Project Caretaker. Part 4. Chassic

Disclaimer: I have no background in mechanical engineering, robotics, industrial design, and related disciplines like strength of materials and metrology passed me by. Therefore, when designing, I follow the principle of redundant reliability.

Now we're going to invent the wheel. I had high hopes for 3D printing, and spoiler alert - I wasn't disappointed. Everything that could be printed, including tracks, was printed.

For small devices, the choice of motors seems quite limited.

There are motors without gearboxes, for which you need to assemble one - such are used in Rover 5.

There are motors with gearboxes in various formats, but with plastic gears and output shafts.

And there are motors with fully metal gearboxes - GA-12-N-20, available in various gear ratios, which suits our needs.

From the pictures on marketplaces, even with dimensions provided, it was difficult to realize how small it would actually be:

Usually in tracked chassis, motors are located inside the body, and rollers rotate on their axles. But I couldn't figure out how to eliminate the lateral skew of the roller during tensioning (present in Rover 5) or how to extend the axle.

However, I calculated that the motor with its axle and gearbox is exactly equal to the planned width of the tracks. Then I thought it would be interesting to build a wheel around the motor.

And to prevent any chance of skewing - the motor would sit on 2 bearings. All that remained was to determine their size and layout.

Initially, I ordered bearings of the minimum size that would accommodate the motor - 6802 (15x24x5mm). But I didn't account for the motor's gearbox not fitting within these dimensions. Therefore, the layout could only be like this:

This option didn't quite meet my reliability criteria, although it was theoretically functional. Therefore, the bearings were replaced with the next size up - 6803 (17x26x5mm), and the original ones were used for wheels without motors.

Several more iterations were spent on adjusting clearances for assembly and improving the outer part of the wheel.

You can also notice that the wheel design itself was modified - grooves for tracks appeared, and the outer shell was divided into interlocking parts that fix all parts in place - this makes it easier to print and assemble, and the parts should be held together reliably by the fit and the track.

Through trial and error, I found the clearances for a zero-backlash fit - the wheel assembles with some force, but by hand.

The bracket doesn't deserve special attention, it was modeled with a focus on reference design and connection to the body. It attaches to the wheel and body with bolts and threaded inserts. Inside there's a channel for routing the motor power cable.

Since designing the body would clearly take a lot of time, I temporarily modeled a flat platform for mounting and testing - and here I managed to get the tolerances right on the first try, the platform turned out to be quite stable:

The concept is quite workable and I even like its appearance. The article turned out to be short, but in reality, there were several iterations of design, printing, and reinvention.

Thanks to all those guys who upload precise 3D models (of bearings, motors, and other ready-made components) to the public domain - only thanks to them was it possible to save an enormous amount of hours. The main source of such models is grabcad. And you can't do without CAD - Fusion 360, which I switched to from SolidWorks, is very easy to learn but quite a technological piece of software.

The evolution of versions looks something like this: