5.2.9 Gearbox functions
The gearbox (GB) is a very simple mechanism, which consists of a body (and it, in turn, of several parts) and shafts with gears inside. The thrust from the engine is supplied on one side of the gearbox, and on the other side, the already increased thrust is transmitted further to the main gear and wheels.
The gearbox also serves to change the tractive effort on the wheels of the car and to obtain reverse gear, as well as for a long-term separation of the engine thrust from the transmission to the drive wheels. This disconnection is necessary in order not to keep the clutch release pedal constantly depressed.
Flexibility is one of the functions of the gearbox. Its essence is as follows: the tractive effort on the wheels, necessary to overcome all the resistances that arise when the car is moving, must change depending on the operating conditions of the car.
In case if the car is moving on a flat road at a low speed, then the traction, which is required to overcome air resistance and losses due to rotation of wheels and all mechanisms, will be small. In order to obtain this tractive effort, only a small fraction of the power that the engine can deliver is needed. All excess engine power is accumulated until the driver needs to accelerate the car in order to obtain a higher speed of its movement. But when the car is driving on a bad road or uphill, the resistance to movement increases significantly. In order to overcome these resistances, the tractive effort on the driving wheels must be increased accordingly. Also, when the car starts moving, the tractive effort on its wheels should be especially large, since this requires, in addition to the resistance of all mechanisms, to overcome the inertia of the car, which is associated with its mass.
Note
At a certain engine power, its torque and the number of revolutions of the crankshaft, various tractive forces on the driving wheels of the car can be obtained by changing the ratio of the number of revolutions of the engine crankshaft and driving wheels. When the number of wheel revolutions decreases if compared to the number of revolutions of the crankshaft, the tractive effort on the wheels increases, but the speed of the vehicle decreases (recall the example with a bicycle). With an increase in the number of revolutions of the wheels, the tractive effort on them decreases, and the vehicle speed can be increased.
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