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So I'm gonna make a robot arm at some point, so I made some planetary servo drives.
It has a 4.333:1 gear ratio and with the servos I am using this gives me over 80kg-cm of torque, nice and slow, and zero backlash as far as I can tell too! No wiggle at all in this thing, it printed very well.
I'm now printing a new servo mount on the bottom to mount 1/4 scale servos so I can get over 160kgcm of torque.
That's pretty neat! The robotic arm I used in my dissertation had 50:1 harmonic drives - a whole different kind of wizardry.
Have you calculated tooth load/life on those gears/shaft under full load? Looks like they're around 1" across? PLA or ABS?
I printed a coupling for the alpha Filastruder out of ABS, sheared pretty quickly at 90 kg-cm (~4 hours or so @2rpm, fatigue life was not so hot).
I was looking into printing harmonic drives but they seem too flimsy when printed. I might actually go for a cycloidial drive instead, I do have a printed one that works, just waiting for bearings.
I have not actually done any of those calculations, but with an arm they might last longer if just because they won't wear out via having to rotate in one direction all day. What are the calculations for figuring those things out?
ABS plastic, I have yet to actually try PLA
As for the output shaft that also is not a final version. I ordered some bearings that are 20mm bore, so that will allow me to have much more robust outputs to the gearbox.
Finally, the planetary gear itself is 2" across, the smaller gears are roughly 3/4" across.
Tooth wear/fatigue life is tough to calculate accurately, especially for 3D printed stuff where the surfaces aren't smooth and the material isn't homogeneous. The mechanical engineer in me cringes, but I vote RTF. Not worth the time to model it accurately. If you start wearing teeth, make all the teeth wider. (make the gearbox deeper)
You'll also do fine in the setting you describe because the majority of the load will likely come from acceleration. If you start to notice wear, you can just move the arm more slowly.
Yeah, that's what I was thinking. And its easy to make the gears bigger, just print them taller, get bigger versions of those 5mm shoulder screws (I think thats what they are called) and then stack more bearings. Possibly a shaft that goes all the way through. I could make it two inches tall if I wanted, lol
Have you thought about trying to implement helical gears in it? as it would give you a more even gear strength concerning the teeth.
You know, I have not..
I use sketchup as my modelling software, so making normal involute gears is easy, but I don't know how well helical works. I know you can twist the top and bottom to make a helical, but that isn't perfect, since it should be cut at an angle, not skewed sideways. What do you use?
Which reminds me that I really need to try PLA at some point.. just need to get some, I guess.
Is there anything you need to do to an SD2 to make it print pla, or just don't turn the bed heater on and leave the door open?
I'm excited to be getting my Ultimaker2 soon. I need to make hobby servo shafts come from the center of the servo rather than off to the side. I expect this requires a bracket that comes around all sides of the servo (eg Hitec 645MG and PowerHD 1235MG) including the shaft side. Then a cog winding the opposite direction would mesh teeth with the shaft spline. Into this would go a rod.
Centering the shaft will allow me to fit the servo in the middle of the wrist.
Can you guys help? Is there an STL file I could download that's already been made. I've just started using TinkerCad.com today and have never made anything before.
Relevant:
Makes sense. Many people make the blanket statement that "ABS is better", but it's more complicated than that as per usual. Everything I've seen with a vague semblance of science to it suggests PLA wins on stiffness and hardness (wear resistance), so for gear teeth / meshing interface is a better option. It is however not as tough, and the ultimate tensile strength usually ends up lower than ABS (basically it cracks before it deforms; it's more brittle).
Has anyone considered making a gear where the hub & web are ABS but the rim and teeth are PLA (or for the advanced material scientists amongst us nylon)? That could be a pretty creative option, using the best properties of each material where they're needed the most. Be careful of the different thermal coefficients and PLA's cold blood - it would have to be a room temp application.
ABS wins on retaining it's mechanical properties above about 40°C though, which I consider to be a big deal. YMMV.
Maybe the krakken extruder will get some use.. heh.
As for stl's for that kind of gearbox, I don't know of any or have any on hand, but it doesn't sound hard.. Can you draw it out?
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