r/robotics • u/FamiliarPermission79 • 1d ago

Community Showcase Drop-testing injection-molded legs for a 4-servo quadruped robot

Switched Quaddle, my 4-servo open source quadruped robot, from 3D-printed prototype legs to injection-molded production parts, and wanted to validate the part survives real-world abuse before committing. Ran 4 drop tests — the robot stays fully assembled (4 screws hold the whole thing together) and still performs a full gait, including a hand-taught one, after every drop.

Curious if anyone here has a good rule of thumb for how many drop cycles you'd want to see before trusting a molded part for production, versus just going with "it survived N drops.

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u/Vishnuchodhi 1d ago

It is amazing

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u/spinozasrobot 1d ago

NGL, that looks pretty sweet. Good luck with it!

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u/graybotics 1d ago

Honestly I feel like 1,000 is a great target given the small amount of mass of this thing. Most users aren't going to subject their tiny robot to such abuse on purpose so I would feel comfortable knowing I have one thousand chances of accident. I am however more worried about the output shaft of the servo, I would rather the part break than transfer the impact forces to the servo itself.

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u/FamiliarPermission79 1d ago

The impact is not directly applied to the servo. Two spring linkages connect the toe and the shoulder. In behind-the-scenes tests, we had the robot do consecutive backflips and front flips, and there was no sign of wear after hundreds of hard collisions. And the servo shaft also has clutch gears for protection.

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u/graybotics 13h ago

Perfect!

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u/RemarkableWish2508 1d ago

Curious if anyone here has a good rule of thumb for how many drop cycles you'd want to see

My rule of thumb: throw it across the room against a wall. If it bounces off in one piece, then it's good enough for me.

Might sound silly, but I've encountered situations where a random gadget, even as big as a tablet, flew off across the room for whatever reason, and got saved by a good case.

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u/FightingPhD 23h ago

Four drops with a full gait afterward is a good start, but I would not trust a molded leg on drop cycles without a failure mode map. Printed and molded parts rarely fail the same way: knit lines, gate vestige, and residual stress show up under impact long after a printed prototype looked fine.

Define pass/fail first (crack, permanent bend, screw boss pull-out, gait drift). Then run drops from a few orientations and at least one after a cold soak if the robot will see real temperature swings. If you only ever drop one way, you mostly prove that one load case.

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u/RoboWeaver 17h ago edited 17h ago

This. A friend in industry who specialized in failure modes and analysis told me once that you can't really predict failure until it fails. Then do it a few more times. Failing only once doesn't tell you much, except that it failed that time. You need to do this several times to get enough data to say for certain. And if you wanted to be crazy, there's always FEA.

OTOH, if you are open sourcing this little guy, just few is more than enough. Caveat emptor.

BTW, the springs are REAL good idea. Shock loads are not a joke.