Fail Of The Week: Hard Lessons In 3D-Printed Bushings For A Giant RC Car

Can you turn 47 pounds (21 kg) of PLA filament into a gigantic working 3D-printed RC car? No, no you can’t — at least not if you eschew proper bearings in favor of printed bushings.

That’s the hard lesson that [Joel Telling] learned with his scaled up version of the OpenRC F1 car, an RC car that can be mostly 3D-printed. The small version still has its share of non-printed parts, mainly screws and bearings. In his video series documenting the build of the upsized version, [Joel] elaborates on some of the reasons for going with printed bushings rather than bearings, which mainly boil down to hoping that the graphite lubricant powder he added would reduce friction enough to prevent the parts from welding themselves together.

The car came out looking great, and even managed to scoot about nicely for a few seconds before its predictably noisy and unhappy demise. But what was unexpected was the actual failure mode. The plastic-on-plastic running gear seemed to handle the rolling loads fine; it was the lateral force exerted on the axle by the tension of the drive belt that was too much for the printed bushing to bear.

As [Joel] rightly points out, it’s only a failure if you fail to learn something, so kudos to him for at least giving this a try. And all that PLA won’t go to waste, of course — everything else on the car worked fine, so adding one bearing should get it back on the road. He should check out our primer on bearings for a few tips on selecting the right one.

Thanks to [Keith O.] for the tipoff.

18 thoughts on “Fail Of The Week: Hard Lessons In 3D-Printed Bushings For A Giant RC Car

        1. Of course I still watch. Your videos are invaluable to learning how product designers fail at user interface design.

          Watching Dave Jones try to use a PC is just as tedious to me, and at the same time indicative to developers of many other people’s frustrating experiences.

      1. …or like me (and others), he likes to see what happens when attempting something that will almost assuredly fail. (The reality is that you get no new information from a success other than that it works. Failure is always more interesting and educational.)

        1. Some people just can’t be told “That won’t work. See all these other people that did *exactly the same thing*? They failed in *exactly the same way* your attempt will.” And when they fail in exactly the same way, some will complain “Why didn’t you tell me…” *sigh* You were told, you just refused to listen.

          People like that do stupid things like getting drunk or taking drugs, or constantly drive way over the speed limit until they get a ticket. To them other people’s knowledge and experience isn’t valid, they think they’ll somehow have a different result.

  1. I wonder if a planetary gear system with many small planets would work as a bearing. You could try printing roller bearings, but as soon it starts slipping it’s over soon. Toothed rollers (planet gears) would prevent that. If you’d use angled teeth or herringbone teeth it could take the axial load, which will be small at those speeds anyway.

    1. I think you’re on the right track here. A planetary gear can eliminate the need to put a radial load on the bearing by eliminating the belt. It was belt tension that destroyed the axle. However, I’m not quite sure how best to do that. One way would be to use a separate motor for each drive wheel, which would allow the drive motor to be on the same axis as the output shaft, but that of course requires two motors.

    2. I agree fully. Plus printing a planetary gear of any type is a must in 3d printing, especially the ones you print, complete, in place that could not be taken apart because of its geometry. They are amazingly durable since rolling action has much less friction and consequently produces lest heat. Also from my experience, any bumps in the gears tend to wear down and cause the assembly to roll even easier.

      This highly respected planetary gear system even has a customizer. It would require some major redesigning but would make the car usable. It would also represent the benefits of scaling up the project. It would be hard to print your own planetary gear bearings at normal size but at the size that Joel chose, sure!!

  2. Big misunderstanding of radial/lateral bearing loads. The failure was still due to radial load and not lateral, the fact that the radial load coming from the belt tension is much greater than the radial load coming from gravity would probably not surprise a lot of people.

  3. Dry graphite film spray would work much better than powdered graphite. Could make roller bearings that use lengths of 3mm filament. With the bearing races printed flat so the rollers run crossways to the layer lines, friction would be low. Lube it with silicone grease and it should work.

    Print the outer race with a lip on both sides to retain the rollers. The inner race should have a lip on the inside and a ring that fits onto the axle to keep everything together.

    Another possibility is a ball bearing using Airsoft pellets, if they’re round enough. Copy and enlarge bicycle wheel bearings.

    Another thing that increased the stress was the solid axle. It should either be 1 wheel drive like smaller go-karts or have a 3D printed differential. At that scale there’s plenty of room and the stress should be low.

    Some front wheel drive cars use a differential that’s all planetary gears, no bevel gears. IIRC the General Motors transaxle as used in the Toronado and Eldorado and GMC motorhome with the longitudinal mounted V8 engines used that type.

    Alter the 3D printed version to use herringbone instead of helical gears.

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