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Gear Note

A project log for Servo50: Scaling Servo-Based Projects

There seems to be an empty spot in the market between hobby servos and professional models - this should partially fill that gap.

nylesNyles 04/13/2018 at 19:464 Comments

I've been trying to find the inspiration for this style of gearbox - I think it was somewhere here on Hackaday, but I can't find it any more.  If you check out the Wikipedia article on Epicyclic Gearing, there's a small note on a version of what I'm doing that was done during WWII for the drive box for a portable radar unit.  It might not be exactly what I've been doing, which is:

This results, in my current design, in around a 150:1 gear reduction in something a bit larger than my fist.  It could be far smaller, but my 3D printer doesn't do well at anything smaller and the amount of output torque scales down as well.

Getting all the gear ratios to match up is a bit of a challenge; not every desired gear ratio is possible.  :)

Cheers.

Discussions

Nyles wrote 04/23/2018 at 17:15 point

Cool link, thanks - one of my inspirations for single sided support has been modern motorcycle swingarms - they transmit a ton of torque and force, don't get squirelley, and better yet don't break.  :)

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Florian Festi wrote 04/20/2018 at 07:33 point

I first thought you have the stepped planets the wrong. For a normal epicyclic gear with stepped planets you drive the bigger side of the planets with the sun and have the smaller side rolling on the fixed ring gear. This way the ratio in the stepped planets is added (multiplied into) to the reduction ratio. Also the ring gear fits in part within the overhang of the larger planet sides. This reduces the overall diameter of the gearbox. Of course this is limited to reductions in the order of 1:50.

Then I realized you were probably talking about this type of gear with two ring gears: https://www.thingiverse.com/thing:2054378 or https://www.thingiverse.com/thing:2220506

While these are very popular on Youtube I have never seen them actually been used. My guess is that there is a good reason for that especially given the very tempting properties (very large but very compact reduction with very small number of parts). I'd guess that efficiency is the problem as the planets have to spin around at input speed (divided by the ratio of the first epicyclic gear train) but have to carry the output torque. This increases the friction by a factor of the gearing ratio provided by the two ring gears - which typically is a two digit number.

Otoh may be efficiency is not that important for a servo anyway.

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Nyles wrote 04/20/2018 at 17:13 point

Hi Florian,

What you're saying makes perfect sense - and I do have a working model that's been through many days of break in with no appreciable tooth wear:

https://www.youtube.com/watch?v=4NBoySjv8uw

Using a ring gear as the output drive requires a bit more structure for it to be useful.  There's a dual bearing race that seems to do a good job of locating and directing the force of the output, and the ring gear is part of the mounting bosses where you bolt the leg/arm/effector/dish as well as the mating bearing races.

The second thingiverse item you posted may be similar to what I'm doing.  :)

One of the hardest parts of this build was getting the ball bearing holders to print properly.  It's also good to put the holding cups at the axis of rotation of the balls, rather than at the point of maximum friction.  Visualizing things properly really does help.

I do care about efficiency - have a feeling that the construction/stiffness of the planetary system has a lot to do with this, as any racking due to torque deflection will cause the teeth to scrub.  In any case, it's going to be an iterative process as I've learned a lot to date thanks to all the past fails.

Thanks again for your input, stay tuned!

               Cheers,

                     Nyles

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Florian Festi wrote 04/22/2018 at 08:43 point

Yes, supporting the planets is another thing that needs close attention. Not only do the planets also carry the full output torque but the torque tries to tilt the planets to the side. So a very strong planet carrier is needed and the planets should probably also be supported by ball bearings - although that can be difficult to do as there is very little room. In theory the planet carrier can be free floating as the tilting torques cancel out within the carrier. You still need planet gears that locate themselves.

There is a way of using 3 ring gears so the planets are supported on both ends and the tilting torque cancels out within the planet itself. But as you need to ground the top ring somehow you cannot have a fully traversible arm attached to the driven (middle) ring gear.

https://www.thingiverse.com/thing:2153317 is an example of that. But it is probably not what you want. It is very interesting to drive a pulley or gear train that will only need access to the output shaft from one side.

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