I would like to make a planetary gear with inner gears and center gear

[ArtF]

there are a few rules to that... I used to build them by feel , but heres the rules.. Ill be adding a module eventually to
do this for you.

zC = outside rings toothcount;
zA = center gears toothcount.
zB = planetary gears toothcount.

1) zC = zA + 2zB;
2) (zA + zC) / (numberofPlanetGears) = INTEGER
3) ((zA + zC) * AD) / 180 = INTEGER ---- AD = 0.5 times the placement angle so since we'll place 4 at 90 degrees apart, AD = 45.

So using those numbers lets build one with 2 knowns.

Zc = 60; zA = 16;


so.. Zc = Za + 2Zb is
    60 = 16 + 2zb
    60 - 16 = 2zb
    44 = 2zB
    zB = 22; ( rule 1)
    (16 + 60) / 4 = 19 ( an integer so good for Rule #2)
    ((16+60) * 45) / 180 = 19 ( an integer so good for Rule #3)

   
   
    So we have 60, 16 and 22.
   
  1)  So start by placing a 60 tooth internal on the project screen.
  2)  Set the pinion to 22 teeth. Press add to project and make sure you click the ring ( it will be blue when its being pointed to..
      Place the gear at 0 degrees.
      If you accidentally select the wrong gear, just press "escape" to cancel the placement and try again.
      Place a 22 tooth at 0, 90, 180 and 270 degrees exactly.
  3) Set the pinion tooth  count to 16, press add to project. Select any of the four planets you set down.
      Roll the gear to center. When it hits center notice its shaft center is now bang on center. Also, all teeth
      on all gears now mesh up. Press simulate. Congratulations, you have your first planetary gear system.     
 
 
  Art

[Ken_Shea]

Art,
Using your example, how would the gear ratio be determined?

Thanks
Ken

[ArtF]

Ken:

  The ratio depends on what you drive .. or what the input is and what the output is..
and which of the three components you hold still...
So we have zRing = 60, zPlanet = 22, zSun = 16.. from out example.

For example, you can drive any of the 3 types, if you take the sun gear in the center as your
input and look at the planetary gears  as your output with the ring held stationary  the ratio is

  1 + zRing / zSun =  1 + 60/16 = 4.75:1

or, if you drive the planet gears and hold the center sun gear stationary the ratio is:

  1/( 1+zSun/zRing) = .7936 : 1

or , if you hold the planetary gears still and rotate the center gear, the output ring will give you a ratio
of
    -zRing / zSun = 60/16 = -3.75:1

  Now if you hold any 2 gear set stationary, you then have a 1:1 ratio. By using mechanical means to switch
which gears you hold stationary you can make a transmission that switches ratios on demand without any gears having
to unmesh or remesh... but thats out of scope of this discussion..

Theres many ways o use a planetary gear set, I think it will need a module and new simulator code to
allow you to lock any part of it to make it fully understandable. In the meantime the above formulas
can help you figure out the ratio of what youve made. Of course we can then use all of the formulas from this
and the previous question to figure out what tooth counts to use to get a ratio your looking for...
but thats another question and another day. :)

Art

[Ken_Shea]

Art,
Thanks for that information, saved it to the PC.

Ken

[John T]

Hi Art,
I've tried planetary gears before but without great success, so I decided to try and follow your posting.

The planetary works!  I do have one question - as you can see from the photos the teeth are quite "pointy" and the sun gear seems to be points to points.  Is this a function of the planetary button on the spur gear tools?  I checked the epicycloidal and the involute and the only one thats active is the involute.

Makes for a neat model - but curious about the tooth shape,

John

[ArtF]

John:

Those teeth are far too small. Looks like youve added backlash and left it on, is the Width setting set to .5? Something is wrong
for sure, the planetary I described should work nicely and tight fitting as an involute. Looks to me as if you tooth profile is being crushed..


Art

[John T]

Hi Art,
I went back to the design and didn't discover a backlash setting it was as 0.5  never the less I reloaded the program and started fresh.  This time it works as advertised.  Must just be a gremlin (operator) at work somewhere.

