What is transmission and how it works


A smooth start of movement, acceleration without bringing the engine to the maximum speed and comfort during these processes - all this is impossible without the car's transmission. Let's consider how this unit provides the mentioned processes, what types of mechanisms there are, and what main units the transmission consists of.

What is transmission

The transmission of a car, or gearbox, is a system of assemblies consisting of gears, shafts, friction discs and other elements. This mechanism is installed between the engine and the drive wheels of the vehicle.

What is transmission and how it works

Purpose of automotive transmission

The purpose of this mechanism is simple - to transfer the torque coming from the motor to the drive wheels and change the rotation speed of the secondary shafts. When the engine is started, the flywheel rotates in accordance with the crankshaft speed. If it had a rigid grip with the driving wheels, then it would be impossible to start moving smoothly on the car, and every stop of the vehicle would require the driver to turn off the engine.

Everyone knows that battery energy is used to start the engine. Without the transmission, the car would immediately start moving using this energy, which would lead to a very rapid discharge of the power source.

What is transmission and how it works

The transmission is designed so that the driver has the ability to disconnect the drive wheels of the car from the engine in order to:

  • Start the engine without overspending the battery charge;
  • Accelerate the vehicle without increasing the engine speed to a critical value;
  • Use coasting motion, for example in the case of towing;
  • Choose a mode that would not harm the engine and ensure the safe movement of transport;
  • Stop the car without having to turn off the internal combustion engine (for example, at a traffic light or to let pedestrians walking on a zebra crossing).

Also, the car's transmission allows you to change the direction of the torque. This is required for reversing.

And another feature of the transmission is to convert the engine crankshaft speed into an acceptable wheel speed. If they were spinning at a speed of 7 thousand, then either their diameter had to be very small, or all cars would be sporty, and they could not be safely driven in crowded cities.

What is transmission and how it works

The transmission evenly distributes the released engine power so that the moment of transformation makes possible a soft and smooth start, movement uphill, but at the same time allows the use of the internal combustion engine power to accelerate the vehicle.

Transmission types

Although manufacturers have developed and continue to create various modifications of gearboxes, all of them can be divided into four types. Further - briefly about the features of each of them.

Manual Transmission

This is the very first and most popular type of transmission. Even many modern motorists choose this particular gearbox. The reason for this is a simpler structure, the ability to use the chassis of the car instead of a starter to start the engine if the battery is discharged (for how to do this correctly, read here).

What is transmission and how it works

The peculiarity of this box is that the driver himself determines when and what speed to turn on. Of course, this requires a good understanding at what speed you can upshift or downshift.

Due to its reliability and relative ease of maintenance and repair, this type of transmission remains in the lead in the gearbox rating. For the manufacture of mechanics, the manufacturer spends not as much money and resources as for the production of automatic machines or robots.

Gear shifting is as follows. The gearbox device includes a clutch disc, which, when the corresponding pedal is pressed, disconnects the engine flywheel from the gearbox drive mechanism. While the clutch is disengaged, the driver shifts the machine to another gear. So the car accelerates (or decelerates), and the engine does not suffer.

What is transmission and how it works

The device of mechanical boxes includes a set of gears and shafts, which are interconnected in such a way that the driver can quickly change the desired gear. To reduce noise in the mechanism, gears with oblique teeth are used. And for the stability and speed of the elements engagement, synchronizers are used in modern manual transmissions. They synchronize the speed of rotation of the two shafts.

Read about the device of mechanics in a separate article.

Robotic transmission

In terms of structure and principle of operation, robots are very similar to mechanical counterparts. Only in them, the choice and gear shifting is carried out by the car electronics. Most robotic transmissions have a manual mode option where the driver uses the shift lever located on the mode selector. Some car models have paddles on the steering wheel instead of this lever, with the help of which the driver increases or decreases the gear.

What is transmission and how it works

To improve the stability and reliability of work, modern robots are equipped with a double clutch system. This modification is called selective. Its peculiarity is that one clutch disc ensures the normal operation of the box, and the second prepares the mechanisms for activating speed before switching to the next gear.

Read about other features of the robotic gear shifting system here.

Automatic transmission

Such a box in the rating of such mechanisms is in second place after mechanics. At the same time, such a transmission has the most complex structure. It has many additional elements, including sensors. However, unlike the robotic and mechanical counterpart, the machine is devoid of a clutch disc. Instead, a torque converter is used.

A torque converter is a mechanism that works on the basis of oil movement. The working fluid is pumped to the clutch impeller, which drives the transmission drive shaft. A distinctive feature of this box is the absence of a rigid coupling between the transmission mechanism and the engine flywheel.

What is transmission and how it works

An automatic transmission works on a similar principle to a robot. Electronics itself determines the moment of transition to the desired mode. In addition, many machines are equipped with a semi-automatic mode, when the driver, using the shift lever, instructs the system to switch to the desired gear.

Earlier modifications were equipped only with a torque converter, but today there are electronic modifications. In the second case, the electronic control can switch into several modes, each of which has its own gearshift system.

More details about the device and system of operation of the machine were described in an earlier review.

Continuously variable transmission

This type of transmission is also called a variator. The only box in which there is no step change of speeds. Torque distribution is controlled by moving the walls of the drive shaft pulley.

