Transfer case is an integral part of any all-wheel drive vehicle. Despite the general purpose - changing the amount of torque and its distribution between the rear and front axles, the transfer case device is highly dependent on the type all-wheel drive. Consider the principle of operation of the main types of handouts.
gear transfer case
The mechanical transfer case is the most archaic and at the same time the most reliable type of design.
As we can see in the VAZ-2121 all-wheel drive diagram, the transfer case is the link between the manual transmission and the drive shafts that transmit torque to the rear and front axle differentials. In the case of the Niva, the transfer case is a separate transmission unit, but on most cars its body is screwed to the gearbox, so there is no cardan to the front axle as such. An all-wheel drive system with such a transfer case is called Full-time.
Principle of operation
Schematic device on the example of VAZ 2121. 
When driving on stable surfaces with high traction properties, permanent all-wheel drive is not necessary, therefore, the sliding clutch (8) through the hub (7) articulates the drive shaft (12), which receives torque from the engine, with the top gear (6). The splines of the top gear are in engagement with the countershaft constant mesh gear. From it, the torque is transmitted to the driven gear (29) and differential (23).
The purpose of the differential is to redistribute torque between the rear and rear shafts. front axle. It is extremely difficult to understand its principle of operation and the device without a visual representation, so we recommend watching the video.
In this mode of operation, the distribution of torque is determined by the level of rolling resistance of the wheels on the front and rear axle. Uniform distribution tractive effort along the axes is carried out by the mechanism of blocking the center differential (23). Acting on the transfer case control lever in the cabin, the driver moves the sliding clutch (31), thereby articulating the front axle drive shaft (32) with the teeth on the differential housing. Thus, the satellites are no longer involved in the operation of the differential, which leads to an even distribution of torque between the front and rear universal joints.
Down row
The driver, changing the position of the lever, moves the clutch (8) through the hub (7), provoking blocking of the drive shaft (12) with the lower gear (9). The difference between the drive shaft gears is in the gear ratio, which, when downshifting, provides:
- stable minimum speed;
- increase in torque transmitted to the wheels.
Part Time
The transfer case in the Part-Time all-wheel drive system does not have a center differential. By default, such cars have a leading rear axle. Front-wheel drive is connected forcibly at the request of the driver. The absence of a differential means that all-wheel drive can only be turned on on surfaces with a reduced friction coefficient. If this advice is neglected, the forces arising from the different wheel speeds when turning will very quickly disable the transfer case. Video with disassembly of the handout Part-Time.
To summarize, the processes inside the transfer case can be reduced to the movement of sliding clutches that change combinations of rotating gears. The movement of the forks of the sliding sleeves can be carried out both directly through the levers in the cabin, and by pressing the keys on dashboard. In the latter case, the movement of the clutch is carried out by a servo, which is attached to the transfer case.
The Part-Time system is equipped with: Toyota Land Cruiser 70, Opel Frontera, Great Wall Hover, UAZ and many other SUVs. 
Why Part-time and Full-time are no longer relevant
Mechanical transfer gearboxes have one undeniable dignity – great resource. The device, in which the distribution of torque between the drive axles is carried out due to the engagement of gears, has no equal in terms of reliability. Therefore, this type of handout is installed on off-road vehicles designed for operation in severe off-road conditions, military equipment.
But since for SUVs and SUVs, which are used mostly in urban areas, reliability mechanical interlocks is not so important, then it becomes impractical to put up with the rather serious shortcomings of an archaic transfer case. Negative sides:
Advantages of modern all-wheel drive systems
Modern all-wheel drive systems make it possible to implement blocking of interaxle and interwheel differentials using an ECU and sensor equipment. By detecting wheel slippage with the help of sensors, the system slows down freely rotating wheels with pads. 
The rolling resistance force is equalized on both sides of the axle. Therefore, the torque can always be transferred to the drive axle drive with a high friction coefficient.
This is how the electronic simulated differential lock works. Briefly consider the device and the principle of operation of transfer boxes modern systems full drive.
