I've recently been working on, no assembly required 3d printed wind-up cars and have been using Shapeways strong and flexible plastic (SLS printer) to print them (i don't have a 3d printer).
In this Instructable I will go through how to design your own 3d printed wind-up gadget as well as giving you some files (including STLs) to help you get started - including a very simple pull back car that you might be able to print in one piece on an extruder-type printer using supports (serious points to you if you manage to get it working!).
Step 1: Designing the Spring
I designed the spring using Adobe Illustrator's Spiral tool I then saved it as an SVG and imported it to turn it into a 3D object.
When using the Spiral tool click and hold the ctrl and or alt key and drag (on a PC) to change the number of turns and the shape of the spiral.
The material you use and thickness of the spring will determine how big and how many spirals/how spaced out they are. For Shapeways nylon (WSF) plastic a spring of about 1mm thick was best. I experimented with different numbers of turns in the spring and found a more spaced out spring was better as it allowed you to wind up the spring further (and therefore unwinds longer).
Included below is the STL (Spring.stl) file for a spring, and the SVG files for a solid line (SpringLine.svg) spring and a spring outline (SpringOutline.svg).
Step 2: The Gears
There is about a 4:1 ratio between these two gears you can stack these two gears to increase the input/output ratio (or you could design your own) .
Because of the large distance required between parts for 3D printing you can't (with standard printers) print gears this fine already interlocked, to get around this and to maintain it's 'no assembly required-ness' the gears are printed in place and then slide together to interlock.
The STL file for these gears is below
Step 3: Putting Together Your Gearbox
Depending on what you want your wind-up gadget to do, the complexity of the wind-up gear box will change. I recommend trying to keep it as simple as possible - this will ensure there are less moving parts and therefore less things that could go wrong.
One of the options you have for your wind-up gear box is whether you want to be able to have your spring unwind as soon as you let go or to have a sort of 'catch' to keep the spring wound until you press a button. I designed this 'catch' as simply as I could. A '+' looking arm (which is attached to the top of the primary gear, which is in turn connected to the spring through a shaft) spins as you wind up the spring, clicking past another thin arm (the thin arm is designed to bend a little bit) which is set in place (unable to turn with the + arm). As the spring is let go the + arm will spin back around and hit the thin arm which it will be unable to click pass as a small stick is stopping the the thin arm from bending backwards. To let the spring unwind, you press the thin arm downwards (by pressing the button on top), releasing the + arm.
*shown above is my first working 3D printed wind-up gear box including pictures of it after i dissected it*
*available below is the basic layout in an STL file (WPBasic.zip) - zipped for your convenience*
Step 4: Free-roll Mechanism
If you're designing a wind-up car (or something like that) you'll want to include a free-roll mechanism that will allow the car to roll with it's own momentum once the spring has unwound. You don't have to include this in your design, though if you don't your car will bounce back once the spring has unwound.
In my design this free-roll mechanism is in the front drive wheels. As the axis connected to the gears spins forwards the Z shaped arm (which is connected to the axis and is free to spin inside the wheel) connects with a long thin arm (connected directly to the wheel) pushing the wheel to turn. Once the spring has fully unwound the wheel will continue to spin under the car's momentum. The long thin arm will click past the rounded back of the Z arm and continue to spin. It is important to make this long thin arm flexible so it doesn't offer much resistance when the car is free-rolling.
*Available below is the basic layout of the free-roll mechanism (FreeRoll.stl)*
Step 5: Implementing Your Gearbox
Once you've designed your basic gearbox you can now implement it in whatever gadget you want!
You can add more springs and/or leavers to achieve different actions, but as i said earlier, if you want it to print in once piece, try to keep it simple. Make sure you keep clearances are large enough to allow moving parts (this will also limit the overall complexity of your design).
The above video is of my first 3D printed, no assembly required wind-up toy which you can check out on Shapeways here - (with alterations made to fix a few problems)
You can also check out 3dprint.com's article for more information about my car here
There is also a simpler pull back only version here
I won't be making the files for these cars available for download, but hopefully from this Instructable you should be able to design your own! And i'll be happy to answer any questions you have!
Step 6: Summary
I hope this Instructable helps in designing your own 3D printed wind-up car or other gadget, as i'd love to see growth in this area of 3D printing!
Included below is the STL file for a simple 'spring only' pull back car for you to use. It is untested but i am confident that it will at least work on an SLS printer - it was designed with a extruder- type printer in mind (Though i don't have one, i have had some first hand experience using them) so if you manage to print it out and get it working, i'd love to see it (the spring may be a bit awkward to print out i know).
Side note - a suggestion that was made to me to increase traction of the wheels was to put rubber bands on them. It greatly helps the movement of the car and stops the wheels spinning out on smooth surfaces
This Instructable is entered in the 3D printing competition so if you enjoyed this instrucable please vote for me!
If you have any questions about this Instructable or about about designing your own wind up gadget please just ask!