Parametric Planetary Gearbox
by chowderhead, published
Change sun and annulus (ring) teeth count, the number of planetary gears and (almost) everything else is done for you. Uses gear calculator (http://www.thingiverse.com/thing:43878) and the awesome involute gear library (http://www.thingiverse.com/thing:3575).
The stls are mixed now (sorry) those suffixed _new are generated with the latest version of PlanetGears_V2 and those with prefix herring_ are (obviously) for printing a herring bone version, including parts for intermediate stage. The two stage herringbone gearbox will yield a ratio of almost 30:1.
Other STLs will build a spur gear, three-planetary 5.45 gearbox to be mounted to a NEMA17 motor and having a NEMA17-like output to attach to whatever. The SCAD is the handy item, though.
Pictures of single stage spur design and two stage herringbone geared design.
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Thank you very much Jeff, I completely missed that one. I've just got used to sketchup and it's easy. been spending some time trying to learn openscad and I'm getting there but I'm old and slow.
When I import STLs into Sketchup, it assumes the units are inches, so I use the scale tool, click on a corner, then type in 0.03937008 to scale models down to mm.
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Published a gear calculator as a separate thing (http://www.thingiverse.com/thing:43878) since it can be used for more than this.
01.17.13 - completed assembly of a herringbone geared, two stage planetary gearbox. Still some tuning to be done, but the process (below) went surprisingly well.
01.16.13 - found a fatal flaw in the herringbone implementation and finally resolved it. The v2 scad has been corrected. Verified that herringbone multi-stage gearboxes can be assembled, but not a task for the impatient. Elaborate below.
01.15.13 - uploaded v2 of planetgears.scad which now has the ability to produce double helical gears (self-aligning, a nice feature coupled with the next feature). Since the design is stackable, I figured it would be nice to make it easy to build multiple stages. There's a module for rendering intermediate planetary carriers that have a built-in sun gear.
For the noob (like me): Print annuluses (annuli?) singly and slowly (30mm/s) with lots of cooling. Group like-height gears (i.e. intermediate and sun) and again print slowly with even more cooling. Print the planet gears on a raft and simply shave off the support - lots of cooling, again. Doing this saves a lot of knife and file time.
Note that the script tests whether the design can be assembled or not. If any of the rules are violated (echos "false" to any rule) the design can't be assembled. The script will happily render parts that can't be made into a working gearbox, so make sure the design is valid before rendering the final version and printing. Changing the number of teeth in the sun or annulus or the number of planetary gears will allow you to arrive at a design that works. Visit wmberg.com/tools/ and use their handy scripts to check if designs can be assembled or just render the script with various combinations of teeth.
The present design uses a 624 bearing for each planetary and a 608 for the output, though all is adjustable. There are a stack of 3-4 washers under each 624 to provide clearance between the carrier and the planet gears. The output shaft is an M8 bolt and there is a washer between the carrier and the output bearing.
Haven't done a lot of testing with this, yet, but don't foresee any serious trouble. The first one is going on the caulk extruder (http://www.thingiverse.com/thing:39954).
A single stage is comprised of:
3x M4x16(maybe 20mm?) screws
3x M4 lock nuts
9x or 12x M4 washers
M8 bolt of desired length
2x M8 washer
3x 624 bearings
M3 screws of length and count for assembling case
For each additional stage:
3x M4x16(maybe 20mm?) screws
3x M4 lock nuts
9x or 12x M4 washers
If necessary, clean the gears up with a small screwdriver blade, razor knife and some sort of abrasive - makes a huge difference in ease of assembly and improved performance. Careful printing will largely eliminate the need for this.
I've developed the habit of assembling the gearbox without the motor end attached and then rotating the sun gear by hand to assure that everything works as hoped.
Construction of single stage with spur gears is pretty straightforward.
Herringbone gears are a bit trickier to assemble. The annulus is split and the two halves are "spun" (not as easy as I make it sound) onto an assembled carrier that includes the sun gear/intermediate gear.
Assembling multiple stages is trivial with spur gears, not so with herringbone gears. Get ready for many traverses across the fun/frustrating border:
With herringbone gears, start at the output end; press the M8 bolt into the output carrier first. Put the planetary gears on loosely and then work the sun/intermediate gear into them so it meshes and then tighten the planetary gear screws. Place the output half on and work the planetary gears to mesh with it. Put the output end on with a washer on both sides of the bearing. Spin an M8 nut onto the output shaft to keep the whole thing together.
If multi-stage, assemble the intermediate planet gears with bearings, washers and screws so they're ready for placement on the intermediate carrier and set aside. Rotate the intermediate so that the nut traps are exposed and put M4 lock nuts in place. From here on out, keep the gearbox oriented output-up or the nuts risk falling out. During assembly, align the index mount to keep the annulus gear teeth oriented properly. Mesh the input intermediate half annulus with the planet gears without dislodging the M4 nuts. Do the same with the output intermediate half annulus with gear teeth down. Carefully screw on all three planetary gears onto the intermediate carrier and meshing with the intermediate output half annulus - don't tighten the screws yet. Work the sun gear into mesh with the planet gears as above, once meshed, tighten the screws. Spin on the input half and rotate the sun gear so that the set screw is facing the motor mount slot. Put on the assembled motor end, push screws through the mounts and tighten the whole package. Tighten the set screw.
Would appreciate input on proper gear design (clearance, lash, etc.); I ain't no mechanical engineer, just a hack. Also some guidance on choosing a good lubricant would be appreciated; currently using white lithium.
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