Author Topic: Planetary gearbox - an attempt to make sense of how it works  (Read 6589 times)

Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #75 on: June 11, 2019, 12:34:47 AM »
Don, yes I tend to calibrate the build plate rather close to the nozzle(under 0.10mm). I do it manually, as I've found that the automatic calibration in Finder is useless.

As you mentioned the bed adhesion, it is indeed quite good, even though the Finder doesn't have warmed build plate. I might do some testing about the top layer and 'elefant feet'. Thanks for the tips.

What comes to the transmission, it is still far from perfect, but here is the mock-up of it with middle support:   


There is something strange between 3rd and 4th gears though, which I haven't been able to trace so far. It feels like they are missing. Maybe it is too obvious for me to see:


At the lower left corner there is a diagram of 'kraftfluss'(power flow?), but I'm not sure, how to interpret it.

Anyways, here is the drawing of current version of the transmission:

Offline jtrain

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #76 on: June 11, 2019, 05:16:25 AM »
This is not a transmission but someone that made planetary gears for a different purpose.


John

Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #77 on: June 11, 2019, 09:09:41 PM »
John, that's a wonderful video. Too bad he doesn't show details of that broaching device. Not that I have a mill or lathe sturdy enough to attach broaching machinery, but it would be interesting to see his take on it.

 

Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #78 on: June 13, 2019, 12:19:54 AM »
Here is the test setup for the transmission so far. Even if there are missing gears (third or fourth), I'll give it a shot, and continue testing:


The steel rod axle on the front will have cams for each brake. When turning the axle, there should then be a sequence, where one brake is locked at a time.

I have no idea, does that concept work, but as always, there is only one way to find out.

The brakes are to be held in position with screws, that are attached to the front 'post', or that's the plan anyway:


There are six brakes(one being 'neutral', as I found out) to fit in, and i'm sure the idle friction between the brakes makes it already quite tricky challenge. But then again, there are plenty of possible alternative options to test. 

Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #79 on: June 16, 2019, 01:07:31 AM »
Testing of the 0.6 mod transmission proved again at least one thing: multiple stages of gearsets multiply any, and all possible, even minor defects, which the printed parts have.

The result was (yet again) a grinding machine, with lots of friction and wobbling.

So far, what I have experienced with my entry level printer, it is highy unlikely to make such complex contraption just by printing the parts out and assemble them, and expect it to work smoothly.

The printed gears must be machined after printing, or formed by other means, to ensure better meshing.

One last thing, that came to my mind, is to 'roll' the printed gears, using the same module metal gear with some heat.

I don't have any 0.6 mod metal gears, so I'll have to skip the current version, and go back to the 1.0 mod one.

I have some mod 1.0 steel gears, that came with lathe. This one has 40 teeth, which I used to form the 12 teeth planet gear:


The heat source:


Result after 'forming'. Left one is straight out from the printer, while one on the right is also printed, but post-formed:








Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #80 on: June 19, 2019, 11:45:06 PM »
I'll have to give up with the current transmission, as there was similar problems with both the smaller and bigger version. Simply too many stages, and too many gear sets.

Next version will be Ravigneaux-based one, as it has far less parts, and uses only one ring gear.

At first I will be looking for the very basic version of it, to see, if I can figure out, how to attach required clutches and brakes to change the gears.

So far, the printed one is straight from the Youtube video:
 


It has gears, that have 45 degree helix angle. There is not any specific reason for doing that, other than to see, how they grind against each other, when compared to plain spur gears. There is some minor friction between the gears, but the main source for the friction is the tube, that is a part of small sun gear, which goes through the planet gear carrier's body. It took me a while to find that out.

Again an example, of how poorly the PLA-plastic parts slide against each other. So for the lubrication, I tried WD-40 and silicone spray. Neither worked. With graphite powder applied, the friction decreased, but only temporarily. Also, if the parts are tested with higher speeds, the surfaces, that face more friction, tend to swell. The planet gears on the above picture have 4mm holes in them, which I had to re-ream couple times, as they got sticky.       

Besides that, there is an extension for the Ravigneaux gear set, called Lepelletier planetary gear set. I find it even more interesting, than the Ravigneaux gear set itself, as it allows the gearbox to have more gears, by addind a single planetary gear set in front of it.

