Saturday, July 16, 2011

The Evolution of the Screw Drive

Ok so I saw this video and I thought to myself "I could design a really awesome R/C version..."

Badass, right?

So a couple months ago I started designing on the best free CAD program ever (along with a little bit of help from the most complex CAD program ever) and behold...

Screw-Drive v1.0.

It's got a ridiculously over-sized front skid-plate just because, why not? It looked stupid to me at first but it grew on me. And here's the drive train:

I decided once and for all that I was going to learn how to make gears and belt/pulley assemblies in Inventor. It's pretty easy once you get the hang of it and actually learn all of the gear terminology used when the program's asking you what you want. And if you're anything like me, it's super satisfying to design your own gears for projects. It can get annoying having to design everything around existing gears instead of just designing your own.

Anyways, I suppose I'll explain the whole screw-drive concept a bit. The two huge "pontoons" or "screwy thingies" or "twisty barrels"...whatever, are "threaded" in opposite directions, therefore when they both turn in opposite directions, the whole vehicle "screws" its way straight through terrain. Then when one pontoon is stopped and the other is turning, the vehicle will either rotate right or left. There's one funny little quirk about this thing that I noticed too...

Ok so...both pontoons rotate in opposite directions to move the vehicle forwards or backwards right? So when they rotate in the same direction, the vehicle should turn in place like a tank, riiiiight? Not really. In the case where both pontoons are rotating in the same direction, they actually function as sideways wheels, since...that's basically what they are. So the vehicle crab-walks side to side when both pontoons rotate in the same direction. BUT, the "threads" on the pontoons still do something and rotate the vehicle just a little. So what ends up happening is the vehicle moves sideways through the path of a large circle, constantly facing the center of the circle. If the vehicle were stuck in super thick mud or floating in water, then it would rotate in place, but that's the only case when that would happen.

Oh by the way, those pontoons are actually hollow. They're not big chunks of material, don't worry.

So yeah, each motor powers one pontoon through my little drive train I designed, and they're powered by the two Lipo batteries which are the big bricks you see on top of the chassis, which is a big flat plate of carbon fiber. So that's Screw-Drive v1.0. All of the components are placed really high on the chassis, the drive train is exposed, and there's just a lot to be improved. Enter Screw-Drive v2.0:

Before I say anything, I would like to point out that, for whatever reason, I decided to design this model using standard units instead of metric. Never again. Metric>Standard.

So you probably notice a few improvements right from the start. For one, I was a lot more meticulous with this model and actually included screws and everything. Also, the pontoons, besides being larger in diameter, are brass and absolutely beautiful. I actually wouldn't have them made out of brass, it's just pretty looking. They'd probably be made out of aluminum or delrin or something.

Moving onto actual improvements, there is now an upper and lower carbon fiber chassis plate, adding a LOT more much-needed rigidity to the overall design. The batteries are stacked differently so that they can be closer to the center, and are supported by the lower chassis plate. Then of course there's a cutout in the upper chassis plate for them to stick out. This arrangement allows for a lower center of gravity while also automatically restraining the batteries' movement in the X and Y direction, so now I only have to worry about holding them down in the Z direction (zip ties: man's new best friend). Nahh...I'll actually make some neat little professional-looking mount to hold them down or something.

Another neat thing about this chassis arrangement is that there's now a little area in between the upper and lower chassis plates to store other stuff like, oh I don't know, all of the electronics. So the two speed controls and receiver will be hidden in between the two chassis plates, out of sight and completely protected from debris.


Here's another picture, this is your hint:

Starting to figure it out yet?

So I got the idea to mount the motors inside of the pontoons from one of this guy's projects. Let's have a look inside:

Pretty neat, huh?

The thing on the left is just the end cap for the front of the pontoon, but on the right you will see the same exact motor that powered one pontoon from Screw-Drive v1.0, but instead of being nine feet in the air and attached to a space-hogging set of gears and pulleys sticking out the back, it's attached to one of these compact 5:1 planetary gearboxes and then shoved inside of the pontoons. Then of course that shaft sticking out is what drives the pontoon, and there is a gearbox mount that routes all the way out the back of the pontoon and onto the rear "pontoon brace" (that's what I call them at least).

I guess a simpler way to think of it is that I just stuck a motor and gearbox directly onto the back of each pontoon, and then recessed them into the pontoons a little. Or...a lot. By the way, the motor/gearbox being inside of the pontoons is why the pontoons on Screw-Drive v2.0 had to be larger in diameter. I think that's probably for the better either way though.

So yeah...I was really happy with v2.0. Now the vehicle has a much lower center of gravity and is much simpler overall, which in turn contributes to a greater durability and a more attractive, minimalistic look. I should work for Apple or something.

However, I had one more little improvement I wanted to add.

I was able to eliminate those little pontoon end caps that were on the previous designs by instead sticking the mother of all ball bearings within the front of each pontoon. Okay, I guess they're not that huge, considering this whole vehicle is only 13 inches long, but relative to the vehicle they're quite large! So then the centers of those bearings are supported by the front pontoon brace and bam, a whole bunch of complexity is eliminated, while also adding durability because of the huge ball bearings which are probably are probably more at home in a lawn mower engine or something.

Oh yeah, I also came up with a silly-looking body real quick, just because. I plan on actually building my own vacuum-forming machine and making my own lexan bodies eventually, but I'll save that for another blog.

All of the parts for this vehicle can be made/purchased with great ease...except for those incredibly complex pontoons. The center could always be turned on a lathe and then the "threads" can be welded on somehow, but I'd really like to have them machined on a 4-axis mill (or 5-axis might actually be required). Hopefully after I finish my suction mobile thingy I can start working on making parts for this beast and figure this all out.

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