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Home >   How To >   How to Repair Steering >   Transmission Gear Box

Transmission Gear Box


The transmission gearbox is used to provide the gear reduction needed to transform the high speed of the engine to the requisite speed to drive the wheels. The gear box is the chief component of the transmission system. The gearboxes are different for manual and automatic transmissions. Manual transmissions usually have five or six gears in the gearboxes. When the driver presses the clutch down, the sliding gear gets engaged with the appropriate gear. There are higher and lower gears which when engaged with the sliding gear provide high and low speeds, respectively. Modern manual gearboxes employ a diagonal gear that keeps the sliding gear synchronized with the main gears. This design prevents the gears from clashing with one another.


Automatic transmissions use an automatic gearbox that allows the transmission to select the right gear, without having the driver to choose. A hydraulic system monitors the pressure of fluids in the engine and engages the appropriate gear with the help of a torque converter, with respect to the engine fluid's pressure. The torque converter engages the higher or lower gears, depending on whether the fluid pressure is high or low, respectively.




The gearboxes in the front and rear wheel drive units are different.


Front-wheel drive (FWD) gearbox: The typical FWD unit is compact and will house the gears (known as the gear set, kit or cluster), the final drive and the differential. Typically, there will be two shafts with gears on them- an input shaft and below it, an output shaft. Each pair of gears is constantly meshing, but only one pair is fixed to the shafts at any given time.


Rear-wheel drive (RWD) gearbox: A typical RWD gearbox will have three shafts - input, lay shaft and output. The input and output shafts run in line with each other but are mechanically separated. The lay shaft sits beneath the two and overlaps them. The input shaft constantly drives the lay shaft through a pair of meshing gears: this is known as “constant mesh”. The lay shaft and output shaft have a pair of gears rotating on them and from the constant mesh gearing onwards, it behaves in a similar way to the FWD gearbox. The main extra aspect is that if the box features a direct ratio, usually fourth, there will be a system to lock the two shafts together, bypassing the gear cluster.


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