There are plenty of fancy logic charts and diagrams online about it, but how the actual working model of that could be accomplished, I'm not so sure about, as I'm not the brightest one, what comes to logic.

Instead, I'll go ahead and proceed with 'hands on' -procedure.


Offline Admiral_dk

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #81 on: June 20, 2019, 09:26:46 PM »
Your last picture looks like nice parts - so it is a shame that you still have trouble with too much friction  :-\

I hope that you will end up with a successful gear train before long ....

Best wishes

Per

Offline ddmckee54

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #82 on: June 20, 2019, 10:08:46 PM »
I don't know how compatible it is with the PLA, but have you tried a lithium grease for lubrication?  The corrugations that you naturally get from printing the layers with an FDM 3D printer should work to your advantage as they'd tend to hold onto the grease.

It'll be interesting to see how the helical cut gears are going to hold up over time.  I would think that those same FDM layer corrugations will grind against each other as the gears rotate.  Maybe the gears will wear in or maybe they'll just wear out, only time and testing will give you that answer.

Don

Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #83 on: June 22, 2019, 12:16:22 AM »
Thanks for checking in, guys. The friction between certain printed parts is really a major issue. Perhaps not so much, if you plan on using the parts at very low speeds, like hand-cranked devices.

I like to machine all the mating surfaces(except gears, for now), so that there is a bit better chance to get them centered, or aligned with each other.

Although the mating machined surface(s) may seem smooth, they develop friction between each other, and therefore heat.

Not much heat is needed to make them gall, and the reaction, if allowed to continue, makes even more heat, to the point, where mating rotating parts, being of the same material, start to fuse together.

Then comes the question about lubrication. As mentioned before, I tried WD-40 and silicon spray, plus vegetable oil, vegetable glycerin, graphite powder, and even water and soap.

As far as I have tested, graphite powder, for some reason, gives the best results so far. That leads to conclusion, that perhaps the machined, smooth surfaces are actually porous. 

And if the machined surfaces are porous, the lubrications in fluid form would fill the pores, and stay there, allowing the mating surfaces to gall after all, without forming any constant film between surfaces.

But enough of that babbling. Here is the part with most friction(red through the orange part):


At first, I smoothened the surface more with fine sandpaper. Then I used a glass rod to rub the graphite powder to it:


Result:


Finally, here is the 'run-in' -setup. While motor is running, I hold the green part, which is the planet gear(s) carrier. 


By using the graphite powder as a lubrication between the red and orange parts, motor speed is up to 2000 rpm with no problem.

As Don mentioned, lithium grease could be a good one, as well as something like ptfe lube. I don't have those at the moment, but might consider buying them for testing.

Offline sorveltaja

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #84 on: July 24, 2019, 09:20:44 PM »
An update: It's been quiet over a month now, but the project isn't forgotten. But still, unfortunately there comes time, when you have done a lot to make the 3D-printed thing(a model of an automatic transmission) to work smoothly, and finally, ran out of ideas.

Good thing is, that I have (hopefully) demonstrated most of the challenges, that one faces on project like this.

Bad thing is... no, there isn't anything bad in learning, after all.

Offline Vixen

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #85 on: July 24, 2019, 10:11:18 PM »
I have enjoyed making this journey with you, trying to help and encouraging you when I could.

Well done, You sure gave it a good run for it's money.

Cheers

Mike
It is the journey that matters, not the destination

Offline ddmckee54

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Re: Planetary gearbox - an attempt to make sense of how it works
« Reply #86 on: July 24, 2019, 10:22:17 PM »
sorveltaja:

Don't worry too much about "running out of ideas".  I've had projects sitting on the back burner for months before I got new inspiration of how to solve the problem.

When I built my wooden excavator models I worked on track link prototypes for over a year and went through I don't remember how many different designs before I got one that was buildable, reliable, and that I was mostly happy with.  I was only mostly happy with it because each track link had 7 individual parts and each model required 90 track links.

One of the models was a proof of concept model, much like your transmission is a proof of concept model.  I built 4 of the models for others and for various reasons 3 of the models were returned to me.  With 2 of the models the owners passed so the models were returned to me, and the 3rd owner downsized into a small condo and didn't have the room for a 1/16 sized excavator anymore.

Don