Aluminalis Crankshaft

Thursday, February 27th, 2014

One of the funnest, but most challenging parts of building Aluminalis (our 16-legged walking robot) has been the construction of the two crankshafts. Each side of the robot has a motor that rotates a 10” long, multi-link crankshaft, which drives 4 pairs of legs. The leg pairs need to be kept 90-degrees out of phase from each other in order to produce the walking gait. Our initial vision for the crankshaft was to build it out of 1/8” aluminum round shafts, custom crankshaft arms we made on our CNC, and tiny 4-40 set screws, but when we put it all together for real-life testing, the rotational forces were so high that the set screws couldn’t hold the round shafts, the crankshaft arms slipped out of place, and the entire crankshaft tore apart (not a good day). We went back to the drawing board. We needed a new design. We had the idea of using a square shaft to prevent slippage and guarantee that each of the pairs was 90-degrees out of phase with the others. At first I thought maybe it was a silly, impossible idea. The crankshaft ran through a series of round holes in the body of the robot, so how could a square shaft rotate smoothly in a round hole? Then I realized we could use ball bearings to do that. Our hope was that the square shaft would not only guarantee the 90-degree angle, but it would also give our set screws a flat area to take hold. So, in version two, we used a combination of square shafts, round shafts, larger 6-32 set screws, and bulkier crankshaft arms (that gave the set screws more thread length to take hold). The results were fantastic. The new crankshaft works great in all our real-life tests. Runs strong and smooth. Note that the square shafts are made out of high-strength copper rather than aluminum.

Aluminalis Crankshaft 1

Aluminalis Crankshaft

Comments (7)

  1. Andrew Terranova
    February 28th, 2014

    This is really cool. I’m working on a similar robot, but I’m trying to use as many stock ServoCity Actobotics parts as possible.

    My version of the robot has only six legs on each side, so the crank shaft needs 120 degree rotation between cams, rather than 90 degrees.

    I might try square bar and some custom cut hubs with set screws to build it. Thanks for the inspiration.

  2. Camille
    March 1st, 2014

    Good to hear from you, Andrew. Yes, we love Actobotics parts, too. Good luck with your project. Please keep us posted on how it goes. We’d love to see it. We have Aluminalis working now. We’re now working on a video and pictures so we can make a post.

  3. Chris Beatty
    March 1st, 2014

    Excellent out of the box thinking. At first it seems like “square pegs and round holes” but the pictures are beautiful.

  4. Julie Holtan
    March 2nd, 2014

    Way to think outside the box Camille! (Or is that thinking inside the box by making the shafts square?)

  5. Paul Brown
    March 2nd, 2014

    Loctite inserts or patches on set screws would provide added security.

  6. Ron
    March 3rd, 2014

    This is a novel design, but I’d like to offer a couple of suggestions. The square shaft is great for insuring rigidity and insuring the timing remains constant. However according to your pictures I still see some round shaft sections where the connecting rods and bearings run. May I suggest that you also make these sections from the square stock to guarantee the rigidity. If fact, make the entire crankshaft (all sections) of the square stock. Then where you need a round surface such as at the bearings and connecting rods, simply also insert a part (machined of a round outer surface and a square inner surface) right over the existing square core. A ball bearing could then be slipped right over this “square to round” adapter and you will still be insured of having rigid and more robust timing. Simple setscrews could hold all the assemblies together, and of course use Locktite on the threads to insure that don’t work loose.

  7. Renuka
    October 26th, 2015

    Each another is connected for rolling purpose without using the bearing elements.

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