by 
The Curious Bag
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"Spider gear" is an old phrase used to describe the inner workings of an automotive differential. These gears allow one drive tire to turn faster than the other for easier turning, and are found in most diffs.
Differential Requirement
When a vehicle turns, the outer tire must cover a greater distance than the inner tire, meaning that it must turn faster to prevent slipping.
History
Opinions vary as to who invented the geared differential, but the first to be documented appeared in the Book of Song. This history claims that the Chinese first employed the arrangement between 1050 B.C. and 771 B.C.
Operation
A driven pinion gear rotates a large ring gear, which is set at a 90-degree angle to it. This ring gear rotates a central sun gear, which distributes power evenly between the wheels.
Disadvantages
Spider gears route drive power through the path of least resistance. This means that if a tire is already spinning, the vehicle will not move.
Options
Vehicles that encounter wheel-slippage often use some sort of locking differential, which helps to send power to the slower-spinning wheel.
Source:
Resource:
Differential Animation (Video)
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