A car is a complex system of components that must work as one mechanism. The gearbox is considered one of the most important. The acceleration speed, smoothness and much more depend on the type of device. Not surprisingly, each type of transmission has its own best option.

When it comes to four-wheel drive, the only viable option is a transfer case. Among motorists, it is simply called a razdatka. It takes on two main functions:

• distributes torque
• adapts the system when driving off-road.

Quite often, a transfer case is installed on special equipment. In this case, its design allows you to connect additional equipment.

Perhaps you have a logical question, but where does the traditional gearbox go? In reality, the dispensing system and the conventional system work together. It's just that the first device is installed additionally.

Device and principle of operation

To understand how the transfer case works, it is necessary to consider its structure in detail. But a little clarification needs to be made. Automotive designers create different designs that, first of all, adapt to specific technical tasks. Despite this, its purpose remains unchanged.

Therefore, we can single out a certain base, which is typical for almost any transfer case:

• drive shaft,
• differential lock,
• drive shaft,
• reduction and gear transmission.

All these elements fit into the body. They are the very basis on the basis of which more complex structural manipulations are carried out. The lubricating fluid plays an important role in the system. It avoids premature wear of parts.

Attention! Typically, transmission oil is used as the lubricating fluid.

The gear oil ensures stable lubrication of the entire transfer case assembly. This, in turn, avoids overheating, which means that increased loads are permissible.

A simple gearbox and transfer case work closely together. Take torque, for example. He is with The checkpoint is transferred to the razdatka. The drive shaft is used as a bridge. This, in fact, is the basic principle of the transfer case.

From the transfer case, the torque goes to the center differential. This device does not have any precise design. It largely depends on the specifics of the vehicle on which the transfer case is installed.

If we consider the old transfer cases, then their differential does not have a lock. In all modern devices, such a control is available.

Center differential

The center differential is the most important part of the transfer case mechanism. It is he who is responsible for the distribution of torque. Without it, the entire system would simply be inoperative.

Attention! The transfer case differential must optimally redistribute the torque between the axles.

If the transfer case differential does not have a lock, then the axles can rotate at different speeds. Otherwise, distribution is compulsory. The proportions directly depend on the road surface.

Self-locking of the transfer case differentials is due to the following devices:

• coupling,
• separate blocking,
• friction clutch.

In most cases, modern car manufacturers use a viscous clutch in the design of the transfer case. The fact is that this mechanism is not particularly difficult to manufacture. Therefore, its price is more than affordable.

The viscous coupling monitors the angular velocities of the axes. As soon as one of the four parameters increases, the blocking is activated. The torque for the axle with the minimum parameter is increased in parallel.

Attention! The transfer case clutch is based on a fluid, the viscosity of which can change.

Unfortunately, we couldn't do without drawbacks. The main thing is no manual blocking. This can lead to overheating during prolonged use. Although we must admit that incomplete automatic blocking is still present.

An example of a successful locking mechanism is the Torsen differential. From an engineering point of view, it is practically a masterpiece. But it cannot be installed on SUVs due to its excessive fragility.

Attention! But if we talk about the range of the transfer of torque, the Torsen differential shows very good results.

Nevertheless, the friction clutch is still considered the most advanced. It combines two designs. This makes it possible to carry out both manual and automatic blocking.

The discs are responsible for the operation of the friction clutch. They work thanks to the force of friction. As soon as one of the drive axles slips, the discs are compressed. This leads to a complete or partial differential lock.

The location of the drive shaft depends on the drive shaft. Moreover, on the front axle, the shaft rotates with the help of a gear transmission. A downshift is used to increase torque. Therefore, a similar structural element is installed in almost all transfer cases for SUVs.

Types of transfer boxes

Transfer cases are best classified into types according to the way power is distributed. Now there is such a classification of devices:

1. without the possibility of disconnecting the driving axle,
2. with the ability to disable the front axle,
3. the system allows manual connection of the second bridge.

If the transfer case has an inter-axle drive with interlocking, then the second bridge cannot be connected. More precisely, this is not possible when there is a high adhesion to the supporting surface. Simply put, this cannot be done when driving on asphalt or dirt road.

To understand why this is so, consider an illustrative example. Imagine that the car changed direction or entered a hill. In this case, the wheels travel unequal sections of the path. As a result each wheel has its own speed of rotation. But this does not happen, since the shafts in the dispenser rotate at the same speed.

