I have seen them on this forum a lot for different applications, but I was never sure what it did for the machine/vehicle.  Is there a mechanical advantage? adds torque/speed?

first,drive shaft and output shaft inline. useful in small space.

second, planetray gear mecanisen has little backlash. this also useful for need some accuracy machine.

Some vehicles use them in place of a portal axle; the builder uses a turntable and 8z gears inside to reduce the speed of the driven wheel and increase the torque after the driveline. Real vehicles use planetary gear sets quite a bit. For example, most automatic transmissions operate using several planetary gear sets; the various rings can be clamped by the transmission to make the different ratios. In fact, the Ford Model-T used a planetary gearbox operated by pedals. Also, many older cars (e.g. an MGB) had a manual transmission and a planetary overdrive operated by a switch. In that case, it lowered engine rpms -- so added speed.

Subtractors also could be said to rely on planetary gearsets, and those are useful for tanks and other things with caterpillar tracks. Those allow tracked vehicles to be controlled using a motor for driving and a motor for steering (see some of Sariel's excellent tanks and this: http://sariel.pl/2009/01/subtractor/).

Lastly, if the right part of the planetary gear set is held in place (the center with idler gears), it can be used to rotate two things in opposite directions, as in this set: http://www.bricklink.com/v2/catalog/catalogitem.page?S=8960-1#T=S&O={} The drill has two counter-rotating parts done using a planetary gear set.

Does that answer anything?

There are multiple reasons for using a planetary reduction set vs. other options.

1. Planetary sets offer relatively high gear reduction ratio than others

2. They're often used in 'final drives/reductions' for many construction machines.

3. You can also find them in typical automobiles transmissions (automatic only)

Gear are made to transmit power, power = force × speed = Torque × Angular Speed. Often, the gears/shafts/bearing sizes are dictated by the load applied on them. As such, if you have an engine driving a vehicle, you want to keep the components' sizes as small as possible for the same power. Thus, let the speed be high. But once you get to the tires (or tracks), you need very high gear reduction, and that is why planetary sets are often used in wheel/hubs of large construction machines. In lego though, we rarely talk about torque and thus, replicating a planetary drive is just a curiosity for many of us mechanical engineers to try and reproduce real life mechanisms.

They are also used in the Power Functions motors. 

 

Small space for the high reduction. and I think they can handle alot of power aswell. 

A feature that i think it is important of the planetary gears in vehicles is that you can put them very close to the wheels and they wont affect the position of the wheel compared to the axle, and so, the whole transmission from the engine to the planetary gears doesn't need to be too big or robust like it would if it was directly connected, this is why most of the trucks which use these planetary gears in the axles tend to have big tires and a very good ground clearance, for example, M.A.N KAT, MB Zetros and some Tatras, these planetary gears can multiply the torque many times and they need a considerable small space to place them; as well, the planetary gears are used in some other kind of machinery, gearboxes, electric motors (like the Lego ones), and many other things.

Planetary gearboxes are useful in many real life applications. I used to work for a company which makes gearboxes for knife drives on harvesters. The rotary movement of a belt pulley is being converted into a pure linear movement of the rail where the knives are attached:

 

Another interesting mechanism is this:

Both planetary gearboxes near the center diff regulate (by intelligent software) the acceleration/braking on the outer/inner wheels which allows improving a car´s performance through cornering curves, depending on the radius and current speed.

The push apart forces between the gears are balanced when you use a planetary gearbox, so there are lower bearing loads, so the bearings can be smaller/run faster/last longer. Very useful when you get to high loads, so they are used a lot in winches. You also get a much higher reduction ratio compared to spur gears (standard Technic gear arrangement), again useful in winches. 

so basically its down gearing in the compact system? If I have a high output, and add a planetary gear, the output shaft will be slower with more torque

4 minutes ago, aminnich said:

so basically its down gearing in the compact system? If I have a high output, and add a planetary gear, the output shaft will be slower with more torque

Exactly, and it could also be used in the opposite way, to make the output shaft with less torque but faster. 

Just now, Imanol BB said:

it could also be used in the opposite way, to make the output shaft with less torque but faster. 

That is what I was looking for, can you post a picture of something like that? 

