Differential gear and why is it necessary in automotive?

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Why is the differential gear used?

Look at the picture, in the image, you could notice the outer wheel has to travel more distance than the inside wheel. It means the left wheel has to rotate at the higher speed compared to the right wheel. If these wheels were connected using a solid shaft, the wheels would have to slip to accomplish the turn. This is exactly where the differential comes in handy. The skilful mechanism in a differential makes both the wheels to turn at different rpms, while transferring power to both wheels.

Parts of differential

Pinion drive gear: This transfers power from the driveshaft to the ring gear.

Ring gear: transfers power to the differential case assembly.

Spider gears: Spider gear is the heart of differential gear system. The specialty of spider gear is, it rotates in two directions: one along with the ring gear (rotation) and the second on its own axis (spin). The spider gear is meshed with 2 side gears.

Differential case assembly: holds the ring gear and other components that drive the rear axle.

Rear axle bearings: These are ball bearings that fit between the axles and the inside of the axle housing.

Differential operations

Case 1: The vehicle moves straight

In this case, spider gear turns along with the ring gear but does not rotate on its own axis. Hence, the spider gear will transmit power to both the side gears and both will turn at the same speed. In short, when the vehicle moves straight, the spider gear assembly acts as a single solid unit.

Case 2: the vehicle takes the right turn

In this case, the spider gear plays the most important role. Spider gear rotates along with the ring gear on its own axis. With the proper mesh side gears and spider gear, the side gear will have the same peripheral velocity as the spider gear. While the vehicle turns to its right, the spider is spinning as well as rotating, peripheral velocity on the left side of the spider gear is the sum of the spinning and rotational velocity. Whereas on the right side, it is the difference between the two, since the spin velocity is in the opposite direction on this side.

Case 3: The vehicle takes a left turn

When we compare with the previous condition, if the spider spins in the opposite direction, the right side gear will have a higher speed.

For heavy load carrying vehicles are preferred to carry more spider gears. Note that the spider gears should spin in opposite direction to have the proper gear motion. A four spider gear arrangement is used usually for vehicles with heavy loads. In such cases, the spider is free to spin independently and is connected to the ends of a crossbar.


This narration of differential gear and its working may make you know how your car works while turning. And you will be amazed to know so many things are integrated into your vehicle. Having a healthy gearbox shows the good health of your vehicle. So, don’t neglect its operation.

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