Designing a Planetary Gearbox to increase speed?

[scuba1000]

I have been researching ways to design a planetary gearbox. It seems that the main configuration of the gears is to have the ring gear fixed, output the rotation of the planet gear carrier, and use the sun gear as the input. This would produce a reduction.

I have found little information about using a planetary gearbox the other way around to multiply speed. Is it commonly done or is there some other alternative to this? The design would be to have the ring fixed, the gear carrier as the input, and the sun as the output. Is the ratio just the reciprocal of the normal ratio.

I did see that the Wikipedia article on planetary gearboxes said that the ratio for what I'm talking about is 1+ Nring/Nsun. But it said a citation is needed, so I just wanted to double check.

[MechEng83]

The most common example is an old-school pencil sharpener (although maybe not as common anymore). The handle is attached to the carrier and the planets spin relatively fast compared to the crank handle, which causes several rotations of the sharpening cylinders around the pencil. There is no sun gear in this case, as the desired output is shearing of pencil pieces, rather than shaft output speed.

Yes, the ratio with a fixed ring gear with the sun as the output is 1 : (1+Nring/Nsun). You can do any of the 6 combinations of sun, carrier, or ring fixed with the other 2 being input and output. The math gets kinda fun, and I'm sure I could dig up an old textbook to cite. Certain configurations even produce backwards rotation.

[Tom Ore]

Ns * Ss + Nr * Sr = (Ns+Nr) * Sc

where N is number of teeth and S is speed, s, r, and c are sun, ring, and carrier.

If the speed of the Ring (Sr) is zero then:

Ns * Ss = (Ns + Nr) * Sc

or

Sc= Ns / (Ns + Nr) * Ss

(This is for a simple planetary)

[scuba1000]

Excellent. Thank you both.

[IKE]

Originally Posted by Tom Ore

Ns * Ss + Nr * Sr = (Ns+Nr) * Sc

where N is number of teeth and S is speed, s, r, and c are sun, ring, and carrier.

If the speed of the Ring (Sr) is zero then:

Ns * Ss = (Ns + Nr) * Sc

or

Sc= Ns / (Ns + Nr) * Ss

(This is for a simple planetary)

As Tom is showing, technically, all 3 can spin at a given rate. One of the more recent and famous versions of this is the drivetrain of a Prius:
http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive

In your scenario, you are used to seeing a PSR or Planetary speed reducer. This is usually because you have a high speed source (like an electric motor) and want high torque to do some useful work.

Windmills are the opposite and have PSI or planetary speed increasers as they blades produce a lot of torque, but the generator wants higher speeds.

[DonRotolo]

Originally Posted by scuba1000

I have found little information about using a planetary gearbox the other way around to multiply speed. Is it commonly done or is there some other alternative to this?

While it can be done, in FRC it usually isn't, because our motors are generally too fast and we need to slow the speed down (without losing power). All the info above is good.

[Clem1640]

To increase speed, you need to drive the carrier (with a stationary ring). The Sun gear would be output. This is the opposite of the normal FRC arrangement.

[Foster]

If you want to play with one to see how they work, look for the Tamiya Miniature Planetary Gearbox Kit with Motor. They are about $20 and come with 4 sections to give you multiple reductions / increases.

(General plug, Tamiya makes a series of motor/gears/transmissions/etc. that are easy to assemble and watch how they work. )

[Paul Copioli]

If you want to learn more than you ever wanted to know about multiple moving parts of a planetary transmission, please look here: http://www.chiefdelphi.com/media/papers/1361

We did this in our 2002 robot and never got it working like we wanted until Championship. It worked like a charm then.

This was the very first year the CIM motor was introduced.

[kevin.li.rit]

Originally Posted by IKE

As Tom is showing, technically, all 3 can spin at a given rate. One of the more recent and famous versions of this is the drivetrain of a Prius:
http://en.wikipedia.org/wiki/Hybrid_Synergy_Drive

From Wikipedia

Originally Posted by Wikipedia

This limited gear-ratio set, forces the engine crankshaft to rotate at speeds where the ICE is less efficient, i.e. where a liter of fuel produces less joules.

Does this make sense? Does a limited gear-ratio set on a ICE produce incomplete combustion?

[DonRotolo]

Originally Posted by kevin.li.rit

Does this make sense? Does a limited gear-ratio set on a ICE produce incomplete combustion?

I did not check the article, but it is a fact that certain IC engine speeds are more efficient than others. Depends on the engine of course but efficiency generally peaks around 2500-3000 RPM. If you put a stupid gear ratio in you can force efficiency down.

[kevin.li.rit]

Originally Posted by DonRotolo

I did not check the article, but it is a fact that certain IC engine speeds are more efficient than others. Depends on the engine of course but efficiency generally peaks around 2500-3000 RPM. If you put a stupid gear ratio in you can force efficiency down.

You can force efficiency down, but does a litre of fuel produce less joules as stated? I assume this is only possible due to incomplete comubstion?

[Ether]

Originally Posted by kevin.li.rit

You can force efficiency down, but does a litre of fuel produce less joules as stated?

No. It produces fewer joules.

I assume this is only possible due to incomplete comubstion?

The joules being referred to are mechanical output joules at the engine crankshaft, not joules of combustion heat.

[DampRobot]

Originally Posted by Ether

The joules being referred to are mechanical output joules at the engine crankshaft, not joules of combustion heat.

I'd presume that fewer joules of the chemical are released at all--IE some of those joules are turned into entropy.

[Ether]

For a given required mechanical power output at the engine crankshaft, there is a continuum of engine_speed + fuel_flow_rate combinations which will produce that power. Only one of those pairs minimizes the fuel_flow_rate.