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I have just bought this set and wonder the purpose of the white gear in the middle:
How can you use it in your Lego Technic projects? Is it available in other sets?
When your function (such as an extending crane boom) gets to one limit or the other, this clutch gear ratchets instead of binding up the motor and all the gears in between.
ETA: Forgot the second question. It has appeared in many sets, most recently the Fairground Mixer.
Required, but never shown