Planetary Gear Question 2

[Lowe12]

Sorry if this seams really simple, but it is my first time working on a project involving planetary gearing. I have a sun gear with a diametrical pitch of 20 teeth/in and 16 total teeth, planetary gears with the same pitch and 12 teeth each, and the annulus with 40 teeth and the same pitch.

As you can see in the attached image, I cannot get the gears to line up so that they all mesh with the annulus. Is there something that I am missing? I plan on having the sun and planetary gears stationary so that the outer housing rotates in the opposite direction relative to the center shaft.

Sorry if this is something really simple. It may just be something that is going right over my head.

http://tinypic.com/r/14abmg5/5

 

[JohnRBaker]

While I've never had to actually design and build a planetary gear train myself, I suspect that perhaps your problem may be in assuming that the three 'planetary' gears must be at a 120 degree spacing.

John R. Baker, P.E.
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[SNORGY]

I am definitely not an automotive or machinery expert, but from the diagram, I think the sun gear has to align with the annular gear in such a way that, at three points along its circumference, the centerline of one tooth is colinear with the centerline of one tooth on the annulus, and tha corresponding midpoint of each of those lines sets the shaft centerline of the other gears. You would then adjust the affected pitch and profile and, if necessary, diameter of all gears to suit that arrangement, as well as check for backlash etc.

 

[Timelord]

Also not a gear guy, but try 4 or 5 planet gears. The alignment between the sun and ring must be the same at each planet. 20 and 40 are divisible by 4 & 5, but not 3.

Timelord

 

[HPost]

Have you got the same tooth module on all of the elements?

The annular gear seems different. That will affect the PCD and therefore the number of teeth, which appears to be the problem.

 

[gearguru]

Lowe12,
the planetaries look simple, but there are some restrictions when designing them.
One of those restrictions is how many EQUALLY spaced planets (sometimes called pinions) can be allocated in the assembly.
For simple planetary (your case) you can calculate how many pinions can be put there:
(No_of_teeth_sun + No_of_teeth_annulus) / Number_of_pinions has to be integer.
In your case there can only be be 4 equally spaced pinions. (16 + 40) / 4 = 9

If you select an assembly with 16 teeth sun, 44 teeth annulus, 14 teeth pinion
you can locate 3, or 4 or 6 equally spaced pinions. Try your math...

gearguru

 

[gearguru]

In a 16/44/14 teeth planetary as mentioned above there would not be enough space for 6 pinions. Only 3 or 4 can be allocated equally spaced.
(that's another restriction - enough space for the pinions).
gearguru

 

[tbuelna]

For a simple planetary like your example, one option is to do what Timelord suggests and switch to a planet configuration of 2 or 4 that evenly divides into both the sun and ring gear tooth numbers. A second, more complicated option, is to perform some calculations to see if there is any arrangement of uneven angular spacing of the planet centers that will allow your gear set to assemble and mesh. The AGMA documents contain the necessary equation for making these calculations

Good luck to you.
Terry

 

[gearguru]

In my first posting there is an obvious error - it should be (16+40)/4=14 and it is an integer. Otherwise everything what I mentioned is valid.
Happy New Year to all
gearguru

 

[Lowe12]

Thank all of you guys for your help. Gearguru, it appears that your method works, and I've been able to get the gears to to mesh correctly with the three planet gear configuration by your calculations. If I need to change anything, I will definitely know how to do it now. Thanks

 

[gearguru]

Lowe12,
the formula I mentioned is mathematically derived, it has to work. The no's of teeth on sun and annulus each do not have to be divisible by no. of planets. Just their sum has to be.

 

[tbuelna]

AGMA 6123-B06 section 6 gives guidance on the design of non-factorizing epicyclic gear sets.

 

[gearguru]

I agree with Tbuelna, that standard is a good guide to design quality planetaries. Also the "hunting" relations between the teeth numbers of sun, pinion and annulus can be evaluated, especially if the planetary will be produced in larger quantities. That's also covered in that standard (at least in the older version, I do not have the newest one).
PF 2014