John

[ArtF]

Hi John:

  Thx for the update, yes, thats much nicer looking. On the bright side of all this , youll now remember that if a planetary
is too tight for you, just slightly lower that .5 to like.. 0.48 and it can make things run much more smoothly..

Art

[Colin Angevine]

A very helpful discussion! Thanks so much.

I have a related question: I'm hoping to build an object that will have a few layers of planetary gears like these. All layers will have common shafts for the planets, and a common drive shaft for the suns (as input). I am struggling to figure out how to set the size of the gears properly to make sure that there is consistent shaft spacing at each level. Can you offer any guidance?

thanks,
Colin

[ArtF]

Colin:

  Ahh yes, I think I know what you mean.

  You need to look at the center distance of the first sun and planetary and maintain that through all the rest of the layers.
There is a calculator on the round gear tab that can help you figure out a module to keep random tooth counts at a set
center distance, thats one way, though it means each layer may have a different module for its gears.

  If you wish to maintain the module so all gears are the same, I think its a matter of manipulating the formula's we spoke of
so that the end toothcount totals are the same. ( I cant verify that though yet..).

I can see form theinterest we'll need a module to make these and hypocycloidal gearboxes as well eventually...

Art

[Colin Angevine]

Fantastic! Thanks so very much for your help. I'm getting very close to something that works, but have two follow-up questions:

1 - How can I specify the angle at which I would like to place the planet around the sun? When I am adding the gear to the general sketch, I don't see where I can specify 0 / 90 / 180 / 270 degrees to place the planet.

2 - How can I add the outer / orbital gear to the sketch so that it is driven by the planet? When I add it, it automatically gets driven by the central shaft rather than being driven by the outer planetary gears. (And when I try to change a linkage, it says that there are no more undriven gears... but I want the orbital gear to be driven by the planetary gear, which is driven by the sun gear.)

I made sure to check the 'notify me of replies' box this time -- I'll be sure to reply more quickly this time around! Thanks for your help,
Colin

[ArtF]

Colin:

  The angle is arbitrary, if you have 4 , they must be 90 degrees apart, but it doesnt matter where, so they can be placed anywhere so long as
they are 360/nPlanets apart form each other.

As to the simulation Im afraid you cant. While you can simulate at the moment based on placement order, G2 doesnt have
the final degree of freedom necessary to allow for a full simulation of planetaries. They have multiple possabilites based on
whats locked in position. It is planned to add another degree of freedom at some point, but you cannot do that as yet..

Art

[Colin Angevine]

Thanks so much, Art!

[wfrancis]

I found this thread very helpful and a I wrote a small python program which will give you valid possible combinations given a desired ratio, desired range of teeth and number of planets. I put in a little bit of a fudge factor from the desired ratio that you can adjust as well. I looked at a couple of the browser embedded python apps but the ones I found don't seem to support the is_integer() for floats.

#!/usr/bin/env python
"""

Rules taken from here:
http://gearotic.com/ESW/FavIcons/index.php?topic=1074.0

"""

desired_ratio = 2.50
ring_max = 75
ring_min = 40
sun_max = 75
sun_min = 10
planets = 4

print "Ratio, Ring, Sun, Planet"
for rteeth in range(ring_min, ring_max):
    for steeth in range(sun_min, sun_max):
        ratio = 1 + (float(rteeth) / steeth)
&nbsp; &nbsp; &nbsp; &nbsp; if (ratio * .98) < desired_ratio < (ratio * 1.02):
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (((float(rteeth) + steeth) / planets).is_integer()):
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if ((float(steeth) + rteeth) * ((360 / planets) / 2) / 180).is_integer():
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; print ratio, rteeth, steeth, (rteeth - steeth) / 2

[Mooselake]

there are a few rules to that... I used to build them by feel , but heres the rules.. Ill be adding a module eventually to
do this for you.

Art, if you ever get a chance to revisit this can you consider adding appropriate backing plates for the various combinations of stationary elements and make them 3D printable?

Kirk