What is a transmission and how does it work

A smooth start of movement, acceleration without bringing the engine to 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 are, and what basic 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.

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 speed of rotation of the secondary shafts. When the engine is started, the flywheel rotates in accordance with the speed of the crankshaft. 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 driving using this energy, which would result in a very rapid discharge of the power source.

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, when 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 sports, and they could not be safely driven in crowded cities.

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 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).

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 of 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 into another gear. So the car accelerates (or decelerates), and the engine does not suffer.

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 an oblique arrangement of teeth are used. And for the stability and speed of the elements engagement in modern manual transmissions, synchronizers are used. 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.

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.

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 shift 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 stepped gear change. Torque distribution is controlled by moving the walls of the drive shaft pulley.

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 to use the potential of the gearbox effectively; 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.
automatic	Comfortable 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 repairs; 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 drive	The smoothest gear shifting without bringing the motor to higher revs (which prevents it from overheating); Increased driving comfort; Careful use of the engine resource; Simplicity in driving.	Expensive maintenance; Sluggish acceleration (compared to previous analogs); Does not make it possible to use the engine in an economical mode in terms of fuel consumption; You cannot 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

Чем отличается механическая коробка передач, автомат, вариатор и роботизированная

Watch this video on YouTube

Mechanical transmission

The peculiarity of a mechanical transmission is that the entire process of switching between gears occurs solely due to the mechanical intervention of the driver. Only he squeezes the clutch, interrupting the transmission of torque from the flywheel to the clutch disc. It is solely through the actions of the driver that the gear changes and the resumption of the supply of torque to the gears of the gearbox occurs.

But the concept of a manual transmission should not be confused with a manual transmission. The box is a unit with the help of which the distribution of traction forces occurs. In a mechanical transmission, the transmission of torque occurs through a mechanical transmission. That is, all elements of the system are directly coupled to each other.

There are several advantages to the mechanical transmission of torque (mainly due to the gear connection):

Maximum efficiency, since there is no loss of power for the operation of associated mechanisms, as, for example, in a torque converter;
The cost of a mechanical transmission is the lowest among identical analogues with a different type of torque transmission due to a simpler design;
The same simple design allows manufacturers to create units with small dimensions.

Hydromechanical transmission

The device of such a unit includes:

Hydrodynamic converter;
Mechanical gearbox.

The advantages of such a transmission are that it facilitates the control of gear changes due to the automated gear change. Also, this box provides additional damping of torsional vibrations. This reduces the stresses on the machine parts at maximum loads.

The disadvantages of hydromechanical transmission include low efficiency due to the operation of the torque converter. Since the unit uses a valve body with a torque converter, it needs more oil. It requires an additional cooling system. Because of this, the box has increased dimensions and more weight compared to a similar mechanic or robot.

Hydraulic transmission

The peculiarity of such a box lies in the fact that gear shifting is performed using hydraulic units. The unit can be equipped with a torque converter or hydraulic coupling. This mechanism connects the required pair of shafts and gears.

The advantage of the hydraulic transmission is the smooth engagement of the speeds. The torque is transmitted as gently as possible, and torsional vibrations in such a box are minimized due to the effective damping of these forces.

The disadvantages of this gearbox include the need to use individual fluid couplings for all gears. Due to its large size and weight, the hydraulic transmission is used in rail transport.

Hydrostatic transmission

Such a box is based on axial-plunger hydraulic units. The advantages of the transmission are its small size and weight. Also, in this design, there is no mechanical connection between the links, so that they can be bred over long distances. Thanks to this, the gearbox has a large gear ratio.

The disadvantages of a hydrostatic transmission are that it is demanding on the quality of the working fluid. It is also sensitive to the pressure in the brake line, which provides gear shifting. Due to the peculiarities of the checkpoint, it is used mainly in road construction equipment.

Electromechanical transmission

The design of the electromechanical box uses at least one traction motor. An electric generator is installed in it, as well as a controller that controls the generation of energy necessary for the operation of the gearbox.

Through the use of an electric motor (s), traction is controlled. The torque is transmitted in a wider range, and there is no rigid coupling between the mechanical units.

The disadvantages of such a transmission are the large size (a powerful generator and one or more electric motors are used), and at the same time the weight. If we compare such boxes with a mechanical analogue, then they have a much lower efficiency.