What is transmission and how it works

The drive and driven shafts are connected using a belt or chain. The choice of the gear ratio is determined by the transmission electronics based on information received from the sensors of various vehicle systems.

Here is a small table of the pros and cons of each box type:

Box type:Advantages:Disadvantages:
Manual transmission (mechanics)High efficiency; Allows to save fuel; Simple device; Inexpensive to repair; High reliability.A beginner needs a lot of training in order to effectively use the potential of the transmission; Compared to other gearboxes, this does not provide as much comfort.
"Robot"Comfort when shifting (there is no need to reach for the lever every time you need to switch); Electronics will determine the most optimal moment to shift to the desired gear (this will be especially useful for those who find it difficult to get used to this parameter).There is a delay during gearshifts; Up / downshifts are often jerky; Prevents the driver from saving fuel.
AutomaticComfortable gear shifting (smooth and almost imperceptible); When you press the gas pedal sharply, it downshifts to accelerate the car as quickly as possible (for example, when overtaking).Expensive maintenance and repair; Does not save fuel; Not economical in terms of oil consumption; Difficulty in repair, which is why you need to look for an expensive service, not every mechanic is able to correctly adjust or repair the mechanism; You cannot start the engine from the tug.
Variable speed driveThe smoothest gear shifting without bringing the motor to higher revs (which prevents it from overheating); Increased ride comfort; Careful use of the engine resource; Simplicity in driving.Expensive maintenance; Sluggish acceleration (compared to previous analogues); Does not make it possible to use the engine in an economical mode in terms of fuel consumption; It is impossible to start the engine from a tug.

For more details on the differences between these types of boxes, see this video:

What is the difference between a manual transmission, an automatic transmission, a variator and a robotic

Vehicle transmission units

Regardless of the type of transmission, this mechanism consists of several components that ensure the efficiency and high efficiency of the device. These are the components of the gearbox.

Clutch disc

This element provides a rigid coupling of the engine flywheel to the main drive shaft. However, if necessary, this mechanism also separates the motor and gearbox. The mechanical transmission is equipped with a clutch basket, and the robot has a similar device.

In automatic versions, this function is performed by a torque converter. The only difference is that the clutch disc can provide a strong connection between the motor and the transmission mechanism, even when the engine is turned off. This allows the transmission to be used as a recoil mechanism in addition to the weak handbrake. The clutch allows you to start the engine from the pusher, which cannot be done automatically.

What is transmission and how it works

The clutch mechanism consists of the following elements:

  • Friction discs;
  • The basket (or the case in which all the elements of the mechanism are located);
  • Fork (moves the pressure plate when the driver presses the clutch pedal);
  • Drive or input shaft.

Clutch types include:

  • Dry. In such modifications, frictional force is used, due to which the friction surfaces of the discs do not allow them to slip during the transmission of torque;
  • Wet. A more expensive version that uses torque converter oil to extend the life of the mechanism and also make it more reliable.

main gear

The main task of the main gear is to receive the forces coming from the motor and transfer them to the connected nodes, namely to the drive axle. The main gear increases the KM (torque) and at the same time reduces the revolutions of the driving wheels of the car.

What is transmission and how it works

Front-wheel drive cars are equipped with this mechanism near the gearbox differential. Rear-wheel drive models have this mechanism in the rear axle housing. The GP device includes a semi-axle, drive and driven gears, semi-axial gears, as well as satellite gears.


Transmits torque, changes it and distributes it to non-axial mechanisms. The shape and function of the differential differs depending on the drive of the machine:

  • Rear wheel drive model. The differential is installed in the axle housing;
  • Front wheel drive model. The mechanism is installed in the gearbox;
  • All-wheel drive model. The differential is located in the transfer case.
What is transmission and how it works

The differential design includes a planetary gearbox. There are three modifications of the planetary gear:

  • Conical - used in the cross-axle differential;
  • Cylindrical - used in the center differential of an all-wheel drive car;
  • Worm - is considered a universal modification that can be used both in interwheel and inter-axle differentials.

The differential device includes axial gears fixed in the housing. They are connected with each other by a planetary gear, which consists of satellite gears. Read more about the device of the differential and the principle of operation. here.

Cardan transmission

A cardan drive is a shaft consisting of two or more parts, which are interconnected by means of a hinge mechanism. It is used in different parts of the car. The main application is in rear-wheel drive vehicles. The gearbox in such vehicles is often lower than the gearbox of the rear axle. So that neither the gearbox mechanism nor the gearbox experience additional stress, the shaft located between them should be divided into sections, the connection of which would ensure smooth rotation when the assembly is deformed.

What is transmission and how it works

If the gimbal is faulty, then during the transmission of torque, strong noises and vibrations are felt. When the driver noticed such an effect, he should pay attention to repairs so that the transmission mechanisms do not fail due to increased vibrations.

In order for the transmission to serve as efficiently as possible and for a long time without repairs, each box must be serviced. The manufacturer sets its own scheduled maintenance period, which the car owner is informed about in the technical documentation. Most often, this period is in the region of 60 thousand kilometers of car mileage. Maintenance includes changing the oil and filter, as well as resetting errors, if any, in the electronic control unit.

More details about caring for the box are described in another article.



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