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Need advice on creating planetary gear

Kunstmaan

Alibre Super User
LS,
I am a litle bit stuck here as Iam not a gear expert.
Need to create a planetarygear wiht:
Reductionratio = 21:1
Sun theet=23 & OD=18mm.
I have been looking on the internet for some examples, but found nothing simple.
So I need some help here how to calculate the Planets and the outer-ring.
Would like to use the create-gear script in the AlibreExpert examples

Thnx in advance,
Berdien
 

albie0803

Alibre Super User
Hello Berdien,

23t seems a lot for a sun gear

a ratio of 21:1 with a 23t sungear has a ring gear with 483 teeth in it

a sun gear with Ø18 OD equates to 0.75 Module

The sun gear would have an internal diameter of 361 mm and the planetary gears would have 230 teeth!

1650710702374.png
 

albie0803

Alibre Super User
This is a 2 stage 25:1 gearbox that we overhaul here at work. What you want will have to be along these lines to get the reduction you want.

1650781866914.png
 

Oldbelt

Senior Member
PlanetGear 1-5.png
1:21 can only be made by two gear in "series" : 1 : 3 and 1:7
It better to made two similar gears as gears as Albie propose.
Hear example at a 1:5 planet gear.
Using my parametric gear templates.
 

Attachments

  • PlanetGear (1).AD_PKG
    759.2 KB · Views: 10

albie0803

Alibre Super User
It's still not that simple. In a stacked set like I have shown, the ring gears would be fixed, so movement is transferred to the second gearset by the revolving of the planetary carrier that has the sun gear for the second set attached to it. Also, just to make things interesting, using Oldbelt's 5:1 gearset, the sun gear has to actually turn 1 more time in order for the planetary carrier to do a full rotation, making it a 6:1 ratio. This seems to be consistant with any ratio.

Use your catalyst link and set the following values:
1 planet
Solve for Planets and Carrier
Sun 10
Planet (15)
Ring 40
Sun Speed 0
Carrier Speed 0
Ring Speed 0

Note the position of the sun gear dot
Set the sun gear running (Change sun gear speed)
You will see that the sun gear needs to rotate 5 times to bring the planet back to the start position, making it 5:1 even though the gearset ratio is 4:1 (40/10)

My example runs the second set the same way with the output rotation driving from the second carrier.

What are you trying to achieve? Do you need a moving or fixed ring gear? Is the ring gear to be driven or a shaft?

We really need a lot more information to give any more real help.
 

Oldbelt

Senior Member
Sorry to say don't let you confuse,, use my solution, the baseplate is fixed and the ring gear is the output axel.
My post isn't a full design, axels, ball bearings ect. had to be added.
You most finish the ring gear with a "bottom" and an axel with the sun gear for next planet gear.
Try my solution and turn the sun wheel 5 rev. with the curser and the ring gear take 1 rev.
Another solution is to make a 1:4 and a 1: 5 gear in Serie, that's nearer your wanted 1:21 , it gives 1:20.
Why do you want to use Alibre script?. Nearly every thing can be done direct in Alibre, the theoretical stuff can be handled in
the Parameter form. The theory shall also be known if one want to made a script.
Try my templates there are few control input needed : amount of teeth's, module, pressure angle, width of gear.
Have fun :D
 

Kunstmaan

Alibre Super User
View attachment 35890
1:21 can only be made by two gear in "series" : 1 : 3 and 1:7
It better to made two similar gears as gears as Albie propose.
Hear example at a 1:5 planet gear.
Using my parametric gear templates.
That looks great, but if the ring is stationary and the planets are the output do whe have the same ratio?
 

Attachments

  • carrier plus planets.AD_PKG
    907.6 KB · Views: 7

Oldbelt

Senior Member
Miles the carrier is fixed in my proposals, if you had opened and down loaded my Pck.- file . and tried it, you would know.
If Kunstman want the ring gear to be stationary, why haven't she told,, then it another story
and you are right Miles.
 

Kunstmaan

Alibre Super User
Miles the carrier is fixed in my proposals, if you had opened and down loaded my Pck.- file . and tried it, you would know.
If Kunstman want the ring gear to be stationary, why haven't she told,, then it another story
and you are right Miles.
Sorry OldBelt,

I should have given more info.
Input is the sungear wiht 23T on a 8mm dc motor axle
The output is de planetgear-carrier on a 12mm axle
The unit is used to control resistor bank that controls a high-voltage dc-motor.

