several ways to do this, some involving gears, some not. You could go with a fixed-ratio transmission, avoiding this complication, but if you do, you’re likely to lose to a more efficient design
....
I imagine the winning car will be an interesting, and possibly surprisingly weird machine. Best of luck to you, hongher – may the winning car be yours (or your team’s)!

Aye, there's the rub! The carpet. The long & the short of it is that the vehicle needs a granny gear. Speed is not a component of the contest, rather it is distance that wins.

So, how to accomodate that ever-changing pull of the rubber band with a granny gear? Why with a flywheel of course, which evens out that irregularity. Is making it hard? Of course! But we don't do these things because they are easy, we do them because they are hard.

Virtually all wind-up action toys use a flywheel and gears arrangement, and for one simple reason; it is the most effective and efficient. I can't find any other drawings but the one I posted earlier, but take any little wind-up toy you have and take it apart to see how it works. Measure the parts, count the gear-teeth, wind it up and let it go as you watch how it works. One you have internalized the system, apply the principles to your car.

Since the rule writers put 'flywheel' in, then that's what they expect to see.

[hongher2005]

hmm ill work on it..

so the flywheel.. is there any specific way it works? or do i just add it to the car instead of wheels.. or is the flywheel inside the wheels?

[hongher2005]

Originally Posted by CraigD

Pardon the crappy mouse-sketch, but hopefully even a badly drawn picture can communicate the idea better than words.

So in the drawing, the blue is the thread correct? and the rubber band is pulling the thread, spinning the wheels right?

also, would putting something above and below the rubberband, slightly holding it, reduce the energy it gives? it would definately reduce speed..

[CraigD]

Originally Posted by hongher2005

So in the drawing, the blue is the thread correct? and the rubber band is pulling the thread, spinning the wheels right?

Yes.

You can try this basic scheme on any toy car you can find that has wheels solidly attached to an axle. Duct tape can be used to attach a thread or string to the axle, and can be layered to make spools of different sizes. You can skip the thread/string and attach a rubber band (especially one bigger than you’re allowed to use in the contest) directly to the spool/axle, though this is inefficient, since the part wound around the spool wastes energy slipping as it unwinds, and doesn’t contribute much of its force to moving the car.