The transfer case (or transfer case) is an integral part of each. Its function is to distribute torque (hereinafter KM) along the axes of the machine, as well as to increase it when driving off-road or bad roads.
The transfer box has the following device:
The drive shaft transmits torque (hereinafter KM) from the gearbox to the transfer case;
The center differential is needed to distribute the KM between two axles;
The mechanism that locks the center differential provides a stronger connection between the rear and front axles. Blocking is carried out manually or automatically;
Chain (or tooth) transmission, rear and front axle drive shafts, reduction gear.
For a fuller realization of the possibilities all-wheel drive car a mechanism is used that blocks the center differential, that is, its partial or complete shutdown. Modern self-locking mechanisms are a friction multi-plate clutch, a self-locking differential and a viscous coupling.
A viscous coupling (another name is a viscous coupling) is the most inexpensive and simple device. Its work is based on the appearance of a blocking element at a certain difference in the speeds of the axes. The transfer case is most often equipped with a viscous coupling.
The self-locking differential is a special design, which consists of worm gears - driven and driving. Locking becomes possible due to friction. Since such a differential has limitations in strength, it is not used on SUVs.
The friction multi-plate clutch distributes the KM between the axles depending on external conditions. In this case, the CM is redistributed to the axis that has better grip with the road. To ensure the operation of such a clutch, the transfer case can be equipped with hydraulic or electric drive as well as the electrical control system.
Forced (or manual) blocking is possible only with the help of the driver. In this case, hydraulic, electric or mechanical drive.
The function of the chain drive is to transfer the KM to the front axle of the machine. It includes the driven and leading as well as the drive chain. Instead of a chain, the transfer case can be equipped with a spur gear. On modern machines looks like a planetary gear.
On some vehicles with a 4x4 system, it is possible to connect front wheel drive manually (for example, some designs have permanent drive systems with the ability to disengage the front axle.
The transfer box can operate in several modes: the rear axle is on; both axles of the machine are included; both axles work at the center axle; both axles work in a reduction gear when the differential is locked; both axles work with differential lock.
Switching modes occurs using a lever, a rotary switch or buttons on the panel.
Cars. Thanks to this unit, there is a distribution of torque along the axes, as well as its increase when the vehicle moves through terrain with difficult terrain.
A look into the past
The first all-wheel drive vehicles appeared at the beginning of the twentieth century. These were cars designed for road and off-road racing. The public reacted coolly to the invention, considering it pampering, and was not going to overpay big money for dubious pleasure.
It seemed that the idea had died, no one needed it. But already in the tenth years of the last century, the designers revived it by developing all-wheel drive trucks.
The developers faced the question: how to distribute the torque between the two drive axles. It was clear that a conventional gearbox could not solve this problem.
After some manipulations appeared committed new unit who performed the following tasks:
- created a series of gear ratios that contribute to the coordination of the speed of rotation of the crankshaft, the torque coming out of it, and the speed of the car;
- distributed torque between drive axles;
- did not prevent movement back.
Out of habit, this new mechanism has long been called a gearbox. But, unlike the familiar one from non-all-wheel drive vehicles, this box had two output shafts.
But this development was not successful due to the high cost. Perhaps she would have sunk into oblivion, but then the first World War. And, as you know, military operations are conducted not only along paved roads, you have to drag guns off-road, and even in the spring thaw. This is where the finest hour of all-wheel drive vehicles has come.
The purpose of the transfer case
The transfer case serves to distribute the force to all the drive axles of the vehicle. In addition, it is used to turn on and off the leading front axle. The transfer case, as a rule, has a two-stage gearbox. Under its influence, the gear ratios change, and the number of gears of the car doubles.
First row gear ratios obtained when you turn on the direct (higher) gear. The second row is activated when downshifting. This allows you to use vehicles in different road conditions.
In car design high cross there is a device that does not allow downshifting at the moment when front axle included. Such a device protects the rear axle from overload with a large torque.
Transfer case types
1. With coaxial drive shafts leading bridges. 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 don't have an intermediate shaft. Compactness, noiselessness, high efficiency– here are the main advantages of this type transfer box. In addition, it has a low metal content.
3. With a blocked drive axles. Allows you to use the full traction force without wheel spin. With such a transfer case, the front axle is switched on only on difficult sections of the road. When driving on hard surfaces, the front axle is disengaged to save fuel and reduce tire wear.