Attention! Compensation is carried out by slipping or slipping.

Other popular classification systems

Naturally, transfer cases can be classified in more than just the way power is distributed. Modern automotive engineering includes many popular classification systems:

• by the number of gears,
• by drive type,
• according to the location of the shafts.

All transfer cases can be classified according to them. This makes their use more appropriate.

Outcomes

The transfer case is often used in SUVs and special vehicles. It allows you to stabilize the work of the wheels when you have to drive on uneven surfaces and makes it possible to connect additional equipment.

The transfer case is an integral part of any 4WD vehicle. Despite the general purpose of changing the amount of torque and its distribution between the rear and front axles, the transfer case design is highly dependent on the type of all-wheel drive. Let's consider the principle of operation of the main types of handouts.

Gear razdatka

A 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, which transmit torque to the differentials of the rear and front axles. 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. The all-wheel drive system with such a transfer case is called Full-time.

Principle of operation

Schematic device using the example of the VAZ 2121.

When driving on stable surfaces with high adhesion 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 mesh with the constant mesh gear of the intermediate shaft. From it, the torque is transmitted to the driven gear (29) and the differential (23).

The purpose of the differential is in the redistribution of torque between the shafts of the rear and front axles. It is extremely difficult to understand its principle of operation and device without a visual presentation, so we recommend watching the video.

In this operating mode, the torque distribution is determined by the rolling resistance of the wheels on the front and rear axles. The uniform distribution of tractive effort along the axles is carried out by the center differential locking mechanism (23). By acting on the transfer case control lever in the passenger compartment, 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 work of the differential, which leads to an even distribution of torque between the front and rear driveshafts.

Descending row

The driver, changing the position of the lever, moves the clutch (8) through the hub (7), causing the drive shaft (12) to block with the lower gear (9). The difference between the gears of the drive shaft in the gear ratio, which, when the downshift is engaged, provides:

• stable minimum speed;
• an increase in the torque transmitted to the wheels.

Part-Time

The transfer case in the Part-Time AWD system does not have a center differential. By default, these vehicles have a driven rear axle. The front-wheel drive is connected forcibly at the request of the driver. The lack of a differential means that all-wheel drive can only be engaged on surfaces with a reduced grip coefficient. If you neglect this advice, the forces arising from different wheel speeds when cornering will very quickly destroy the transfer case. Video with disassembly of the Part-Time handout.

To summarize, the processes inside the transfer case can be reduced to the movement of sliding clutches, which change the combinations of rotating gears. Moving the forks of the sliding clutches can be carried out either directly through the levers in the cabin, or by pressing the keys on the dashboard. In the latter case, the movement of the clutch is carried out by a servo drive, 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 indisputable advantage - a large resource. The device, in which the distribution of torque between the drive axles is carried out due to the meshing of gears, is unmatched in terms of reliability. Therefore, it is this type of distribution box that is installed on SUVs intended for use in difficult off-road conditions, military equipment.

But since the reliability of mechanical interlocks is not so important for SUVs and SUVs, which are used mostly in urban areas, it becomes impractical to put up with the rather serious shortcomings of an archaic transfer case. Negative sides:

Benefits of modern all-wheel drive systems

Modern all-wheel drive systems make it possible to implement blocking of center and cross-axle differentials using ECU and sensor equipment. By detecting wheel slip with the help of sensors, the system brakes freely rotating wheels with pads. The rolling resistance forces are equalized on both sides of the axle. Therefore, the torque can always be transferred to the drive axle with a high coefficient of adhesion.

This is how the electronic simulated differential lock works. Let's briefly consider the structure and principle of operation of transfer cases of modern all-wheel drive systems.

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 fluid. When there is a sharp difference in the angular velocities of the metal and friction discs, the silicone fluid changes its viscosity, provoking a blocking of the clutch and an even distribution of torque.

Lack of viscous couplings in delayed actuation, weak force holding the friction and steel discs interlocked. Therefore, the viscous coupling overheats during active operation.

The viscous clutch can be installed in the transfer case or at the rear differential.

Torque on-demand

At the moment, the Torque on-demand all-wheel drive transmission is the most advanced for use on. Features of the device of the most common types of transfer cases TOD:

• • electronically controlled hydrodynamic clutch. There is no center differential in the design of the transfer case, but there is a reduction row. The inclusion of both axles is carried out by a running clutch, which presses the clutch pack. The pressing force depends on the magnitude of the current applied to the solenoid valve. The transmission of torque between the gear wheels of the input and output shafts is carried out by a chain gear. The device and principle of operation of the transfer case with a chain drive are perfectly shown in the video;

For a better understanding of the operation of the transfer cases of modern all-wheel drive systems, we recommend that you familiarize yourself with the article "".

