Hackaday Fail: A $3000 Prototype That Doesn’t Work

[Dan Royer] is hard at work building his own personal robot army. Robots mean motors, and motors mean gearboxes. In [Dan]’s case, gearboxes mean $3000 wasted on a prototype that doesn’t work. Why doesn’t it work? He doesn’t know, and we don’t either.

[Dan] would like to use small but fast DC motors for his robots coupled to a gearbox to step down the speed and increase the torque. The most common way of doing this is with a planetary gear set, but there’s a problem with the design of planetary gears – there is inherent backlash and play between the gears. This makes programming challenging, and the robot imprecise.

A much better way to gear down a small DC motor is a hypocycloid gear. If you’ve ever seen the inside of a Wankel engine, this sort of gearing will look very familiar: a single gear is placed slightly off-axis inside a ring gear. On paper, it works. In reality, not so much.

[Dan] spent $3000 on a prototype hypocycloid gearbox that doesn’t turn without binding or jamming. The gear was made with incredible tolerances and top quality machining, but [Dan] has a very expensive paper weight sitting on his desk right now.

If anyone out there has ever designed or machined a hypocycloid gearbox that works, your input is needed. The brightest minds [Dan] met at the Bring A Hack event at Maker Faire last weekend could only come up with. ‘add more lasers’, but we know there’s a genius machinist out there that knows exactly how to make this work.


2013-09-05-Hackaday-Fail-tips-tileHackaday Fail is a column which runs every now and again. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

113 thoughts on “Hackaday Fail: A $3000 Prototype That Doesn’t Work

  1. It looks like the design is highly hyperstatic . The use of 3 satellites need very high precision , the gear have to be cut on a Fellow machine ( for example ) . It would need a floating design of some parts. An industrial sketch ( 3 orthogonal views or more ) with dimensions could help to have a better view of the design . A classical problem is a phase default between the teeth of the satellites , the 3 of them may need a different phase between the teeth . First idea use only only satellite !

  2. From the linked article:
    … Torque on either the output shaft or the input shaft causes the shaft to bend off-axis, away from the point of contact between the inner gear and the housing. As soon as this happens the interior gear starts to tilt. A tilted gear is a jamming gear, and I don’t mean in a One Love kind of way.

    If I understand correctly, a naive way to address that problem is to use multiple eccentrics. Two lobes, 180 degrees out of phase should give first-order balancing, three lobes at 120 degrees will be better, and four lobes with relative phases of 180-0-180 should balance almost perfectly.

  3. If the goal is to use cheap motors, and also avoid the complexity/cost of sensors for gathering positional feedback, why not just accept backlash as a design trade-off, and compensate for it?

    Determine the worst-case bounds of the error (the magnitude of the lost motion), and use software-based backlash compensation.

  4. Hi Dan,

    I hope you even had a better time in the last Year with this drive. In the off chance you are still working on this, I would be happy to share email correspondence with you about your hypocycloidal reducer. I work for a research laboratory which is in the process of developing these reducers and can share some pointers about designing these mechanisms. They have their downsides, but require many careful considerations to operate correctly and are excellent alternatives to harmonic drives when well designed.

    I am also willing to share knowledge with anyone else who is interested. Dan or others, leave a comment below and I’ll get back to you.

    Good luck!

  5. I have developed and tested first ever prototype of two-stages planetary gearhead having zero-backlash.
    The design is patent pending and cannot be presented for this discussion.
    But a few details can be provided.
    1. The gearhead prototype was developed for smal size of 1.58 diameter and low torque only 40 in-lbs.
    2. The meaured backlash was Zero.
    3. The gearbox was backdrivible and having very good efficciency.
    4. The Torsional stiffness was gaduadully stiffer with the torque encrease.

  6. recliners off higher end car front seats are quite commonly continuous eccentric recliners ( i believe you are referring to it as a hypocycloid gearbox) go get one from an old audi/jag/bmw etc fyi 95-2000 era volvos and jags used the sun and planet system but they moved to continuous eccentric drive for the same reason – freeplay.

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