I like this stage of a project.
It's the point where something that seemed like crazy pipe dream is sitting on the kitchen table.
Over the past few months I've designed four separate mechanisms: a radial engine (currently for sale at mechanicalgifs.com), a transmission, a differential, and a steering mechanism. Each of these is a nice, self-contained model of similar complexity to all the models I currently sell.
But then I thought it would be cool to put them all together into...drum roll...a whole car. I spent several weeks thinking this was a dumb idea. It was just beyond my present capacity for designing 3-dimensional models, and surely it wouldn't really work, and I can't be wasting that kind of time right now. But....if I could do it...wouldn't that be so cool?
About a week ago I posted about the first draft of this idea, which combined engine, steering, and differential in a rather clunky and inelegant way. Now I've finished V2 of what is turning into a most pleasing device. I re-designed my transmission model to make it fit into the scale and shape of the car (and to work around various logistical issues that come about when you try to link together multiple mechanical stages without using any universal joints or shaft passers)
Here's a video overview of the car in action:
The differential and steering are the same as before. You can see more videos of them in my earlier blog post, but here's a new angle on the rack-and-pinion steering mechanism. (The tires, by the way, are red Tygon micro fuel line slit down its length. I'm not sure I like them, but they do give the wheels at least a bit of traction.)
I'm particularly pleased with the transmission. (OK, the current version is a bit sticky, but I know why and I can easily fix it in the next iteration.) Here is a top view showing how it shifts from low gear to neutral to high gear, and then back down again.
And here is a view from the back, through the differential. I like the pale blue acrylic used for the big gears, because it's dark enough to really stand out when viewing the gear on edge, but from the side you can see very clearly through it.
I did my best to use colors and shapes to expose and bring clarity to the dual-clutch mechanism that lies at the heart of this kind of transmission. Notice that all the gears are always fully engaged with each other and in constant motion: when you shift badly and "grind the gears" you are not actually grinding any gears.
Instead the shifting is accomplished by moving a clutch plate that locks alternately one or the other output gear to the output shaft. The lower gears spin freely on the output shaft: only the center spinning disk is on a square shaft that rotationally locks it to the output shaft.
When shifted all the way to the left or right, green teeth on the central disk engage with pink teeth on the output gears to lock one or the other of those gears to the output shaft. The "grinding" is grinding of these clutch plate teeth: it happens when they are not spinning at close to the same speed when they are pushed into each other.
(This is a two-speed transmission for simplicity. In 3, 4, or 5-speed transmissions they simply have more gears of different sizes, and more clutch plates. Reverse gear is done with an extra gear off to the side that reverses the direction of rotation before transferring the motion to the output gears.)
You can sort of "drive" the car across a table, either by turning the engine:
Or by pushing it along, driving the engine from the wheels. (You can do this with a real manual-transmission car too: it's one way of starting a stalled car with a dead battery, but only works if you have a hill or a bunch of people to push, and some luck.)
Aside from the addition of a transmission, the biggest difference between this version and the previous one is that, instead of an awkward plate across the whole top of the car, I've got a pair of side rails, which even shift into different planes on both sides of the transmission to account for the different widths of the differential vs. the steering block. This took a long time to figure out, but of course it's nothing compared to the complexity of designing a real car—let alone something insanely complicated like an airplane.
I remember reading that there was a team of a dozen engineers who worked for several years on the design of one door for a new passenger jet. This is complicated stuff. And while we all marvel at the software on a modern iPhone, take one apart and the mechanical complexity alone will make you question whether's it's even realistic to think that human beings designed it.
But the way these seemingly impossible things are done is always the same—in layers. You break down the task into smaller and smaller units, define interfaces between them, and then work out the details one step at a time. To build this car, I first had to design an engine, a steering mechanism, a transmission, and a differential. Each of those is a problem that can be solved on its own. (And each contains sub-problems, like designing a gear, or a square frame to hold four gears, etc.)
When it comes time to integrate the separate sub-systems, there is the exciting possibility of merging and integrating parts that were designed separately. This meta level of design is often the way in which more refined, evolved products differ from first-generation models (in the real world as well as in my world of pretend cars).
For example, the first generation of my differential and transmission each had two "bearings" (holes in acrylic plates) that the drive shaft went through. That's necessary, because without two bearing, the shaft won't stay where it belongs. So the first version of the car (on the left) had a total of four walls supporting what had become a single drive shaft running from the transmission to the differential. This is not only overkill, it also made for a very sticky shaft.
An intermediate stage (not shown) had only three bearing points, and for the final version I realized that I could actually get it back down to just two bearings by completely blending the transmission and differential (right view):
I have a suspicion that is is basically what's called a transaxle, used in front wheel drive cars, but I don't actually know much about automotive engineering so I'm just guessing here. EDIT: I have since been informed that this is in fact exactly a transaxle, and it's used in both front and real wheel drive cars. The Wikipedia article on transaxles has a nice picture of one that looks exactly like mine, except made of metal and more complicated.. You can see the same gears, clutch plates, fork for pushing the clutch plate, and differential housing. (At least I assume it's a differential housing, and the article claims it is, though it looks rather small to me.) Here is a cutaway drawing of a transaxle also pretty much just like mine.
Another real-world example of this sort of integration is in the transition from chassis-based cars to unibody cars. The first cars all had a strong steel frame, or chassis, that the mechanical components were fastened to. When the whole chassis is finished with the guts connected, then the body panels are bolted on as a sort of decoration. (Trucks and serious off-road cars are still built this way.)
In a unibody car there is no separate chassis, and instead the mechanical components are integrated and supported by the body itself. This is much harder to design, but it allows for greater flexibility in the shape of the car, makes it lighter, and cheaper.
It's fun re-living these stages of the industrial revolution, and the mechanical evolution of the modern world, in miniature form sitting next to my magical laser cutter.