Epicyclic Gear Train, why is net radial force near 0 2

[morris9791]

Dear Experts,

I have a query regarding tangential and radial forces present in an epicyclic gear train.
I understand that the tangential force is responsible for transferring torque between the mating gears, while the radial forces tend to push the gears apart etc.

I read articles stating that the radial force is irrelevant in epicyclic gear trains but does not give a concise explanation why.
Can anyone shed some light on this query?
Even better, are there good books/ web sources that explain loading on epicyclic gears? I have machinery’s handbook and Machine Design by Robert Norton but they are not useful for epicyclic systems.

Something is telling me the radial forces cancel each other outer at the ring / planet and planet/sun interfaces but I need solid proof in terms of references etc.

Any information will be appreciated.


Kind Regards
Ed

 

[Timelord]

morris9791,

You list your field as mechanical, so it should be obvious. Draw a free body diagram and resolve the forces at the gear tooth faces into tangential and radial components for the gear train. This will demonstrate that the radial forces on the planet gears cancel.

Timelord

 

[morris9791]

Timelord,

Thanks you for your response. Yes, I have drawn the FBD of planet gear mating with the internal ring gear and the sun gear.
I have resolved the applied force acting along the line of the pressure angle into its radial and tangential components. I can see that the radial forces cancel but the tangential forces also eliminate each other since they are opposite and equal.

Perhaps I am missing something.

Regards
Ed

 

[BobM3]

The tangential forces are in the same direction (not opposite) for the planets.

 

[tbuelna]

Technically, I believe a single planet or asymmetrically indexed multiple planets would produce an unbalanced radial force result. Right? Theoretically, the gearset would work. But I can't think of any good reason to construct a gearset in such a manner.

Good luck.

Terry

 

[gearguru]

tbuelna: the planets are always radially balanced. What you mean is, that the sun and annulus are pushed away from the single planet, correct?

 

[plasgears]

A simple spur gear mated pair will exhibit a separating force and tangential force because of the pressure angle, typically 20 deg. Now when you have several of these interactions around the sun gear, the separating forces will cancel. Of course, the tangential forces are additive and contribute to torque.

 

[gearguru]

plusgears - I agree, but only if the planets are equally spaced around the sun gear.

 

[tbuelna]

gearguru,

You are correct. I should have been more specific.

Regards,
Terry

 

[gearguru]

And I should be more specific too... There are configurations where the radial forces eliminate each other even if the planets are not equally spaced. One example would be if we remove two planets from the original 6 planet planetary. Now the 4 remaining are located at 120/60/120/60 degrees and the sum of radial forces will still be zero.
The free body diagram is always the key to the solution.

Now when we covered all configurations (I hope :-D ) let's put the turkey in the oven... Happy Thanksgiving to all fellow US eng-tips members!