Types of car transmissions

As for the classifications of automotive transmissions, all these units are divided into only three types:

Anterior;
Rear;
Four-wheel drive.

Depending on the type of box, different wheels will be driving (from the name of the transmission it is clear where the torque is supplied). Consider how these three types of vehicle transmissions differ.

Front wheel drive transmission

Front-wheel drive transmission structure consists of:

Clutches (if it is a mechanic or a robot);
Gearboxes;
Main transmission;
Differential;
Shafts that drive the front wheels.

All elements of such a transmission are enclosed in one block located across the engine compartment. A bundle of a box and an engine is sometimes called a model with a transverse motor. This means that the car is front-wheel drive or all-wheel drive.

Rear-wheel drive transmission

Rear-wheel drive transmission structure consists of:

Clutches (if it is a mechanic or a robot);
Gearboxes;
Main transmission;
Differential;
Cardan transmission;
Semiaxes.

Most classic cars were equipped with just such a transmission. With regard to the implementation of the transmission of torque, the rear-wheel drive transmission is as simple as possible for this task. A propeller shaft connects the rear axle to the gearbox. To reduce vibrations, supports are used that are slightly softer than those installed in front-wheel drive cars.

All-wheel drive transmission

This type of transmission is distinguished by a more complex device (for details on what an all-wheel drive is, and how torque transmission is realized in it, read separately). The reason is that the unit must simultaneously distribute torque to all wheels. There are three types of this transmission:

Permanent four-wheel drive. In this version, the unit is equipped with an interaxle differential, which distributes the torque to both axles, and, depending on the quality of the adhesion of the wheels to the road surface, change the forces between them.
Manual connection of four-wheel drive. In this case, the structure is equipped with a transfer case (for details about this mechanism, read in another article). The driver independently determines when to turn on the second axle. By default, the car can be either front or rear wheel drive. Instead of an interaxle differential, as a rule, interwheel ones are used.
Automatic all wheel drive. In such modifications, instead of the center differential, a viscous clutch or an analog of a friction type is installed. An example of how such a clutch works is considered DM all.

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.

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 modification 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 transmission is to receive the forces coming from the motor and transmit 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.

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, side-axle gears, as well as satellite gears.

Differential

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.

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 gear - 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 to 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.

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.

Transmission

This is the hardest part of any transmission, even a manual one. Thanks to this unit, an even distribution of traction forces occurs. This happens either through the direct participation of the driver (manual transmission), or through the operation of electronics, as in the case of an automatic or robotic transmission.

Regardless of the type of gearbox, this unit allows you to make the most efficient use of the power and torque of the engine in different modes of operation. The gearbox allows the car to move faster with minimal engine speed fluctuations (for this, the driver or electronics must determine the appropriate rpm) or subject the engine to less load when driving uphill.

Also, thanks to the gearbox, the direction of rotation of the driven shaft changes. This is necessary to drive the car in reverse. This unit allows you to transfer all the torque from the motor to the drive wheels. The gearbox allows you to completely disconnect the motor from the drive wheels. This is necessary when the machine must come to a complete stop, but the motor must continue to run. For example, a car should be in this mode when stopping at a traffic light.

Among the gearboxes there are such varieties:

Mechanical. This is the simplest type of box in which the distribution of traction is carried out directly by the driver. All other types of boxes can be freely classified as automatic types.
Automatic. At the heart of such a box is a torque converter, and the change in gear ratios occurs automatically.
Robot. This is an automatic analogue of a manual transmission. A feature of the robotic gearbox is the presence of a double clutch, which provides the fastest gear shifting.
Variable speed drive. This is also an automatic transmission. Only traction forces are distributed by changing the diameter of the belt or drive chain.

Due to the presence of the gearbox, you can use the previous engine speed, but change the speed of rotation of the wheels. This, for example, comes in handy when the car overcomes off-road.

Main bridge

Under the transmission bridge is meant the supporting part, which is attached to the frame of the car, and inside it is the mechanism for transmitting torque to the wheels. In passenger cars, axles are used in rear-wheel drive or all-wheel drive models. In order for the torque to come from the gearbox to the axle, a cardan gear is used. The features of this element are described in another article.