Viscous clutch
The viscous coupling is used both to block the center differential and to connect one of the axles. Structurally, the clutch is a set of discs placed in a dilatant liquid. With a sharp difference in the angular velocities of the metal and friction discs, the silicone fluid changes its viscosity, provoking clutch blocking and even distribution of torque.
The lack of viscous couplings in delayed operation, weak force holding the clutched friction and steel discs. Therefore, the viscous coupling overheats during active operation.
The viscous coupling can be installed in the transfer case or at the rear differential.
Torque on-demand
On this moment all-wheel drive transmission Torque on-demand is the most advanced for use on . Device features of the most common types of TOD transfer boxes:
- hydrodynamic clutch with electronic control. The design of the transfer box is missing center differential, but there is a lower row. The inclusion of both bridges is carried out by an incoming clutch, which presses the clutch pack. The pressing force depends on the magnitude of the current supplied to solenoid valve. The transmission of torque between the gears of the input and output shafts is carried out by a chain gear. The device and principle of operation of a transfer case with a chain drive are perfectly shown in the video;
For a better understanding of the operation of transfer boxes of modern all-wheel drive systems, we recommend that you read the article "".
It is installed on all vehicles with all-wheel drive, as well as on vehicles equipped with any additional equipment. The purpose of the transfer case of an all-wheel drive vehicle is as follows:
- distribution of torque between the drive axles;
- increase in torque when driving off-road.
Vehicles with additional equipment mean all kinds of special equipment, such as truck cranes, car lifts, fire fighting equipment And so on. IN this case transfer box provides pluggable drive oil pumps, water pumps and other equipment.
Car transfer box device
In terms of device transfer boxes of different cars differ significantly, but their general purpose remains unchanged. The "razdatka" of a car with all-wheel drive consists of the following components:
- body (crankcase);
- drive shaft;
- drive shafts of the front and rear axles;
- center;
- differential lock device;
- gear or chain transmission;
- downshift.
All elements are located in the transfer case, including the reduction gear and the center differential. , there is a lubricating fluid (gear oil) in the transfer case housing, which provides constant lubrication of all internal components.
Torque from the gearbox is transmitted to the transfer case through the drive shaft. Further, the torque is transmitted to the center differential, which can have various designs. Obsolete transfer cases contain the simplest center differential without a locking mechanism. On modern cars a differential with the possibility of a hard lock or a self-locking differential is installed.
center differential
Main purpose of the center differential consists in the distribution of torque between the drive axles of the vehicle. A differential without a locking mechanism allows the axles to rotate at different speeds. Whereas a self-locking or manual locking differential allows you to forcibly distribute torque between the drive axles, depending on road conditions.
In modern transfer cases, there are various designs of self-locking differentials:
- viscous coupling;
- lock type Torsen;
- friction clutch.
The most widespread viscous coupling design. This mechanism The self-locking differential is the easiest to manufacture and is relatively inexpensive. The principle of operation of the viscous coupling is based on tracking the angular velocities of the drive axles. With an increase in the angular velocity of one of the drive axles, the differential locks smoothly and the torque increases on the axle with a lower angular velocity. The main working body of this clutch is special liquid with variable viscosity. The disadvantages of this design include the lack of the possibility of manual blocking, incomplete automatic blocking, the possibility of overheating during prolonged operation.
Torsen differential has a more advanced design, however, due to limited strength, it is not used on SUVs. The main advantage of such a mechanism is a wider range of torque transfer.
Lock based friction clutch substantially surpasses previous designs, as there is the possibility of both automatic and manual differential locks. The design has friction discs, and the work is based on friction forces. When one of the drive axles slips, the friction discs, compressing, completely or partially block the differential.
The rear drive axle in most cases is located coaxially with the drive shaft. The front axle drive shaft receives rotation from a chain or gear drive. Among other things, the transfer case may also contain downshift, which allows you to increase the torque transmitted from power unit. Most SUVs are equipped with transfer boxes with a reduction gear. The transfer case can also be fitted with a manual or automatic shutdown and all-wheel drive connections.