This is what I think of when I hear planetary gearing

sorry if the picture is too big

for extreme gearing down, at least that's what i used in a large Trial truck, and for tilting robotic arms etc

4 hours ago, Matty04 said:

for extreme gearing down, at least that's what i used in a large Trial truck, and for tilting robotic arms etc

I have done that too, i actually made a planetary version of the portal hubs of the 8110 Unimog which were inspired by Efferman planetary axles, here is a comparison of the standard portal hub part with 3:1 reduction and my own version, which has a reduction of 12:1, both portal hubs have the same distance to the pivot point and have very similar dimensions, the advantage of using the planetary version is the increased torque (By 4 times) and decreased damage to the gears and axles in the hubs, the torque is divided in 4 gears and pins instead of 1 gear and axle, this is a very old MOD that i did, and there are some modern versions of it which have some nice features, but this one is a good example too:

This is how both standard portal hub and the planetary version look compared:

 

And these are the guts of the planetary part:

And the LXF in case anyone wants more details: http://bricksafe.com/files/imanol/other-models/Planetary Axle Comparison.lxf

Edited February 25, 20178 yr by Imanol BB

Wait, so how do you increase speed/decrease torque by using planetary gearing?

it's decrease speed/increase torque... the small 8 tgear operates the another four gears with a ratio of 1:1. the other four geares operate at the same time on the inner teeth of the turntablewith a ratio of 1:4

3 hours ago, aminnich said:

Wait, so how do you increase speed/decrease torque by using planetary gearing?

If you want to increase speed somewhat, hold the innermost gear steady and rotate what holds the middle idler gears. See: http://gears.sariel.pl/

If you look at the picture of the planetary gears that he has, note that keeping the center (red) 16z gear stationary while rotating the 5L beam with the two idler gears on it will cause the yellow outer gear to rotate faster than the 5L beam.

So I can't use a turntable as the main part of the planetary gear if I want to gear up? 

45 minutes ago, aminnich said:

So I can't use a turntable as the main part of the planetary gear if I want to gear up? 

You need to use the 56Z turntable and if it is held stationary then depending on which axle is the input & which is the output you can get either 1:4 step up ot 4:1 step down.

All is explained in Sariel's "Unofficial Lego Technic Builders Guide" Second edition in Chapter 21 - pages 349 - 353 with details on how to build them.

Edited February 25, 20178 yr by Doug72

Here's one I built earlier to figure out how it works.

There are four possible outputs:-

 

1/.    With turntable stationary 1 revolution of Red crank = 4 revolutions at Yellow crank in same direction.

 

2/.    With turntable stationary 4 revolution of Yellow crank = 1 revolution at red crank in same direction.

 

3/.    With Red crank stationary, 4 revs. at Yellow crank =  1 rev. of turntable in reverse direction.

 

4/.    With Yellow crank stationary, 1 rev, of Red crank = 4 revs of the turntable in same direction.

 

Edited February 25, 20178 yr by Doug72

@Doug72 gave a great answer, but I think there are a couple other possibilities unlocked if you don't connect the two halves of the turntable. Have a look at this page: https://en.wikipedia.org/wiki/Epicyclic_gearing

The first picture shows the standard setup. Now imagine the inside of the turntable as the outside (pink) ring. Then place the 8z gears (blue) on a carrier (green). Also, place an 8z gear as the center sun gear. Now if done right, there should be three possibilities of things to hold steady. Doug showed four of the possibilities, but now think of holding the sun gear in place and rotating the carrier. A single rotation of the carrier will spin the ring gear (inside of the turntable) once plus an additional 8 teeth for a total ratio of 1:1.33. When I did this I attached the carrier to one half of the turntable, so I spun that. Of course, it is difficult to hold the sun gear in place while rotating the rest around, but it can be done.

Clear as mud?

Aminnich: heres the extra images you requested.

Note options 3 & 4 are not realy viable as you need to lock the two halves of the turntable together - to do this the yellow 3L beam (gray in previous image) then obstructs the turntable external gearing.

If you only want a 1:4 reduction then my build will work if turntable is fixed into the structure.

Please note my adaption is based on the planetry gear show in Sariel's guide book minus the L motor and with the turntable locked.

Edited February 25, 20178 yr by Doug72

Hmm, I am going to have to try that out.  It makes sense, but I cannot picture how to works.  I think the idea of a gear inside a gear is what is confusing me.  Thanks Doug, I will post my results once it is done.  

50 minutes ago, aminnich said:

Hmm, I am going to have to try that out.  It makes sense, but I cannot picture how to works.  I think the idea of a gear inside a gear is what is confusing me.  Thanks Doug, I will post my results once it is done.  

The Yellow axle is connected to the centre 8T gear.

The Red axle is connected to the two wedge pulleys which have the two 8T gears on them which contact the internal 24T gear in centre of the turntable, and the 8T gear on the yellow axle.

Edited February 25, 20178 yr by Doug72

I built it, it works perfectly, I still don't 100% understand how it changes the gear ratio, but it doesn't matter. As long as it works and it does. 

Thanks for helping me Doug