Differential gears refer to an arrangement of gears used to transmit torque and rotation through three shafts. This type of gear is most commonly used in two ways. The first is to receive input and provide two outputs. The second is to combine two inputs to create an output that is the sum, average, or difference of the inputs. The first way is the most common and typically used as rear end gears in cars, trucks, other self-powered vehicles, and industrial machinery requiring perpendicularly transmitted torque.
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Applications of Differential Gears
The function of differential gears in power transmissions is to enable the rear wheels of the automobile to be driven at equal force, while also being able to rotate at different speeds. Different rotational speeds are mainly used when turning corners. Typically constructed using bevel gears, differential gears may also use spur gears, which are straight-sided along the gear wheel's axis with straight teeth radiating in alignment with the axis. They can also use epicyclic gearing, or planetary gears, which contain one central spur gear, or "sun gear" surrounded by three or more "planet gears." While the use of spur gears is somewhat outdated, since they were often used in early automobiles, the use of epicyclic gearing is being used in more technologically advanced applications. For instance, the Toyota Prius utilizes epicyclic gearing in its automotive drive train in order to apply torque asymmetrically. This type of differential gear has the advantage of being compact along the length of its axis, which is also called the sun gear shaft.
Differential Gear Design and Function
The typical set up of a differential gear includes sets of bevel gears, axles, shafts, and a differential carrier. Bevel gears may have straight teeth, or they may be spiral bevel gears with curved teeth, similar to helical gears. The bevel gears in the differential gear are arranged into epicyclic configurations, which allow various attached axles to turn at different speeds. The spiral bevel pinion gear is encased within the housing of the final unit and driven by torque transmitted from the end of the propeller shaft. The pinion gear interlocks with the large spiral ring gear, which is also known as a crown wheel. The crown wheel is attached to the differential carrier, which contains a cluster of four opposed bevel gears in a perpendicular plane. These four bevel gears work by each gear meshing with its two neighboring gears but treating the third gear differently by rotating counter to it instead of meshing. The planetary configuration of the differential gear is seen by two sun gears and two planet gears. The two sun gears are aligned on the same axis as the crown wheel and serve to drive the axle half shafts that connect the automobile’s wheels. The two planet gears are aligned on a perpendicular axis whose orientation is affected by the crown wheel’s rotation. The crown wheel is rotated as a result of the rotation of the differential carrier. Upon rotation, the changing orientation of the axis transfers the crown wheel’s motion to the sun gear by pushing on them as compared to turning against them. The two planet gears are not restricted from turning against each other, and when they do the sun gears can counter-rotate relative to the crown gear and to each other under the same force.