Transfer case is installed on all vehicles with four-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 drive axles;
• increased torque when driving off-road.

Cars with additional equipment mean all kinds of special equipment, such as truck cranes, car lifts, fire fighting equipment, and so on. In this case, the transfer case provides a plug-in drive for oil pumps, water pumps and other equipment.

What is a handout. The purpose of the transfer case

The transfer case is installed on almost all cars with all-wheel drive (for example, Niva, UAZ) or on cars with any additional equipment that require a separate drive, and according to the principle of operation, it is primarily intended to divide the torque along the axes of the car, hence the name - transfer case. For the first time, the razdatka was applied at the beginning of the last century on passenger cars, which were intended to participate in off-road races. But most consumers did not immediately understand all the benefits of the "gearbox" (this name was given to the transfer case after its invention) and were rather skeptical about this innovation. But after the use of handouts in all-wheel drive trucks, the interest of manufacturers and end users in it increased. Although the main function of this unit is the distribution of torque, most of its modifications also have an additional function in the form of increasing the gear ratio. This function allows you to increase the traction characteristics of the 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. Distribution of torque on the driving axles, to ensure the vehicle's off-road capability, while minimizing the circulating power.
2. Increasing the torque on the driving axles to reduce the rolling resistance of the wheels when operating the vehicle off-road.
3. Ensuring stable movement of the vehicle at low speed when the power unit is operating at maximum power.

Transfer case device. Design.

Usually, the transfer case is mounted on the side members of the car using brackets with pillows (buffers) whose function is to damp vibration loads, but on different cars the layout of the transfer case may have different installation options.
The transfer case, depending on the model, may have a different device, but since the principle of operation of the transfer case, regardless of the model, is practically the same, then its device has common basic elements:
- drive shaft
- a mechanism for locking the center differential
- chain or gear transmission for transmitting torque to the drive shaft of the front axle
- center differential
- rear axle drive shaft
- reduction gear
- front axle drive shaft.
All of these elements are constantly lubricated with transmission oil, which is located in the transfer case housing, thereby ensuring lubrication of all internal elements.

WHY DOES THE TRANSFER BOX HAVE THIS NAME?

They began to name the transfer case because of the proximity of the concepts with the checkpoint. The torque is transmitted at an insignificant vehicle speed, since the transfer case has a range or a downshift. Thus, the car 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 built for off-roading. The demultiplier helps the machine to reduce the rotational speed of the drive shafts. For this reason, no large torque is lost. The lowering row allows the car to have several more gears. In the case of various driving situations, the driver will have more opportunities to correctly determine the required gear ratio.

FAULTS AND REPAIR OF THE TRANSFER BOX

The transfer case is quite an expensive device, so proper care and timely repairs are necessary to increase the service life. Very often, drivers complain about the effort involved in shifting gears. Most often this occurs when the rods and forks are worn out. This problem is often caused by scuffing of the gears of mechanisms. Poor performance of the retainers cannot be ruled out. If the rods and forks are worn out, they must be replaced immediately. When driving a car, automatic gear disengagement is observed. This indicates that the gear teeth are worn out and require urgent replacement.

It also says that the forks may be defective. The high noise level during the operation of the transfer case indicates many factors: poor viscosity of lubricants; damage to gear wheels; faulty clutch; high level of bearing wear. You can only replace lubricants on your own, and for everything else you need to contact 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 here - worn out gaskets. To eliminate this problem, it is necessary to disassemble the box and replace the gasket. The box needs care. The oil level should be checked and replenished periodically. If oil leaks, check all oil seals and tighten the fasteners.

It is better to entrust the repair of all transfer case malfunctions 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 all this requires a significant financial investment. Therefore, regular visual checks should not be neglected. It is best to eliminate all faults immediately at the initial stage, as this will help reduce costs in the future and ensure the correct operation of the box.

CLASSIFICATION OF TRANSFER BOXES

The modern classification includes the following types of transfer cases:

1. by the location of the drive shafts (with coaxial and non-coaxial shafts);
2. for a certain number of gears (one-stage, two-stage and three-stage);
3. by the type of drive of the driving axles (with differential or blocked drive).

