What determines the size of a gearbox


What types of gearboxes are there?

A basic distinction is made between the manual transmission and the automatic transmission.

Manual transmission

In manual transmissions, the following components are used to transmit the torque to the wheels:

  • Starting and disconnecting clutch
  • Synchronized gear step transmission
  • Transmission actuation, which transmits the shift movement from the shift lever to the transmission

With manual transmissions, the engine speed is initiated via the drive shaft. The output speed, i.e. the speed that leaves the gearbox again, results from different gear ratios that result from the different speeds of the meshing gears.

Manual transmissions usually have a pair of gears for each gear, the gears of which are constantly in mesh. Of these gear pairs, however, only one gear is permanently connected to its shaft. These gears are called gear wheels. The opposing gears (ratchet wheels) rotate loosely on their shaft when idling. If a gear is engaged, one of the gear pairs is switched into the power flow. For this purpose, a shift sleeve actuated by the shift lever connects the loosely rotating gear with its shaft.

automatic transmission

In contrast to the manual gearbox, the following components are used to transmit the torque in the automatic gearbox:

  • Hydrodynamic torque converter
  • Shift elements for transmitting the torque during gear changes
  • Planetary gear with planetary gear sets to realize the different gears
  • Mechatronics
  • Gear oil pump (vane pump)
  • Parking lock
  • Selector lever

Instead of the separating clutch, automatic transmissions have what is known as the hydrodynamic torque converter, which takes over the start-up process. It is not necessary to loosen the connection between the engine and the transmission, as automatic transmissions are shifted under load and thus the engine torque is also transmitted when changing gears.

In contrast to the positive engagement of the toothed clutches in manual transmissions (separation of engine and transmission by the clutch), the gears in automatic transmissions are shifted in a non-positive manner. This is done by the switching elements, which automatically adjust the engine speed to the new gear by means of oil pressure (transmission oil pump) (no separation of engine and transmission). The driver can stay on the gas. All five shift elements of the current 8-speed transmission are designed as multi-disc clutches and multi-disc brakes.

The different gears in an automatic transmission are implemented by the planetary gear with its various planetary gear sets, which can be used to achieve slow or fast gear ratios. Several forward gears and one reverse gear can be represented by coupling several planetary gear sets.

In automatic transmissions, the electronic-hydraulic transmission control is designed as a mechatronic module. It is the "command center" of the transmission. The switching programs, adaptation of the switching pressure to the motor torque and safety functions are implemented in the electronics.

Ultimately, unlike manual transmissions, automatic transmissions have a parking lock. This is a mechanical locking of the transmission output shaft, which is engaged in position P of the selector lever and prevents the vehicle from rolling away under all possible conditions.