That Amazing Planetary Gear Set!

How many people really understand a Planetary Gear set, how it works, it’s use, and versatility in the market place?  The planetary gear set is one of the most amazing inventions used in our industrial society!  It’s use is all around us and we don’t even realize it.  Without it, we wouldn’t have all the luxuries and manufacturing processes that we have today…

Well, what is the darn thing?

A planetary gear set is gear assembly that consists of 3 primary components.  A Ring Gear (the outer most gear), 3 planetary gears that are housed by a Planetary Carrier assembly (the middle gears), and a Sun Gear (the gear in the center of the gear set.   All three gear sets are splined in order to connect or mate to a band, clutch, and/or splined to the main shaft.  Through an intricate collage of these various types of connections and brakes, various components of the planetary gear set can be allowed to spin free, be held, or transmit applied power or torque to an output member – commonly referred to as the transmission assembly or power plant.  The bands or brakes can be applied in several ways such as Hydraulics, centrifugal clutches, electronics, servos, etc., and can be done automatically or manually.

Below is an example of a Simple Planetary gear set.

With this Simple Planetary gear set, there are “7″ gear possibilities (8 if you count neutral) in a Simple Planetary Gear Set (meaning only 1 planetary gear set).  Although technically you can have seven gear possibilities, typically only “4″ gears are commonly used due to how the planetary gear components are applied for output.

Below is  chart that shows the seven possible gear ranges in a simple planetary gear set.  Note that if any “two” gears are held together in a planetary gear set, the result is a 1:1 gear ratio applied to the output.

  Click on chart to view

To show you how old and amazing a Simple Planetary gear set is, take a look at this next example – I’m sure we’ve all seen, used, maintained and repaired this next gear set…  An old fashion Pencil Sharpener.

Now that we have discussed a simple planetary gear set, lets discuss the more typical and common use of planetary gear sets.  If you apply two or more gears sets together, sharing a common member between one output as the input for the next, you now have a compound planetary gear set.  By combining two or more gear sets with varying gear sizes and tooth counts, you can create a multitude of gears and gear ratios.  Thus the typical Automotive automatic transmission.  A typical Automatic transmissions use three or more planetary gears sets to achieve the desired gears and ratio results for output members.

However, simple and compound planetary gear sets are not limited to the automotive industry.  They are commonly used throughout manufacturing and in the general market for common household items – you’ve just never looked inside of it to realize it…  In manufacturing, they are used in many of the industrial machines to provide quite, quick, and multiple gear ranges.  There are also many mini gear sets used in many house hold tools and appliances.

Below is one of the most amazing uses of a mini compound planetary gear sets.  A Rechargeable Drill Motor or Screw Driver.  You ever wondered as to when you screw or drill into a hard substance that the drill motor or screw driver slows down and increases torque in order to get the job done, and speed up with less torque when it’s smooth sailing.  This is done with a friction type of clutch that applies or free up members of the compound planetary gear set for gear reduction, one-to-one or over-drive if necessary – not to mention reverse when then reverse switch is applied.

Planetary gear sets is very old technology, but still widely used in a versatile market of products.  One of the most cost effective methods of manufacturing these gear sets is through a process called Powdered Metallurgy.  A leading manufacturer in this process is Fansteel-AST.

The next time you use a rechargeable screw driver, see if you can notice the smoothness and multitude of gear ratios used when drilling or screwing.  It’s truly amazing!

- Jack Miller (Click to View Business Card)