Berdien
 

Oldbelt

Senior Member
Dear Kunstmaan.
I have calculated i little at your project.
A planetary gear with 3 planet wheels must have a sun gear wheel where the amount of teeth's is a multiple of 3 : Z = 21 , Z=24 ; your wheel with Z=23 is impossible to use.
The same rule for ring gear wheel.
The planet gearwheel must have an even number of teeth's.
i = 1+ Z ring / Z sun ; i 1 = 1+ 96 / 24 = 5; i2 = 1+ 96 / 32 = 4 ; I total = i1 * i2 = 5*4 = 20.
i = 21 is impossible to make.
The planet gearwheel shall only "fil" them gab,
I will come back when i have calculated the Planet wheel Z1 and Z2
 

albie0803

Alibre Super User
Um... I'm not sure about your reasoning as the planetary gearbox picture in post #7 has 11 and 23 tooth sun gears in it, neither of which are divisible by 3.
The Planetary gears are 20 and 37 teeth respectively and the ring gears are 97 and 52 teeth.

As shown in post #2, a single stage 21:1 planetary can be created. The number of planets limited only by the space needed to fit them in.
Multi stage planetary gearboxes are created to limit the space needed to build them in.

A major factor we don't know is how large can the ring gear be? This will determine the largest ratio that can be done in a single step and then we work down from there.

I think a possible solution is a 8:1 driving a 4:1, carrier driven to carrier output adds another rotation to each sun gear making it a 7:1 driving a 3:1 which equals 21:1.

23-184 and 20-80 for example. Edit: I'm pretty sure this is wrong
 
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Oldbelt

Senior Member
I have never commented the picture of the gear you uploaded in post #7.
I think a possible solution is a 8:1 driving a 4:1, carrier driven to carrier output adds another rotation to each sun gear making it a 7:1 driving a 3:1 which equals 21:1.
How can the gear you describe in the quote be designed, try to make this gear, to show what you mean, !!!

In the last line in post #18 you propose 23-184 and 20-80 , i = 1+ 184/23 = 9, i2 = 1+80/20 = 5; total I 1:45; !!!!
I can't follow your thinking, it's like you have studied another textbook than I have.
 

albie0803

Alibre Super User
Hey Oldbelt,
To be honest I don't have it clear in my head how to translate the +1. I understand that a gearset with a 10t sun gear and a 40t ring gear is a 4:1 ratio when the ring gear is being driven, but needs 5 sun gear rotations in order to rotate the carrier once. Does that mean I actually need a 5:1 ratio to get the equivalent full carrier rotation or do I just end up with that? Even though it is a 4:1 gear set the actual input rotations to output rotations is 5:1. Thinking as I type, it seems what I actually need is a 6:1 gearset (with a 7:1 input/output) driving a 2:1 gearset (with a 3:1 input/output).
Input driving carrier/input driving carrier/output (as in the model in #7) resulting in a 21:1 reduction.

I freely admit that I may be wrong with this but.......

I didn't agree with your sentence A planetary gear with 3 planet wheels must have a sun gear wheel where the amount of teeth's is a multiple of 3 as the first stage of the gearbox in #7 has 3 planetary gears and a sun gear with 23t, planetary gears with 37t and a ring gear with 97t. The modelled gearbox exists, we overhaul them regularly. Also the planetary gear is not even or the ring gear divisible by 3.

As I said, we really need to know the maximum diameter that the ring gear tooth root can be in order to determine what ratio is possible.

This is a 6:1 ratio with 23t sun gear. 1+138/23 = 7 which has a ring gear root od of 105.975
I've chosen 0.75M as the sun gear works out close to the 18mm given earlier.
1651468804374.png

It's interesting that the modelled gearbox ratio in #7 is 97/23 = 4.217, giving a (+1) 5.217 ratio. Obviously power, torque and tooth strength as well as limiting dimensions all play a part in the final design of a planetary gearset.

I don't have to be right, but I do want the answer to be right. :)
 
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