Depending on the design, the transfer case may have several modes of operation. Main modes:
- connected rear axle;
- both drive axles are connected;
- both axles connected center lock;
- both axles are connected with interaxle blocking and low gear;
- both axles are connected with automatic locking of the center differential.
The transfer case can be controlled directly by levers (mechanical drive), or by buttons that include executive mechanisms acting on the controls of the transfer case. On all-wheel drive vehicles that are not intended for off-road driving, the “razdatka” is controlled automatically in most cases.
The transfer case is a device that is designed to distribute the vehicle's torque directly between the drive axles. In addition, the transfer case itself can also increase the torque that is supplied to the drive wheels. vehicle. Often, the transfer case has a device that is responsible for turning on and off the front drive axle. Quite often you can find designs when the drive of additional vehicle units will be provided from the transfer case itself.
In order to increase the torque that is supplied directly to the driving wheels of the car (and this procedure will be necessary when difficult conditions movement), boxes dispensing type are performed and manufactured in two stages. Wherein top gear will have a gear ratio that will be equal to one, and the lowest, that is, the first gear will have the same number of about two. Two gears, if provided in the vehicle, will increase both the number of steps, as well as the range in the fluctuations in the change in transmission gear ratio vehicle. It is this that will increase the possibility of an advantageous selection of an optimized transmission, which will correspond to all operating conditions of the car.
Transfer case devices with a locking drive are often used, when all axle drives will be tightly and tightly connected to each other, as a result of which their rotation will always occur on the same angular speeds. Such transfer boxes usually have a device that is designed to disable the front axle drive. This kind of function has its own need when the car is moving at good conditions, on a good road, which will significantly reduce fuel consumption, reduce the load in the transmission part, as well as tire wear.
Some designs of transfer boxes have a special mechanism in their arsenal - center differential. This device distributes the torque that is supplied from the engine internal combustion directly to the transfer box. In addition, this kind of differential allows the wheels of the drive axle to rotate at different angular speeds, which will completely eliminate the possibility of their slippage, as well as reduce the load on the transmission and tire wear of the car. Differentials are used with spur and bevel gears. In order to maximize the throughput of the transmission, differentials are sometimes made with a forced locking device.
1. Classification of transfer boxes.
Due to the rapid development automotive technology, manufacturers have already predetermined several types, the main differences of which will depend on the purpose of the vehicle on which they are provided. So, design engineers distinguish three types of the main transfer case:
- behind the location of the drive shafts of the drive axles (with misaligned and coaxial shafts);
By number and amount variable gears(1,2,3-speed);
Drive axles (with differential and blocked drive).
All cars that are equipped in factories all-wheel drive systems, V without fail will be equipped with a transfer case.
2. The purpose of the transfer case nodes.
It is important to note that many transfer cases have many peculiar design differences, however, there are shared devices that make up the distribution box. These include: drive shaft, front axle drive shaft, reduction gear, chain drive, rear axle drive shaft, center differential lock mechanism, center differential.
The inclusion of two bridges and automatic locking of the center differential;
The inclusion of two bridges in a reduction gear, which provides for a locking center differential.
Switching the operating mode in the transfer case is carried out by means of a special control lever, buttons on the instrument panel or a special rotary switch.
3. Typical breakdowns of the transfer case.
Transfer case is quite complicated technical node, as a result of which there may be many reasons why this device will fail. The main signs of a breakdown of the transfer case will be hidden in the howling sound that occurs during the direct movement of the vehicle on high speeds, vibration of the floor of the body, when the car is just starting to move or accelerate. All the noises and the kind that occur when the car is directly turned, wheel slip will also be witnesses to the fact that certain problems have arisen in the operation of the transfer case. most common problem that occurs with the transfer case is the differential lock. In addition, quite often there is another important problem that comes from spontaneous shutdown gears.
