How a Car Gearbox Works How a Car Gearbox Works

A car gearbox is also known as that car’s manual transmission. A car gearbox takes torque from the engine and uses gearing to transfer that torque to the car’s drive wheels. Below is a very simplified discussion of how the average car gearbox works.

Controlling Torque Input

In a manual transmission, there has to be a way to interrupt the input of torque power supplied to the gearbox. This is accomplished through the use of the clutch system. Attached to the engine’s crankshaft is a flywheel. This flywheel has a friction surface. The clutch disc is pressed against this friction surface to transfer input to the gearbox by the pressure plate. When the clutch pedal is depressed, a clutch fork pressed a throw out bearing against the fingers of the pressure plate, which releases the pressure against the clutch disc, allowing input to be interrupted and thus shifting of the gears. When torsional input is interrupted, synchronizers cause the gears in the planetary to stop spinning and correctly line up, allowing smooth shifting.

Creating Geared Torque for Application

The clutch disc transfers rotational torque to the gearbox through a splined input shaft. At the gearbox end of the input shaft is an input gear. Rotating around this input gear is a planetary gear system. At the rear end of the gearbox is an output gear which is attached to the output shaft. On a rear drive vehicle, this output shaft is connected to a drive shaft, which is in turn connected to a differential which translates the torque from rotating longitudinally to rotating transversely so the rear wheels can use them. In the gearbox, the planetary gear system has gears of varying ratios, which allows the vehicle to accelerate from a stop up to freeway speeds by changing gears.

Changing Gears

In order to accelerate and decelerate, you have to change gears, up as you accelerate and down as you decelerate. When your car is equipped with an automatic transmission, this shifting of gears occurs automatically, as the name of the transmission type implies. When the car is equipped with a manual gearbox, the shifting is accomplished by the driver. The driver will release pressure from the accelerator, or gas pedal and depress the clutch. A gear shift lever, either floor, console or steering column mounted, is moved to a position that corresponds to the desired gear. As the driver depresses the clutch and shifts gears, torsional input to the gearbox is interrupted, the synchronizers cause the gears to properly line up for shifting and a fork moves a controlling gear, changing the gear ratio supplied to the drive wheels, allowing an increase or decrease in vehicle speed. For passenger cars, there are from 3 gear ranges up to 6 gear ranges. Large commercial trucks will often have manual gearboxes with up to eighteen gear ratios available.


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