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Gears & Axles

Gears and axles represent an integral part for the stable and continuous movement of the vehicles. These parts provide support in the transmission of the rotational power of the engine to the wheels. Gears inside the differential gear train change the direction of the torque and the axles channel this torque to the wheels.


The tasks of axles include commanding the tires, providing a connection for them, transmitting the driving torque and maintaining the position of the wheels. Axles in modern vehicles have both fixed and moving parts which consist of many different components. Among these, are many supporting components besides the large number of drivetrain components. The structural configuration of the rear and front axle depend on the drivetrain of the vehicle (FWD, RWD, AWD, 4WD). Thus, their installation and functions vary.

There are various types of axles in vehicles: one of them is a dead axle which is a free-rotating axle found in rear wheel drive and front wheel drive systems. This axle is not part of the drivetrain and it is used to bear the weight and centralize the load. Apart from the dead axle, in heavy duty vehicles, a lift axle is used to increase the load capacity by distributing the weight of the cargo over more wheels. Without the extra cargo, this axle is lifted off the ground to prevent wearing on the tires and axle as well as increasing the traction in the remaining wheels.


Trucks and heavy duty vehicles are generally come equipped with a rear wheel drive system. In this configuration, the rear axle consists of two separate axles connected to both wheels with a differential gear train in-between. This differential directs the rotational power (torque) acquired from the gearbox to the two axle rods by changing the axis of rotation 90 degrees. Meanwhile, it can also increase the torque received from the transmission by decreasing the rpm. Differential allows the right and left wheels to rotate at different velocities as well. This is necessary because when taking turns and passing through bumpy roads, the right and left wheels have to rotate at different velocities since they don’t travel the same distance. Otherwise wheels would lose traction, the vehicle would spin and the handling would be unstable which, in longterm, causes the tires to wear out.

Components and the Structure of the Differential

Main components: Axle Housing, Pinion Gear, Ring gear, Differential Case, Spider Gear, Side gear, Oil Plug

In rear wheel drive vehicles, the differential gear box is located at the center of the rear axle. The rotational power received from the transmission is carried to the differential via a drive shaft. There is a pinion gear at the rear end of the driveshaft; this gear transmits the rotational power to the ring gear. And the ring gear transfers the power to the spider gear mounted inside the differential case. Finally, the spider gear channels the rotational power to the right and left side gears.

In the front wheel drive vehicles (the majority of the automobiles) the engine has to be placed in the transverse direction under the hood. For transmissions used in these vehicles, the differential and the gearbox are integrated in a single housing called the transaxle, and axle shafts enter the differential from two opposite directions, so there is no driveshaft in this configuration. One of the axle shafts is longer than the other as the transmission is placed close to one of the wheels and far from the other.