RepRappable replacement for a worm gear?

On a separate blog, aka47 mentioned an alternative gear for the worm gear -- a planetary gear arrangement with a split annular gear..

The concept intrigued me; Wikipedia has a sample image of this kind of gear, and appears to be made completely out of plastic; it seems the entire gear system could be reprapped, should be pretty sturdy if the gears are solid infilled and a bit thicker than normal (.5cm - 1cm)

This seems ideal as a gear reducer for the extruder: Just create a 60:1 version one of these with solid infill and fairly thick gears (for strength), no metal required (except perhaps the screws and mounting plate to an axle on each end.)

This also would seem perfect for a second generation RepolaRap design, depending on exactly how much backlash was present (a larger annulus and more planetary gears should reduce or solve that problem; I'm thinking 4cm radius and 12-15 planetary gears.)

My gears still come out a bit bigger than designed, which is easy to adjust for in a simple spur gear arrangement, but it would take a few tries to get the gear sized just right for a planetary gear. It does look promising though!

I also like the harmonic drive gear -

[en.wikipedia.org]

That might be a bit simpler to reprap; two annular rings, one thinner flexible gear, and maybe a pair of 624 bearings on a bar mounted on the motor shaft to flex the inner gear.

Either of these would solve the positioning problem I was having - my big spur gears don't fit very well inside a Mendel yet.

Wade

Edited 1 time(s). Last edit at 01/25/2010 03:34PM by Wade.

flex spline is a pretty specific part for harmonic drive (cup with solid base but flexible, splined walls), and the wind-up may make it unsuitable for extruders. fascinating application in other areas though

there are already some planetary gearboxes on thingiverse, they certainly warrant playing with. I envision a planetary gearbox with the annulus expanded to a nema17 footprint, for direct attachment to the existing extruder block.

I'm still waiting for some parts to continue with my RepolaRap, so I mocked up the planetary gear reduction parts of what might be printable with a reprap.

This could be something to allow me to be free of belt drive at some point in future repolarap development by mounting the platforms together using the annulus. Still not sure how I'd apply pressure to press everything together, but that can be for me to play with when I can actually print stuff out.

This could possibly also be usable as part of the extruder, by attaching a smaller piece of metal pipe to reduce the ratio even a bit further, or to allow for a smaller gear set and/or fewer thicker teeth to increase strength.


For the mockup I created:

The overall gear box is about 66mm x 66mm x 50mm (2.5"x2.5"x2").

The 5 planetary gears have funny distinct symbols since they must inserted in proper order. I used 19 teeth and a pitch radius of a little under 9.7mm.

The two annulus gears, one with 60 teeth, the other with 59, are pretty big -- a 30mm pitch radius, and ~33mm outer radius. The 60 - 59 gear arrangement will produce an overall gear reduction of 59:1. The annulus half with the square is stationary, and the other annulus the output.

The sun gear would plug onto a stepper or other motor drive. It has 21 teeth, and a pitch radius of about ~10.7mm.

All the teeth are about 1.5mm thick, and use involute profile - I think with either 20 degree or 25 degree pressure angle.

Hopefully the image links in.. not sure exactly how to make it work, and preview doesn't show it.

(Okay, couldn't get image to work.. it's at the bottom of the RepolaRap wiki)

Edited 2 time(s). Last edit at 01/27/2010 09:59PM by BeagleFury.

Wow, that setup makes my brain hurt! Pretty neat! I've been having some success with printed gears with a little less than a 5 mm circular pitch, and I think your planets have a circular pitch of about 3 mm, so I could almost try and print one of those. Maybe once I get my extruder and firmware tuned up.

Here's a shot of my gears:

[gallery.me.com]

Pretty rough still, but they are working so far. More testing tomorrow.

Looking again at the epicyclic Wikipedia image, it looks like the planet gears have holes for toothpicks or something to get the initial timing right - that might be something to add in. Also, the torque reaction on the planetary gears will cause them to try and tumble; I notice that the Wikipedian planetary gears are a lot wider than they are tall, to counteract the torque. Maybe a smaller sun and fatter planets would be more stable?

Looking forward to trying those out at some point!

Wade

Edited 1 time(s). Last edit at 01/29/2010 04:25AM by Wade.

an arm to hold the planets in position would also discourage tumbling, like the green piece in [commons.wikimedia.org]

Triffid_Hunter Wrote:
-------------------------------------------------------
> an arm to hold the planets in position would also
> discourage tumbling, like the green piece in
> [commons.wikimedia.org]
> ear_ratios.png

The planet carrier, required in many epicycle gear trains, is not needed in this one; I am hoping that the longer gears would prevent tumbling by nature of being locked on both sides by gears, with the longer tooth profiles; I could very well have mistaken how the forces would twist everything in the end though; however, until someone actually prints and creates a model to test, I'm not of the effectiveness of tuning it.

Wade, if you want to try printing a test, I'll see if I can make a 5-6mm tooth pitch arrangement, changing the gear ratio to one of 20:1 to 35:1 or so. Until I get a printer, my designs just give me mental cotton candy, not the real thing. In terms of planet tumbling, how thick were the gears you printed? I can set it up to match that (I.E, make the gears larger diameter than thickness). I'll also give you the blender files -- not sure how comfortable you feel with the tool, but it would allow you to further modify it more easily.