4. With differential drive of driving axles. In boxes of this type, it is used that allows the rotation of the drive shafts with different speeds. In a car equipped with such a box, the front axle is constantly on. To increase the cross-country ability, center differentials are made with forced locking.
Transfer box device
Despite the differences in the design of transfer boxes, depending on the system all-wheel drive, they all have common main nodes:
- drive shaft;
- center differential;
- a mechanism that blocks the center differential;
- drive shaft rear axle;
- gear or chain transmission;
- reduction gear;
- front axle drive shaft.
Transfer box diagram
Torque from the gearbox to the transfer case is transmitted through the drive shaft.
Distributes the torque between the axles - center differential. Thanks to him, the axes can rotate with different angular velocities. There are two types of center differential:
- 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 (driver and driven) and drive chain. Instead of a chain, a gear (cylindrical) is often used. In the all-wheel drive system, automatically connected, the transfer case is made in the form of a bevel gear.
The purpose of the downshift is to increase the torque when the car is driving off-road. Has a design planetary gear.
Operating modes
The transfer box operates in five modes.
1. Neutral on.
2. Differential unlocked with overdrive engaged: torque split 1:2.
3. Differential locked with overdrive engaged: torque is divided according to road grip.
4. Differential unlocked with downshift: torque is distributed in a ratio of 1:2.
5. Locked differential with downshift: front and rear axles work as one. Torque is distributed depending on the degree of adhesion of the wheels to the road. In this mode, the car's cross-country ability is the highest.
Video: transfer case.
It is installed on all vehicles with all-wheel drive, as well as on vehicles equipped with any additional equipment. Purpose of the transfer box all-wheel drive vehicle following:
- 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 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 lubricating fluid in the transfer box housing ( transmission oil), which ensures 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 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, a smooth differential lock occurs and an increase in torque on an 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 train. 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.
In today's life, we almost never get on real off-road. Therefore, in the list of models of an average company, cars with a transfer case are less and less common. But still, they are, and we will talk about this most "iron" of off-road units ...
Simplification and uniformity of car design off-road understandable.
The vast majority of new all-wheel drive vehicles are built according to the “passenger” scheme: a transverse engine in front, permanent drive on the front wheels and connected with a disc clutch - on the rear. But in the class of heavy off-road vehicles, there is traditionally a different set of all-terrain values, and the transfer case for them is the main attribute.
ORTHODOX HONEST IRON
The transfer case is an independent, rather weighty and complex unit: a part of the transmission between the gearbox and drive axles stuffed with gears, shafts and couplings. Its first and main purpose is to divide or distribute in one or another proportion the engine torque to both drive axles - front and rear.
In addition to the distribution function, the transfer case has been given one more - to increase the torque on the wheels while reducing the speed of movement. So, in the first downshift, compared to the first upshift, about three times less speed rotation of the wheels and the same amount of traction on them. The gearbox part of the transfer case is sometimes called a demultiplier and is most often referred to as 4L or simply L.
Thus, two devices "live" inside a single case. And if everything is more or less unambiguous with the demultiplier (from model to model, only gear ratio), then there are several principles for distributing torque across bridges. Accordingly, the designs of transfer boxes also differ.
NOTHING MORE PERMANENT...
The first principle - it is also the first and historically - is a permanent drive to the rear axle with a temporary connection of the front axle. Connoisseurs will exclaim "part time" and will be absolutely right. Such a transmission dominated SUVs for more than half a century, largely due to the simplicity and endurance of the design. The primitive connection with a lever was gradually replaced by push-button control, but the essence was preserved.
It would seem, what is simpler: on asphalt, the front axle is turned off, off-road - on. The only catch was the question: when exactly should I connect? After all, late connection of the "front" on the road is fraught with a jam, and even a short movement on asphalt in 4x4 mode for most part-time cars is technically fatal.
Engineers solved the problem range rover, forcing his transfer case to always drive both axles and at the same time creating a second type of SUV. The difference in the speeds of the front and rear universal joints is extinguished by an ordinary center differential. Symmetrical, the same as between the wheels. And in order to equalize the transmission of permanent all-wheel drive in terms of patency with the plug-in, they also installed a locking center differential. Our dear "Niva" - from this opera.
So, both types in the off-road mode rigidly drove the two axles, and on the pavement, the permanent all-wheel drive dominated due to the traction of the four wheels. In addition, constantly all-wheel drive is always fully armed - so to speak, always all-inclusive.
