Help with planetary gearbox design 2

[suren1982]

Hi, I want to design a planetary gearbox with a ratio of 1:2. the output will be used to spin up a 25Kg flywheel. I have read that it is possible to design a planetary gearbox that increases the final rpm, but I am unsure of the geometry/design to achieve this.

This is what I believe will achieve the 1:2 ratio.

Power input will be through the sun gear, output will be through the planet gear carrier (the ring gear is fixed).

Sun gear has 50 teeth, planet gears have 25 teeth, ring gear has 100 teeth. (sun gear rpm: 3000, planet carrier rpm: 6000).

Does this sound right? The more I think about it the more confused I get. There isn’t much information on planetary gearboxes on the net, and so any information/advice will be greatly appreciated. I know this is going to be a difficult task, and I am at the beginning of the design stage, but its the best way to learn… by doing!

Suren

 

[israelkk]

The best way is to get a hold of a book and read. Gear Handbook, or Handbook of Practical gear Design both by Dudley are probably the best. You can look at AGMA publications too.

 

[diamondjim]

Looks reasonable. You might have to stub the
internal ring gear addendum. Chech the mesh of 25 teeth
and the 100 tooth inner gear. Also check the
25 tooth planets mesh with the 50 tooth gear.
I do not think you can space the 3 teeth perfectly
at 120 degrees and have them all mesh.
I think all of the individual components must
be divisible by a prime number. 5 is a prime
number that would work so 5 planets would work.

look at 24, 48, 96 teeth combination with 3
equally spaced planets or 27, 54, 108 teeth
combination.

 

[BobM3]

Wouldn't this give you a 1:1 ratio? With the ring gear fixed I don't think it's possible to drive the carrier faster than the sun.

 

[gearcutter]

What you suggest will give a ratio of 3:1, "Nr/Ns + 1 = Ratio", where Nr is the number of teeth in the ring and Ns is the number of teeth in the sun.
What you're after is a "Star" set where normally the cage is fixed, the sun is the input and the ring is the output, "Nr/Ns = Ratio". This will give you 2:1. I feel if you reverse the configuration, making the ring the input and the sun the output, it should yield what you're after. Direction of rotation between input/output will be reversed.
Remember that it’s best that "Nr + Ns / No. of planets" equals an integer.
Be very careful as the speeds you're suggesting will yield extremely high velocities at the planets & sun gears tips, the planet’s will be rotating at 12,000 rpm. I don't think your proposal could be manufactured economically as grinding of the component’s teeth would be necessary and grinding internal gears is very specialized and expensive.
To help keep the noise and vibration down helicals would have to be considered making the set even more expensive. Then there’s the specialized gear advice you’ll require to calculate the tooth geometry for a “speed increasing” gear set, the helical angle, the power requirements, etc, etc, etc.
Good luck!

 

[unclesyd]

The basics.

http://www.roymech.co.uk/Useful_Tables/Drive/Epi_cyclic_gears.html

The calculator.

http://www.wmberg.com/Tools/epgear.htm

 

[gearguru]

Your planetary creates the desired ratio if the input will go to annulus (ring gear), output to sun( which will rotate in the direction opposite to the input) and the planet carrier will be fixed.
The selected tooth combinations do not allow for 3 equally - 120 degree - spaced planets (pinions). And 5 of would be probably a problem to put there, seems to me there is not enough space for them...
Try another no. of teeth combinations and look in some books.

 

[gearguru]

Please disregard my statement above about 3 equally spaced pinions - you CAN locate them at 120 degrees in your planetary. Sorry!

 

[suren1982]

Thanks everyone for your help. I think what I was planning to design is beyond my funds and my knowledge! To achieve the 1:2 ratio, I will have to drive the ring gear and take the output from the sun gear. This changes the layout I had in mind totally. I do plan to get hold of a copy of the ‘Handbook of Practical gear Design - by Dudley’.

Thanks for your advice.

Suren

 

[gearguru]

suren1982,
One more problem with planetaries: the rpm's of the planets. If you make planetary as mentioned above and drive it from ring gear to sun with fixed carrier, then the planets will rotate 100/25 = 4 times per every rotation of the annulus. Now if your input speed is 3000 rpm, we are talking about 12000 rpm for the planets.
I looked little bit closer at your gear teeth selection. You could instal 3 or 5 or 6 equally spaced planets using 100 teeth ring and 50 teeth sun.
There is an ANSI/AGMA 6023-A88 standard "Design Manual for Enclosed Epicyclic Gear Drives; it is really worth to study it when working with planetaries.
Good luck!