It is important to note that if the motorist is faced with this kind of problem, then you should immediately contact service center, since postponing the solution of this problem indefinitely can "bury" the transfer case of the vehicle alive. Another reason why you should immediately go to repair this unit is the unsafe movement of the car along the highway. This is directly related to the fact that the ability to spontaneously turn off the gear can lead the car into an accident, for example, during any overtaking, at any intersection.
If you look at it, then this device is not too difficult to master and understand, since it has no difficulties in its structural component and mechanical functional. However, its simplicity has nothing to do with its great importance for the successful functioning of the entire vehicle.
Transfer case installed on all vehicles with all-wheel drive, as well as on vehicles equipped with any additional equipment. The purpose of the transfer case of an all-wheel drive vehicle is as follows:
- distribution of torque between the drive axles;
- increase in torque when driving off-road.
Vehicles with additional equipment mean all kinds of special equipment, such as truck cranes, car lifts, fire equipment, etc. In this case, the transfer case provides a plug-in drive for oil pumps, water pumps and other equipment.
What is a giveaway. The purpose of the transfer case
The transfer case is installed on almost all vehicles with all-wheel drive (for example, Niva, UAZ) or on vehicles with any additional equipment that requires a separate drive, and according to the principle of operation, it is primarily intended for dividing the torque along the axes of the vehicle, hence the name - transfer case. For the first time razdatka was used at the beginning of the last century on cars, which were designed for off-road racing. But most consumers did not immediately understand the full benefits of the "gearbox" (such a name was given to the transfer case after its invention) and were rather skeptical of this innovation. But after applying the dispenser in all-wheel drive trucks the interest of producers and end users in it has increased. Although the main function of this node is the distribution of torque, most of its modifications have additional function in the form of an increase in the gear ratio. This function allows you to increase traction characteristics vehicle, thereby increasing the vehicle's cross-country ability, which is very useful when driving off-road.
The main functions of the transfer case can be distinguished:
1. Torque distribution over the drive axles, to ensure the vehicle's off-road patency, reducing the circulation power as much as possible.
2. Increasing the torque on the drive axles to reduce the rolling resistance of the wheels when operating the vehicle off-road.
3. Ensuring the steady movement of the vehicle on low speed when the power unit is operating at maximum power.
Transfer box device. Design.
Usually, the transfer case is mounted on the side members of the car using brackets with cushions (buffers) whose function is to dampen vibration loads, but on different cars the layout of the dispenser may have different variants installations.
The dispenser, depending on the model, may have miscellaneous device, but since the principle of operation of the transfer case, regardless of the model, is almost the same, then its device has common basic elements:
- drive shaft
- a mechanism for locking the center differential
- chain or gear for transmitting torque to the drive shaft of the front axle
- center differential
- rear axle drive shaft
- downshift
- front axle drive shaft.
All these elements are constantly lubricated with transmission oil, which is located in the transfer case housing, thereby providing lubrication of all internal elements.
WHY THE TRANSFER BOX HAS THIS NAME
They began to name the transfer case due to the proximity of concepts with the checkpoint. The torque is transmitted at a low vehicle speed, since the transfer case has a demultiplier or a reduction gear. Thus, the machine is able to move in difficult conditions, in the absence of a road. Crossovers do not have such a device due to the fact that they are not designed for off-road use. The demultiplier helps the machine reduce the speed of rotation of the drive shafts. For this reason, a lot of torque is not lost. The low gear allows the car to have a few more gears. In the case of various traffic situations the driver will have more possibilities correctly determine the required gear ratio. 
FAULTS AND REPAIR OF THE TRANSFER BOX
transfer case enough expensive device, so it is necessary proper care And timely repair to increase service life. Very often, drivers complain about the inclusion of gears with the expenditure of effort. Most often this occurs when the rods and gear forks are worn out. This problem is often caused by scuffing on the gear wheels of mechanisms. Can't be ruled out bad job fixatives. If the rods and forks are worn out, they must be replaced immediately. When driving, there is an automatic disengagement of gears. This indicates that the gear teeth are worn out and require urgent replacement.
