Shelly the Crab

Shelly is a rideable, animatronic hermit crab sculpture.

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Shelly is a rideable animatronic hermit crab with a leg assembly inspired by Theo Jansen's windwalking creatures. The Jansen linkage design spec is very available online, so I was able to use it, and learn a lot about it along the way. By adding a shell to the custom made chassis, a cohesive creature emerged and Shelly was born.

Currently, her motors run off a 36V system with energy stored in Lithium Ion batteries. In the future, I'd like to use a Honda 2200EUi to directly convert AC to DC, through a capacitance system into her speed controller so she can be fueled up without recharging. Her original motors were ridiculously underpowered (250W ebike motors with planetary gearing reducti), and would stall frequently, causing damage to them. I've since upgraded her motors to be two MY1020 800W brushed motors with 2 gearing reductions to get to down to 250 RPM for legs & wheels.

Chassis & Leg Design:

Shelly's chassis and leg assembly was spec'd out in OnShape. You can see the design here

The main constraint's of Shelly's legs design are lateral rigidity, as well as overall assembly width. The ideal leg assembly would include 6 legs, not 4, however each leg needed to have enough lateral stability that it wouldn't buckle when all of the creature's weight was on 1-2 legs, and in order to accomplish the sculptural direction I was intending, I decided to stop at 4 legs. to obtain the correct width.

In order to achieve this stability, each leg it composed of 3 different co-planar leg-struts. Designing in OnShape allowed me to not only ensure each leg had sufficient stability (by making an assumption + using my design instinct), but also measure out how much specific distance would exist between each leg component in the assembly.

The crankshaft was designed independently.

And the final assembly was constructed to verify tolerances. 

Here is a Google Presentation explaining the initial conception of Shelly + her leg design implementation.

Cutting all the leg pieces was very tedious, and many angles were a huge pain in the butt to cut on my bandsaw because they were so acute. Next time I am going to have the components laser-cut out of 16g sheets and I'll use a truss system to mate them together. Ideally it will be rigid laterally, and also lighter/more precise.
The chassis are quite heavy, I don't remember the specific sidewall thickness but I'm fairly sure it's at-least 1/8 inch. This was the initial design. You may notice from my OnShape design that there is a "jog" in the chassis toward the rear. I had to cut off the back rectangle and raise it 2 inches so that the steering caster wheels would fit and the chassis would be oriented correctly in space.
Drivetrain + Motor v1:

Ultimately I abandoned the gas engine + clutch because it wanted to happily run at like 2,500 rpm. Shelly needed to get down to 250 rpm minimum, so my friend Andrew Renz helped me install an electric ebike motor. This allowed Shelly to actually propel herself at a reasonable speed!

Crankshaft Fabrication:

My friend James Erd helped mill out the stainless steel crankshaft components used to push Shelly's legs. It's undoubtedly the corner-piece of this build, along with the motors/electronics. I have a ton of respect of Jame's process, knowledge, and machinery!

My first and only part drawing I've sent to a machinist :)
Originally I intended for these grade-8 bolts to go through the crank-axles directly through the "center" of the axle. James recommended we "shoulder" the bolts along the top of the axle. I am unconvinced if this is better design, however because this was a collaborative process I was happy to do it a different way, especially if the functional result was largely the same. His argument was that more surface area would be in contact between the axle/bolt/crank-riser, and thus shearing would be less likely to occur. If anything, the orientation of the "notches" in the axle makes it so that you know for sure you are installing the components correctly, since if it went through the middle, minor variances in the alignment would be harder to perceive and it would be more difficult to assemble. It was a good design choice.

James taught me to use a rubber hammer instead of a metal hammer when assembling my tight-fitting metal components. It was one of the best pieces of advice I got on this project.

Leg Assembly & Articulation:

So, in order to achieve the lateral rigidity I referenced before, Shelly's individual leg components need to completely sheath the axel-bolts which hold the components together. I purchased round stock with a very thick sidewall and then used an annular hole bit to cut holes out of...

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The final sticker design for 2019

postscript - 1.71 MB - 01/31/2019 at 00:22


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  • RC Components Taking Shape!

    Bree Hoffman02/01/2019 at 19:48 0 comments

    Shelly's new RC control system is taking shape! I'm realizing it will be wise to have a dedicated 12v system for linear actuators, speed controllers, and RC receivers. When using a 36v -> 12v Smakn converted, my 36V batteries would stop working (odd). I assume this is due to come kind of protection circuit on the batteries becoming confused. It was much easier to get everything working when plugging all 12v components into their own 12v battery. 

    Maybe hooking up all the grounds will help....

  • Cleanup Pics

    Bree Hoffman01/31/2019 at 00:11 0 comments

    I finally got a space to work on Shelly again! These are some pics of her as I disassemble her for cleaning/upgrades. Featured in this pic: the heavy duty linear actuator I intend to use to lift the legs off the ground. Her new drive wheels, and also the new 750w drive motors I intend to use.
    All the body panels removed for wiping down and de-rusting.
    The shell will require a lot of work. Some small welds broke, and some rust has become rather aggressive. I need to re-paint it.
    The whole crew :)

  • Converting Shelly to RC

    Bree Hoffman01/28/2019 at 23:12 0 comments

    I'm currently completely disassembling and refurbishing Shelly. Her mechanical components are being cleaned/upgraded as necessary.
    The main piece of work is I am completely re-wiring her brain box. All steering/driving will be by wire, and the signals will be coming from an RC receiver which I can control remotely.

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Enjoy this project?



avai89430 wrote 10/01/2023 at 15:12 point

You did a great job to develop this shelly. Actually I want to embed it with my golf cart webpage you can see here.

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reynolds.garrett wrote 09/22/2021 at 03:20 point

I'm super curious about making a similar sized build and wondering if I could pick your brain on the drive system sometime?

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Starhawk wrote 02/18/2019 at 03:47 point

Hey, this is super cool. I wish I could build stuff like this. Liked and subscribed... also I want one!

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Bree Hoffman wrote 01/31/2019 at 00:04 point

I'll post some pictures, really there's just a lot of rust. I washed the playa off of her ~1 year ago then let her sit in the sun for 4 months before tarping her. I'd almost given up on her because I was completely overwhelmed with life stuff. 

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Mike Szczys wrote 01/30/2019 at 19:43 point

I'd love to see some gory photos of the cleanup effort while you have her disassembled. I imaging the dust from the playa gets in between every set of parts. Anything surprising discovered so far?

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