There’s a planet IN my car???

Welcome back! It’s time to continue our look into the nooks and crannies of Transmissions. But before we lift the hood, let’s do a quick recap & review some definitions.


Previously on Anthony’s Automotive Blog….


Last week, we talked about automatic transmissions. If you wish to do a quick re-read of it, you can find it here. For the quick recap, keep on reading.

The automatic transmission system of a car works by using a series of gears that lock and unlock to shift them up or down in gear. This series of gears is called the Planetary Gear System and is comprised of the Ring Gear, The Planetary Gears and the Sun Gear. They rotate and revolve around each other and work together. It is the combination of locking one or two of these gears with the second and/or third being free that initiates the gear shift in the car.

planetary gear set


Last week, I barely scraped the grime off the surface of these gears and how they work. This week? Grab your rag and let’s get ALL the grime off them this time. Let’s make them shine!


Our Growing Automotive Vocabulary


Here is a recap of some of the automotive words we listed last week just in case there is any lingering confusion. There are a couple of new additions as well.

Transmission – The mechanism by which power is transmitted from an engine to the wheels of a motor vehicle.

Torque Converter – A device that transmits or multiplies torque generated by an engine.

Gear – One of a set of toothed wheels that work together to alter the relation between the speed of a driving mechanism (such as the engine of a vehicle or the crank of a bicycle) and the speed of the driven parts (the wheels).

Automatic – A device or process that works by itself with little or no direct human control.

Automatic Transmission – An automotive transmission that can automatically change gears as the vehicle moves, freeing the driver from having to shift gears manually.

Clutch – A mechanism for connecting and disconnecting a vehicle engine from its transmission system.

Band – Something that binds, ties together, restrains, etc.; a strip or ring of wood, metal, rubber, etc. fastened around something to bind or tie it together.

Epicyclic – A circle whose center moves along the circumference of another, larger circle.

Planetary Gear System – A gear system that consists of one or more planet gears (the gears in the middle area) that rotate or revolve around a sun gear (the gear in the center) and a ring gear (the gear on the outer edge).

Hydraulic – Operated by the movement and force of liquid; specifically, operated by the pressure created when a liquid is forced through an aperture, tube, etc..

Input – The act of putting in; or, what is put in

Output – The work done or amount produced by a person, machine, production line, manufacturing plant, etc., especially over a given period.

Shaft – A long, slender part or object.


The Planetary Gear System


As mentioned before, the Planetary Gear System is comprised of three parts: the Ring Gear, the Planetary Gears and the Sun Gear. The Ring Gear is located on the outer edge. The Planetary Gears (which typically number two or more) are in the middle area and are mounted on a carrier or plate. The Sun Gear is located in the very center. The Planetary Gears rotate around the Sun Gear in the center and the Ring Gear along the outside.

How this system works is by moving one or more of these parts, while keeping one or more locked or still. Here’s an example that should really break this down for you to help you see what I’m talking about.

planetary gear system


Let’s say the Ring Gear is connected to the Input Shaft that is coming in from the engine and the Planetary Gear Carrier is connected to the Output Shaft, which is connected to the wheels. Now, in this example, let’s lock the Sun Gear in place so that it can’t move. As we turn the Ring Gear, the Planetary Gears will move in the same direction along the locked Sun Gear. This causes the Planetary Gear Carrier to turn the Output Shaft. This means that the Input Shaft and the Output Shaft are moving in the same direction. BUT, the Output Shaft is moving at a slower speed. Remember that the Input Shaft is connected to the Ring Gear and the Output Shaft is connected to the Planetary Gear Carrier? So the wheels are moving slower than the power of the engine. This is a car that is in First Gear.

Still with me? Good!

Now, let’s unlock the Sun Gear and lock the other two – the Ring Gear and the Planetary Gears. By doing this, all three of these will turn together as one. You now have the Input Shaft (power from the engine) and the Output Shaft (the wheels) turning at the same speed. This is a car that is in Third Gear or higher.

Makes sense right? Now we’re moving!

Finally, let’s lock the Planetary Gears and leave the Ring Gear and Sun Gear unlocked. By locking the Planetary Gears, you lock the Planetary Gear Carrier, which is connected to what again? The Output Shaft (the wheels). You give some power to the Ring Gear (aka the Input Shaft). This causes the Sun Gear to turn. But it’s going to turn in the opposite direction. And this means?

You guessed it! This is a car going in Reverse.


Food for Thought


There are many, many combinations that are possible when you use two or more planetary gears. They can be connected in various ways and provide many different forward speeds and reverse. There are some very clever gear arrangements that you will find in today’s modern automatic transmissions.


8 speed chevrolet corvette


Nowadays we have four-, five-, six-, seven and even eight-speed vehicles! Now that you know what a first and third gear looks like (Side Note – you CAN substitute the word ‘speed’ for ‘gear’ here to help), imagine trying to understand the combinations that these eight-speed cars must go through as they shift from first gear to their top gear as they head on down the highway. Talk about being complex! It is no surprise that we now have computers in today’s cars.


Ideas for Next Week


I feel that you should now have a good idea of how the gear system works in an Automatic Transmission. Next week, I’m going to build on that and we are going to add another level to this Planetary Gear System and expand your automotive knowledge a little further by talking about clutches and bands.


Until then,

Anthony’s Automotive






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