It also indicates that the fork mounts may be faulty. The high noise level during the operation of the transfer case indicates many factors: poor viscosity of lubricants; damage to gears; faulty clutch; high level bearing wear. You can only replace lubricants on your own, and for everything else you need to turn to specialists. With a small amount of oil, it must be restored to normal. There are cases when oil flows out of the transfer case. There can be only one reason - worn gaskets. To fix this problem, you need to disassemble the box and replace the gasket. The box needs care. The oil level must be checked periodically and topped up. In case of oil leakage, check all seals and tighten the fasteners.
It is better to entrust the repair of all malfunctions of the transfer case to specialists, since many malfunctions cannot be solved on their own. A serviceable transfer case works without extraneous noise. It should not overheat when shifting gears. If these conditions are met, then we can safely say that the box is in good condition. If we talk about the repair of its parts, then you immediately need to understand that this is an expensive procedure. To delay the replacement or repair of the transfer case, it is necessary to regularly inspect and check the entire mechanism. Many drivers who are faced with the repair of a transfer case know that this all requires significant financial investments. Therefore, do not neglect regular visual checks. All faults are best dealt with early on as this will help reduce future costs and ensure good work boxes.
CLASSIFICATION OF TRANSFER BOXES
The modern classification includes the following types of transfer cases:
- by location drive shafts(with coaxial and non-coaxial shafts);
- for a certain number of gears (one-stage, two and three-stage);
- by type of drive axles (with differential or blocked drive).
Transfer case operating modes.
The mode of operation of the transfer box, depending on design features can be divided into several types:
- center differential is locked and both drive axles work
- the differential is automatically locked and both drive axles are in operation
-both drive axles receive torque (like a Niva car)
-only the rear axle works
-both axles in operation in low gear, differential locked. In this mode, the car has the highest cross-country ability.
Switching the transfer case from one mode of operation to another occurs with the help of a lever (on older generations of vehicles with all-wheel drive) and with the help of buttons. The switching itself can be carried out mechanically, electrically, pneumatically, and even with hydraulic drive.
The principle of operation and the scheme of the transfer case.
Surely, many who have encountered four-wheel drive vehicles wondered: how does the dispenser work? The principle of operation of the transfer box largely depends on its design.
The scheme and principle of operation of the simplest transfer case looks something like this: the gearbox transmits torque to the drive shaft of the transfer case (sometimes there is a retarder between the gearbox and the transfer case), which in turn transmits torque to the center differential. The center differential distributes torque between the rear axle drive shaft and the front axle drive shaft (it also has important function- it allows the axes to rotate at different angular speeds). Further, depending on the mode of operation of the transfer case, the torque is transmitted either to both axles, or only to one. Here also important element is a mechanism for locking the center differential, whose tasks include the complete or partial shutdown of the center differential. This function provides a rigid clutch and, accordingly, the same speed of rotation of the drive shafts of the front and rear axle.
All-wheel drive vehicles, unlike mono-wheel drive vehicles, are equipped with such a unit as a transfer case or razdatka. Its purpose is to properly distribute the torque between the axles of the car, and increase it to overcome difficult places (not for all cars). The transfer case is installed after the gearbox or as a separate unit, it can also be a single unit with the gearbox.
Transfer box device
IN various cars its execution may differ, however, in general, the transfer case device looks something like this:
- drive shaft;
- drive shafts of the front and rear axles;
- chain or gear transmission;
- downshift or downshift;
- center differential;
- center differential lock mechanism.
The transfer case scheme assumes a constant connection between the drive shaft and the drive shaft of one of the axles (for off-road vehicles - the rear). Power is transmitted to the front axle drive shaft by means of a gear or chain drive. The lowering row and center differential in some handouts may be absent. For example, all-wheel drive crossovers, not designed for off-road driving, a downshift is simply not needed.