Transfer case operating modes.

The operating mode of the transfer case, depending on the design features, can be divided into several types:
-the center differential is locked and both drive axles are working
-differential is automatically locked and both driving axles are in operation
- both driving axles receive torque (like a Niva car)
- only the rear axle works
- both axles are in low gear, the differential is locked. In this mode, the vehicle has the highest cross-country ability.
The transfer case is switched from one operating mode to another using a lever (on older generations of all-wheel drive vehicles) and using buttons. The switching itself can be carried out mechanically, electrically, pneumatically or even hydraulically.

The principle of operation and the diagram of the transfer case.

Surely, many who came across four-wheel drive cars wondered: How does the distributor work? The principle of operation of the transfer case largely depends on its design.
The scheme and principle of operation of the simplest transfer case looks 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 the moment to the center differential. The center differential distributes the torque between the rear axle drive shaft and the front axle drive shaft (it also has an important function - it allows the axles to rotate at different angular speeds). Further, depending on the operating mode of the transfer case, the torque is transmitted either to both axles, or only to one. Here, too, an important element is the center differential locking mechanism, the task of which is to fully or partially disable the center differential. This function provides a firm clutch and, accordingly, the same speed of rotation of the drive shafts of the front and rear axles.

Cars. Thanks to this unit, there is a distribution of torque along the axes, as well as its increase when the car moves on terrain with difficult terrain.

A look into the past

The first four-wheel drive vehicles appeared in the early twentieth century. These were cars designed for racing on the highway and off-road. The audience reacted coolly to the invention, considering it a pampering, and was not going to overpay big money for a dubious pleasure.

It seemed that the idea had died, no one needed. But already in the tenth years of the last century, the designers revived it by developing all-wheel drive trucks.

The developers were faced with the question: how to distribute the torque between the two driving axles. It was clear that a conventional gearbox could not solve this problem.

After some manipulations, a completely new unit appeared, which performed the following tasks:

• created a series of gear ratios that help coordinate the crankshaft rotation speed, the torque coming out of it, and the vehicle speed;
• distributed the torque between the driving axles;
• did not interfere with movement back.

Out of habit, this new mechanism has long been called a gearbox. But, unlike the familiar from non-four-wheel drive cars, this box had two output shafts.

But even this development was not successful due to the high cost. Perhaps she would have sunk into oblivion, but then the First World War began. And, as you know, hostilities are conducted not only along paved roads, you have to drag guns along the off-road, and even in the spring thaw. It was here that the finest hour of all-wheel drive vehicles came.

The purpose of the transfer case

The transfer case serves to distribute the force to all the drive axles of the vehicle. In addition, with its help, the driving front axle is switched on and off. The transfer case usually has a two-stage gearbox. Under its influence, the gear ratios change, and the number of gears of the car doubles.

The first row of gear ratios is obtained when direct (top) gear is engaged. The second row is activated when downshifting. This allows the use of vehicles in different road conditions.

In the design of cross-country vehicles there is a device that does not allow the inclusion of a downshift at the moment when the front axle is engaged. Such a device protects the rear axle from overloading with high torque.

Transfer case types

1.With coaxial drive shafts of driving axles. This type is widely used due to the possibility of using a single final drive for the front and rear axles.

2.With misaligned driven shafts. They do not have an intermediate shaft. Compactness, noiselessness, high efficiency - these are the main advantages of this type of transfer case. In addition, it has a low metal consumption.

3.With blocked drive axles. Allows full tractive power without wheel spin. With such a transfer case, the front axle is activated only on difficult-to-pass road sections. When driving on hard surfaces, the front axle is disengaged to conserve fuel and reduce tire wear.

4. With a differential drive of the driving axles. In boxes of this type, it is used, which allows the rotation of the drive shafts at different speeds. In a car equipped with such a box, the front axle is always on. To increase cross-country ability, center differentials are made with forced locking.

Transfer case device

Despite the differences in the design of the transfer cases, depending on the all-wheel drive system, they all have common basic components:

• driving shaft;
• center differential;
• a mechanism that blocks the center differential;
• rear axle drive shaft;
• gear or chain transmission;
• reduction gear;
• front axle drive shaft.

Transfer case diagram

The torque from the gearbox to the transfer case is transmitted through the drive shaft.

Distributes torque between the axles - center differential. Thanks to him, the axles can rotate at different angular speeds. The center differential is of two types:

• symmetrical (distributes torque equally);
• asymmetrical (distributes torque in various ratios).

