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Most cars have from three to five forward gears, and one reverse
gear. The transmission changes the ratio of the engine speed and the
wheels by connecting gears in various combinations. If a gear with 10
teeth is driving a gear with 20 teeth, the drive would be said to
have a 2:1 ratio.
First gear connects the engine power to the drive wheels via a pair
of reduction gear sets, which gives increased power and reduced
wheelspeed when the car is beginning to move. This means the engine
is turning much faster than the output shaft, typically around a 4:1
ratio. Intermediate speeds are delivered by changing the gear ratio
closer to 1:1. Final drive is usually accomplished by directly
linking the input and output shafts, giving a 1:1 gear ratio. Using a
moveable set of different sized gears, it's possible to get several
degrees of torque output. The differential pinion, driven by the
drive shaft, turns the ring gear, which acts like a single speed
transmission. This further reduces RPM's and increases torque by a set
ratio.
Gears work exactly like levers. A small gear driving a larger one
gives an increase in torque, and a decrease in speed, and vise-versa.
Transmission gears are heat-treated, high quality steel. They have
smooth, hard teeth, cut on precision machinery while red hot. There
are many types of gear teeth, but most transmissions use spur and
helical gears. Most of the gears are the helical type, because they
last longer and are more quiet than spur gears. There has to be
enough room (a few thousandths of an inch) between the gear teeth for
lubrication, expansion, and any irregularities in size.
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