Transfer case classification
The modern classification includes the following types of transfer cases:
- according to the location of the drive shafts (with coaxial and non-coaxial shafts);
- for a certain number of gears (one-stage, two and three-stage);
- by type of drive axles (with differential or blocked drive).
The purpose of the transfer case nodes
center differential
This unit allows you to distribute the center torque and allows the drive shafts to rotate at different angular speeds. This is especially important when cornering because the wheels travel different distances and therefore need to rotate at different speeds. If the transfer case is not equipped with such an assembly, allow the wheels to rotate with different speeds only possible by disabling one axis. 
Center differentials are symmetrical and asymmetrical. The first works in such a way that the torque is distributed equally to both axles, the second divides it in a certain proportion.
Center differential lock mechanism
In order for the car to fully realize its off-road capabilities, the center differential is equipped with a locking mechanism, the purpose of which is to force the wheels of both axles to rotate at the same speed. Blocking can be either forced or manual, depending on the type of mechanism.
The following types of locks are currently used:
- self-locking differential Torsen;
- friction multi-plate clutch;
- viscous coupling (viscous coupling).
The transfer case of crossovers is equipped with the first two mechanisms due to the shortcomings described below.
viscous coupling
This is the simplest device that allows you to automatically lock the center differential. The principle of its operation is as follows: inside it there are perforated discs immersed in silicone fluid, some of the discs are connected to the body, and the rest to the hub; during the slippage of the wheels of one of the axles, some discs begin to rotate faster than others, the silicone fluid heats up and thickens, as if gluing the hub to the body. 
The main advantage of such a system is low cost. There are many more disadvantages: the operation occurs with a delay, as a result of which the car may have time to burrow with slipping wheels; differential lock is not complete; overheating occurs from prolonged operation; This unit is not compatible with the ABS system.
This is a design consisting of a set of worm gears - leading and driven. Principle of operation this device next: as long as all the wheels “hold” the road well, the differential distributes torque to the axles equally. As soon as one of the axles slips, the moment, due to the friction forces in the worm gear, is transferred to the other axle, the ratio of efforts can reach up to 20:80. The main disadvantage of this solution is the limitations on the strength of the structure. For this reason, Torsen is not installed on SUVs, its destiny is crossovers. 
This is a set of friction discs with a controlled compression ratio. Such a clutch allows you to distribute torque between the axles depending on the terrain conditions. Under normal conditions, the moment is divided equally. During the slippage of one of the axles, the clutch disks are compressed, the differential is completely or partially blocked. To ensure operation, the clutch can be equipped with an electric or hydraulic drive and electronic system management. The center differential can also be locked manually by mechanical, pneumatic, hydraulic or electric drive. Many cars have the ability to lock the differential, both automatically and manually. 
chain drive
The purpose of this node is to transmit torque to the drive shaft of the front axle of the car to ensure the operation of the all-wheel drive. It consists of a pair of gears (driver and driven) and drive chain. In addition to the chain transmission, a gear can be used in the transfer case, consisting of cylindrical gears.
Why is a transfer case called a transfer case?
The name arose as a result of combining the concepts of a transfer case and a gearbox. The fact is that in order to transmit a large torque to the wheels while maintaining a low speed, the transfer case of the car is equipped with a reduction gear or a demultiplier. Such a need arises when overcoming heavy off-road conditions, for example, so that the car can drive into steep slope or pull out another car stuck in the mud. Crossovers do not have this node, since they are not designed to work in such conditions.
The demultiplier is nothing but additional box gears, the work of which is to reduce the speed of rotation of the drive shafts of the transfer case, but at the same time maintain a high torque. The lower row provides for the presence of two or three gears. Thus, the total number of gears of the car is doubled or tripled, and the driver has the opportunity to choose the most appropriate gear ratio in a particular situation.
Thanks to the presence of reduction gears, the service life of the transmission is significantly increased, while maintaining a high transmitted power.