Serves for rigid coupling of the front and rear axles.

The chain drive transmits torque to the front axle. It consists of gears (driving and driven) and a drive chain. Instead of a chain, a gear drive (cylindrical) is often used. In the all-wheel drive system, automatically connected, the transfer case is made in the form of a bevel gearbox.

The purpose of the downshift is to increase the torque when driving off-road. Has a planetary gear design.

Modes of operation

The transfer case operates in five modes.

1. Neutral is on.

2. The differential is unlocked when the overdrive is engaged: the torque is divided in a 1: 2 ratio.

3. The differential is locked when the overdrive is engaged: the torque is divided depending on the adhesion of the wheels to the road surface.

4. The differential is unlocked when the gear is engaged: the torque is distributed in a ratio of 1: 2.

5. Differential locked when downshift is engaged: front and rear axles work as a whole. The torque is distributed depending on the degree of adhesion of the wheels to the road. In this mode, the vehicle's cross-country ability is the highest.

Video: transfer case.

Transfer case serves to distribute torque between the drive axles. In addition, the transfer case can also increase the torque supplied to the drive wheels of the vehicle. As a rule, a device is provided in the transfer case to turn the front drive axle on and off, and sometimes additional units (for example, a power take-off) are driven from the transfer case.

Rice. The device of the transfer case of the KAMAZ car:
1 - input shaft flange; 2 - primary shaft; 3, 4, 8, 13, 75, 17, 40 - bearings; 5 - a leading gear wheel; 6 - top hatch cover; 7 - power take-off gear; 9 - clutch for power take-off; 10 - power take-off box; 11 - oil sump; 12 - gear wheel of a reduction transfer; 14 - fitting; 16 - satellite; 18 - rear axle drive shaft; 19 - rear differential carrier; 20 - epicyclic gear; 21 - the leading gear wheel of the center differential; 22 - sun gear; 23 - front clip; 24 - transfer case housing; 25 - overdrive gear; 26 - transfer case housing cover; 27 - cork; 28, 30, 41 - couplings; 31 - drive gear of the electric speedometer sensor drive; 32 - front axle drive shaft; 33 - plug; 34 - spring; 35 - stock; 36 - diaphragm; 37 - switch; 38 - lock; 39 - intermediate shaft; 42 - intermediate gear

To increase the torque supplied to the drive wheels (which is necessary in difficult driving conditions), transfer cases are usually performed in two stages, with the highest gear having a gear ratio equal to one (or about one), and the lowest (first) gear about two. The presence of two gears increases the number of steps and the range of change in the gear ratio of the vehicle transmission, which increases the possibility of selecting the most advantageous gear in accordance with the driving conditions.

Transfer cases with a locked drive are used, when the drives of all axles are constantly rigidly connected to each other and always rotate at the same angular speeds. These transfer cases usually have a device to disengage the front axle drive, for example when driving in good conditions (on hard surfaces with high traction), which can reduce fuel consumption, reduce stress in the drivetrain and reduce tire wear.

In some designs of transfer cases, a special mechanism is installed - an interaxle differential, which distributes the torque supplied from the engine to the transfer case to the drive axles in the required ratio in proportion to the adhesion weight on these axles. The differential also allows the wheels of different driving axles to rotate at different angular speeds, which eliminates the possibility of slippage, reduces stress in the transmission and tire wear. Bevel gear and spur gear differentials are used. To increase the cross-country ability of the vehicle, center differentials are sometimes performed with forced locking or self-locking.

On some high-speed tracked vehicles, a junction box is installed, which makes it possible to obtain various modes of transmission operation, for example, the simultaneous or separate operation of the caterpillar and water propellers, separate or simultaneous drive of other units (pumps, winches, etc.).

Rice. Transfer case of a four-axle vehicle:
1 - pallet; 2 - differential assembly; 3 - differential lock clutch; 4 - lower shaft; 5 - parking brake caliper; 6 - front axle drive housing; 7, 13 - forks; 8 - pneumatic switch; 9 - intermediate shaft; 10 - upper shaft; 11 - transfer case housing; 12 - gear change clutch; 14 - crankcase cover; 15 - gear wheel of the power take-off box drive; 16 - power take-off box case; 17 - oil pump; 18 - differential driven gear; 19 - rear axle drive cover assembly; 20 - filter; 21 - drain plug