The car can have driving and driven axles. A gearbox is installed in the drive axle, which converts the transverse rotation of the shaft (direction across the car body) into longitudinal rotation (direction along the body) of the drive wheels. Freight transport may have more than one drive axle.

Transfer case

The transfer case is used only in all-wheel drive transmissions (torque is transmitted to all wheels). In it, as well as in the main gearbox, there is a set of gears that allow you to change the gear ratios (demultiplier) for different pairs of wheels to increase torque. This is necessary in all-terrain vehicles or in heavy-duty tractors.

Constant-velocity joint

This transmission element is used in vehicles in which the front wheels are leading. This joint is directly connected to the drive wheels and is the last link in the transmission.

The presence of this mechanism is due to the fact that when turning the front wheels, they must receive the same amount of torque. This mechanism works on the principle of cardan transmission. In the car, two CV joints are used on one wheel - internal and external. They provide a permanent link to the differential.

Principle of operation

The transmission of a car works in the following sequence:

The engine starts thanks to the coordinated work of the ignition and fuel supply systems.
In the process of alternate combustion of the air-fuel mixture in the engine cylinders, the crankshaft rotates.
Torque is transmitted from the crankshaft through the flywheel, to which the clutch basket is connected, to the transmission drive shaft.
Depending on the type of gearbox, the torque is distributed either through the connected gears or through a belt / chain (for example, in a CVT) and goes to the drive wheels.
In a manual transmission, the driver independently disconnects the connection between the flywheel and the transmission drive shaft. To do this, press the clutch pedal. In automatic transmissions, this process occurs automatically.
In a mechanical type gearbox, the change in gear ratios is provided by connecting gears with a different number of teeth and different diameters. When a specific gear is selected, only one pair of gears are connected to each other.
When torque is applied to the differential, traction is delivered to the wheels to varying degrees. This mechanism is necessary because the car does not always move along a straight section of the road. On a turn, one wheel will spin faster than the other as it travels a larger radius. So that the rubber on the wheels is not subjected to premature wear, a differential is installed between the axle shafts. If the car is all-wheel drive, then there will be at least two such differentials, and in some models an intermediate (center) differential is also installed.
Torque in a rear-wheel drive car is transmitted to the wheels from the gearbox through the cardan shaft.
If the car is all-wheel drive, a transfer case will be installed in this type of transmission, with the help of which all wheels will be driven.
Some models use a system with a plug-in all-wheel drive. This can be a system with a locking center differential or a multi-plate friction or viscous clutch can be installed between the axles. When the main pair of wheels begins to slip, the interaxle mechanism is blocked, and torque begins to flow to the second pair of wheels.

The most common transmission failures

The most common transmission problems include:

Difficulty switching one or more speeds. In this case, it is important to repair the clutch, adjust the cable or adjust the rocker.
Noise appears in the transmission when shifting to neutral. If this sound disappears when you press the clutch pedal, then this may be a symptom of a failed release bearing, wear of the input shaft bearings, with an incorrectly selected transmission oil or insufficient volume.
Clutch basket wear.
Oil leakage.
Broken propeller shaft.
Failure of the differential or main gear.
Breakage of CV joints.
Malfunctions in the electronics (if the machine is fully or partially controlled by an electronic control unit). In this case, the motor malfunction icon will glow on the dashboard.
During gear shifting, strong jerks, knocks or grinding sounds are felt. The reason for this can be determined by a qualified specialist.
The speeds are switched off arbitrarily (applies to manual transmissions).
Complete failure of the unit to work. The exact reason must be determined at the workshop.
Strong heating of the box.

Dependence of the transmission on the type of drive

So, as we figured out, depending on the type of drive, the transmission will be structurally different. In the description of the technical characteristics of different car models, the concept of "wheel formula" is often mentioned. It can be AWD, 4x4, 2WD. Permanent four-wheel drive is designated 4x4.

If the transmission is distributing torque to each wheel depending on the load on it, then this formula will be denoted AWD. As for the front or rear wheel drive, this wheel arrangement can be designated 4x2 or 2WD.

The design of the transmission, depending on the type of drive, will differ in the presence of additional elements that will ensure the constant transmission of torque to the axle or the temporary connection of the second axle.

Video: Car transmission. General arrangement, principle of operation and transmission structure in 3D

The device, the principle of operation and the structure of the car transmission are additionally described in this 3D animation: