LEGO Technic Planetary Geared, Drive Assembly

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Introduction to Planetary Gearsets & Gear Ratios:

Any planetary gearset has three main components:

  • The sun gear
  • The planet gears and the planet gears’ carrier
  • The ring gear

Planetary Geared System

Each of these three components can be the input, the output or can be held stationary. Choosing which piece plays which role determines the gear ratio for the gearset. Let’s take a look at a single planetary gearset.

One of the planetary gearsets from our transmission has a ring gear with 72 teeth and a sun gear with 30 teeth. We can get lots of different gear ratios out of this gearset.

  Input Output Stationary Calculation Gear Ratio
A Sun (S) Planet Carrier (C) Ring (R) 1 + R/S 3.4:1
B Planet Carrier (C) Ring (R) Sun (S) 1 / (1 + S/R) 0.71:1
C Sun (S) Ring (R) Planet Carrier (C) -R/S -2.4:1

Also, locking any two of the three components together will lock up the whole device at a 1:1 gear reduction. Notice that the first gear ratio listed above is a reduction — the output speed is slower than the input speed. The second is an overdrive — the output speed is faster than the input speed. The last is a reduction again, but the output direction is reversed. There are several other ratios that can be gotten out of this planetary gear set, but these are the ones that are relevant to our automatic transmission. You can try these out in the animation below:


Animation of the different gear ratios related to automatic transmissions. Click on the buttons on the left in the table above.

So this one set of gears can produce all of these different gear ratios without having to engage or disengage any other gears.

With my LEGO Technic Planetary Geared Drive, the Sun Gear is a 8-Tooth Technic Gear, the Planetary Gears are also 8-Tooth Technic Gears and the Ring Gear is the internal gear ring of a Technics Type-2 Turntable. This arrangement gives a gearing down of 3:1, and results in very high Torque Capabilities at the Output.

Now let build a “LEGO Technic Planetary Geared, Drive Assembly”:

The construction of the ‘LEGO Technic Planetary Geared, Drive Assembly‘ begins with the the Planet Carrier, which is built on too the Technics Type-2 Turntable as shown below.

 STEP 1.
Attach 3x LEGO ‘Technic, Axle and Pin Connector Perpendicular 3L with Center Pin Hole’ by using 2x ‘7 long Technic Axles’. At the pertruding 4x ends of the 2x ‘Technic Axels’, fix a ‘Technic, Axle and Pin Connector Perpendicular 3L with 2 Pin Holes’ as shown.

Planetary Geared Wheel 01

 STEP 2.
Next the Sun Gear’s Shaft, along with the Planet Gear’s Shafts are fitted. In this example the Sun Gear’s Shaft is made from a ‘Technic, Axle 3 long with Stop’. The Sun Gear is a ‘Technic 8 Tooth Gear’.
The Planet Gears shafts consistof two ‘Technic Axle Pins without Friction Ridges Lengthwise’.

Planetary Geared Wheel 02

 STEP 3.
Next the Planetary Gears, consisting of 2x ‘Technic 8 Tooth Gears’ are fitted to the assembly.

Planetary Geared Wheel 03

 STEP 4.
Next we start preparing for the ‘Stub Axle Assembly’ by placing 2x ‘Technic Black Pins’ as shown below.

Planetary Geared Wheel 05

 STEP 5.
Before progressing any further, place a retainor onto the End of the Sun Gear’s Axel to provent it from falling out all the time. A ‘Technic Universal Joint’ or ‘Technic Axle Joiner’ will be fine depending on the use you have in mind for the “LEGO Technic Planetary Geared Drive”.

Planetary Geared Wheel 06A

Planetary Geared Wheel 06B

 STEP 6.
Fit a ‘5 long Technic Beam’ to each of the two black Technic Pins. Next fix a ‘Technic Axle and Pin Connector Perpendicular’ and a ‘Technic, Axle and Pin Connector Perpendicular 3L with 2 Pin Holes’ using a ‘Technic, Axle 3 long with Stud’ for each. Next fit a ‘Black Technic, Pin with Friction Ridges Lengthwise WITH Center Slots’ and a ‘Technic, Pin Long with Friction Ridges Lengthwise’.

Planetary Geared Wheel 07

 STEP 7.
Fit two ‘Technic Wedge Belt Wheel (Pulley)’ to the assembly as shown. Place two ‘Black Technic, Pin with Friction Ridges Lengthwise WITH Center Slots’ into the two Pullies to finalize the Wheel Stud Assemly.

Planetary Geared Wheel 08

 STEP 8
That finalizes the “LEGO Technic Planetary Geared Drive Assembly”, leaving you to fit a LEGO Technic Wheel or Large Technic Sprocket for a Bulldozer type track system.

Planetary Geared Wheel 09

Planetary Geared Drive Assembly

Planetary Geared Wheel 10A

Planetary Geared Wheel 10B

In coming days I will post a follow up article exploring a Bulldozer that I’m working on inconjuction with my son, Anthony.  Two “LEGO Technic Planetary Geared“, Drive Assemblies are used to transfer power from a pair on LEGO Mindstorms Motors, too a pair of ‘Catipillar Tracks‘ on each side of the vehicle. The result is a Bulldozer that will go almost anywhere thanx to the increased torque that is available when using a “Planetary Geared System” to transfer the Motor’s Power to the propolsion Assembly (Tracks).

[youtube]aPacGjEOI7w[/youtube]

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