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Gearboxes are so named because they are essentially like "gearbox gearboxes" (hence the English version of "gearbox"). Their purpose is to transfer the power from the clutch to the differential and from there to the wheels and to create a variable gear ratio with gear engagement of different diameter or with a different number of teeth so that the engine rpm is kept close to the useful area.

Larger diameter means less speed but more torque, that is, "... we lose at speed, we gain in strength." In a nutshell, the boxes change the torque and the pulling power of the engine, isolate the movement (dead) and of course offer backwardness. They are divided into five major categories: mechanical boxes (known manually), classic automatic machines, "robotics", "double-compliant" and CVT type.

Mechanical crates

A simple box is made up of a shell or case, shafts and sprockets (axles). Overlooking the structure of the gears slipping on the axes, we can say that in the fixed clutch boxes there are two large axles, the primary that transfers the torque from the clutch and has a fixed gear (input) with the same rpm as the crankshaft. The secondary is in parallel and is permanently engaged with the primary when the driver does not clutch.

More specifically, the gears are permanently locked together in pairs and turn freely on their shaft ("crazy") until the intermediate engagement gear (collar, connecting ring) locks the movement sliding on the minor axis. Each time the driver moves the lever, the corresponding knob moves and thus the gears are not engaged directly but through the knobs for smoother and silent operation. The above are fixed coupling boxes.

Automated boxes


  • Advantages: do not burden consumption, low manufacturing costs, reliability
  • Disadvantages: relatively slow change in relationships, knocking on change

Here is an automated clutch where the gear changes are made by means of an electric motor that activates the clutch. In addition to the clutch actuator, the electro-hydraulic system also includes one for changing the gears / ratios. Simply put, the pressing of the clutch pedal (which does not exist) is assumed by an electric motor as well as changing the ratios, which instead of being manually performed, is re-performed electro-hydraulically.

So every time the driver chooses the serial program (instead of the automatic D option), a sensor perceives the movement of the the selector {usually up and down with hints (+) for ups and (-) for downloads} e.g. from 2a. Then the electronics unit commands a reduction in engine rpm, the clutch opens, the actuator changes e.g. in 3rd gear and the clutch re-closes to transfer torque. Of course, when the car tends to stop or when the ABS intervenes, it automatically fades.


  • Advantages: automatic change of relationships, reliability, absence of drives and overheating
  • Disadvantages: increased consumption and weight, bulky in dimensions

Transmission is carried out by the hydraulic torque converter acting as a clutch transferring power from the engine to the gearbox. An essential part is the ATF (Automatic Transmission Fluid), the planetary transmission system and the interlocking and electronic management unit.

CVT Boxes

  • Advantages: simple to build, good for consumption, smooth change of relationships, cost, compactness in dimensions
  • Disadvantage: a patinating phenomenon, not so fast, limited power transfer to the limit

These are gearboxes not composed of gears but of a belt system with two pulleys. In hybrids, it is customary to use a planetary fit CVT to combine the MEK motion with the electric motor.


A transmission service is part of your routine maintenance, just like replacing your engine oil.


A complete transmission service should include:

  • removing and examining the sump or pan (where possible)
  • replacing or cleaning the screen or filter
  • cleaning the pan
  • reinstalling the pan with a new pan gasket
  • pumping out the rest of the old fluid and replacing it with new, high quality ATF
  • adding a friction modifier or additive package (model specific)
  • New fluid restores the holding power between the clutches, reducing slip and heat production.

    At the same time, flushing the old fluid washes away tiny particles of clutch material and metal shavings. Those particles clog passages and wedge between moving parts, and thus they cause wear throughout the transmission. Changing the fluid eliminates those particles and the wear they cause.


Technical - Transmission

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