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What is a Differential Gear?

By Parker Brown
Updated May 17, 2024
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A differential gear is a mechanical device used to convert and transfer torque in a variety of manners. Typically, a differential will be used to distribute the torque from a single shaft to two or more shafts. Sometimes, the opposite can take place, where the torque of two or more shafts is transferred to only one shaft. While a differential gear can take many forms, it is most often found in automobiles. In this application, the torque of a single shaft is distributed into two different shafts, each of which turns a driving wheel.

While differential gears can be found in many different mechanisms and machines, the modern automobile tends to make the most use of them. When an automobile makes a turn, the wheels on either side of the car should be turning at different speeds. The wheels closest to the direction of the turn need to rotate slower than the ones facing away from the turn. For a car to do this, each rear wheel needs its own individual drive shaft, yet torque must still be evenly distributed.

A differential gear accomplishes this by evenly distributing torque to each individually-shafted wheel. This is done through a special gear arrangement found at the point where the drive shaft from the engine meets the two drive shafts. The engine inside of the automobile creates torque, which is then increased through the transmission. From the transmission, a single drive shaft rotates the differential gear box.

Differential gear boxes can be found in a variety of arrangements, yet the arrangement itself is relatively universal among car manufacturers. A spiral bevel pinion gear on the end of the transmission drive shaft turns a larger crown wheel gear. The crown wheel gear is relatively large in diameter and has a large hole in the middle of it. While the teeth on most gears come out of the side, the teeth of the crown wheel face up. This way, the pinion gear from the engine can connect to the crown gear.

From the crown gear comes a unique gear arrangement, where torque is evenly distributed among both drive shafts. On the face of the crown wheel is what is known as a differential cage, which houses two beveled gears on either side. The faces of these gears are perpendicular to that of the crown wheel, which allows them to properly join with two other matching beveled gears. Each of these other two beveled gears make up one end of each drive shaft.

Without a differential or two different drive shafts, a variety of adverse effects could occur in the automobile and its components. One of the most notable is tire damage, which is the result of tire dragging. This is caused when the outer tire is unable to rotate faster than the inner tire. Such dragging can also result in drivetrain damage and failure.

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