How to make Planetary Gears

[kimvellore]

How does one make the planetary gears with GM?. I mean being able to make and simulate, with either 4 or 5 internal ones. Something similar to the example in the tutorial for output manager.

Thanks
Kim

[Dan]

Kim,

Fairly simple. Use an internal gear with 3 times the pitch diameter (3 times more teeth) of the planetary gears. Likewise you can use any ratio between the centre gear and the running gears, just remember that the sum of the pitch diameters of two running gears and the centre gear must be equal to the internal gear's.

Dan

[kimvellore]

Dan,
  I tried that it works for 5 inside gears because they are 90 degrees apart and it snaps at 90 degrees but if you want to do the 4 inside gear version where the three gears are 120 degrees apart it is a close placement by nudging the gears, not exact so the side plate cannot be used directly.

Thanks
Kim

[Dan]

Ah... if you want to have 3 gears on the circumfer ence this will be a problem. I think a careful calculati on needs to be done to ensure they all mesh.

Dan

[ArtF]

Hi Guys:

  very true, to place at 120 degrees does take nudgeing, I suspect the best thing woudl be a wizrd to do this in future.. and after bevels are working well, I have an education al build to do for the US school system, then Ill look at wizard capabilit ies.. others have requested a ratio calculato r and a diamteric calculato r for various gears. Ill look at that and start discussio ns on it when I get to a place where I can develop that into the program.

Thanks
Art

[Anthony]

Ah... if you want to have 3 gears on the circumfer ence this will be a problem. I think a careful calculati on needs to be done to ensure they all mesh.

Dan

I think in the example suggested, where the ratio of the sun gear to the ring gear is 1:3 and the planets are the same as the sun, the gears will mesh at 120 degrees as long as the sun gears tooth count is divisible by 3. When the sun and ring gears are not a 1:3 ratio the situation will have to be investiga ted further, I think some will work and some not.

If you create a compound planetary gear train I think you can always get the gears to mesh correctly but the planet compound gears may not be identical . The element meshing with the sun may have to have a customize d rotationa l offset from the element meshing with the ring gear.

Anthony

[Dan]

Not sure about that, Anthony. I think the ratios have to be precisely considere d for them to mesh at exactly 120 degrees apart.

Dan

[dagray]

I thought I remembere d this from my school days, but it took an entire day to find a reference .

In the  Machine Design Data Handbook (McGraw-Hill, 1994), page 25.49, "Condition s of Proper Assembly of Planetary Gear Transmiss ion", It says (I'm paraphras ing):

Two Planets:

Both the sun gear and the the planets may have either and odd or an even number of teeth;

Three Planets:

If the number of teeth on the sun gear (z1) is divisible by 3, then the number of teeth on each planet gear (z2) must be divisible by 3, AND:

If (z2-1) is divisible by 3, then (z2+1) must be divisible by 3, AND:

If (z1+1) is divisible by 3, then (z2-1) must be divisible by 3.

Four Planets:

If z1 is even, then z2 must be even, AND:

If z1 is odd, then z2 must be odd.

ALSO VERY IMPORTANT FOR ALL PLANETARI ES:

The condition for the teeth and spaces of the meshed gears (sun, planets and ring gear) to coincide when the planets are arranged uniformly over the circumfer ence is : (z1+z3)/a = q, where: a = the number of planets, and q is an integer.

I haven't had time to make an Excel spreadshe et which will confirm these (Even textbooks have typos!) but I believe they are correct. Check it out... I think these rules are in the Machinery's Handbook Gear Section also.

[ArtF]

Thank you, I wasnt aware of those rules myself...

  Another nugget of wisdom to remember.

Art

[Dan]

Thank you for the rules. Gearotic proves them true.

By the way, this: If (z2-1) is divisible by 3, then (z2+1) must be divisible by 3 seems a stupid rule. It is a math rule

Dan

[BobL]

lol

 

[Dan]

Yes, Bob. Checked all the numbers from 0 to 100 already and none seem to satisfy that rule... Will keep working on it tomorrow checking all the numbers to 1000, may be I'll find... 

Dan

[Anthony]

One of John Stevenson's tricks is to cut (by hobbing) gears with non-standard pitch circles. As I recall he has posted photos of 20, 21, and 22 tooth spur gears (or some similar set) hobbed with a 20 DP cutter and all having the same pitch circle. I believe the claim is that they will all mesh with each other and with any other standard 20 DP gear.

Can gearotic produce files capable of use for cutting gears with a non standard pitch circle?

Let us suppose, for some reason, you wanted a 33 sun and a 100 internal ring, coaxial obviously . For correct fit this would require 33-1/2 teeth on the planet. If you could cut a 33 on a large PC or a 34 on a small PC it would work. Other solutions, change the tooth count of the sun or ring, or double the tooth counts of both and use twice the original DP 9half the tooth size). Changing tooth counts which change the ratio may not be acceptabl e, same with more but smaller teeth. (I will admit that a 33 / 100 sun / ring ratio would not be a common concept, it's just put forward to illustrat e the issue.)

Anthony