TERMINOLOGY:
Strictly speaking, the phrase "shares the moment between two axles" is only fully correct for those off-road vehicles that actually have two leading continuous axles. Since independent suspensions inevitably replace dependent ones, the term “bridge” in relation to an SUV now means not a continuous beam with two wheels per se, but a set of suspension and transmission assemblies related to the front or rear pair of wheels. With the same success, the word "axis" is used here.
Handle.
There is nothing more reliable mechanical switching handouts. Let the pen vibrate and require effort, but it is visual
Part of heavy SUVs now combines a rear axle with independent suspension front ( Mitsubishi Pajero Sport, Toyota TLC200). Most pickups are built according to this scheme. Some companies in new models completely abandoned solid axles, replacing them with independent suspension (Range Rover, Infiniti QX56, Jeep Cherokee). And only the most conservative models, living out their long life on the assembly line ( Mercedes Gelandewagen, Land Rover Defender, UAZ), are completely faithful dependent suspension with two impenetrable-strong bridges.
NOT EVERYTHING IS GOOD WHAT IS COMPLETE
But there are also disadvantages. The transfer case turned out to be very bulky, heavy and complex. On the same Range Rover to keep big ground clearance, I had to lift it above the floor of the cabin and actually place it in a voluminous "box" - the armrest. Such factors directly affected the price of cars with permanent all-wheel drive. In addition, part-time designs were much more reliable and painlessly "digested" much larger overloads.
The all-wheel drive boom that started by the end of the 20th century forced engineers to make a transfer case with an interaxle differential more compact, lighter and cheaper. There were options with a chain instead of heavy gears ( Nissan Terrano and MMC L200), with a planetary design in the drive low gear. But there was one more problem - the stability of a permanent all-wheel drive car in a slippery turn.
SUPERSELECT
A transfer box with this name is used Mitsubishi. It sounds incomprehensible, but in fact - both systems (part-time and permanent all-wheel drive with a center differential) are "in one bottle." The four SuperSelect positions allow you to drive exclusively on rear wheel drive with the front wheel disconnected, connect the front wheels together with the rear wheels through the center differential (long-term mode), do the same, but with the center differential locked (similar to the connected part-time) and, finally, add the "lower" to the locked "center". From which side you look - a paradise for the jeeper!
FREEDOM, EQUALITY AND ... "EARS"
When you are told that a free differential, they say, “sends” part of the torque to a particular wheel or axle - do not believe it! In systems with a free differential, all the power will always go where there is less resistance.
The classic behavior of such a transmission on an icy forest turn with excess speed: the beginning of the arc under traction - more moment goes back and the car resembles rear-wheel drive. You let off the gas before the steepest part of the turn, and instantly the weight of the car, and with it the share of torque, rush forward - the car became front-wheel drive, slid out! You convulsively parry the drift with the steering wheel and, remembering the front-wheel drive commandments, slightly add gas ... They seize again rear wheels, and a tall pickup or SUV, finally turning around across the road, flies into a ditch! Why is this happening? Because there is nothing in your ATV's transfer case that regulates the direction and proportion of torque. It all goes to where it slides more in a given fraction of a second, and the transfer of power occurs with a meager, imperceptible to the driver, difference in the grip of the wheels.
They tried to overcome the destructive freedom of a symmetrical differential different ways. The systems did it best. electronic stabilization but we are not talking about them now. In addition to symmetrical ones, asymmetric differentials began to be used in the “handouts”, which obviously distribute more torque back. Technically, they are simple, but they allow you to almost always leave the car a little more rear-wheel drive, saving you from insidious power shifts. Such a differential is in the "razdatka" Audi Q7, and in the "Mitsubishevsky" SuperSelect II (33:67%).
POWER TAKE OFF
Quite rarely, some other nodes are given from the transfer case, except for bridges. It could be a winch (on a Toyota part Land cruiser 80), various mounted units special equipment (for example, snow plows). Such a transfer case is often called a power take-off.
A special abbreviation KOM is used for it, additional drives rotate their own cardan shafts. Even the domestic UAZ-469 had a version with a KOM for a brush under the belly. Abroad similar utility vehicles very widely used today.
Transfer case- a unit for distributing torque from the engine to several drive mechanisms, which in most cases also increase the number of gears in the transmission.
Distributes the torque between the drive axles in such a way as to ensure best cross car without the occurrence of a negative phenomenon - "power circulation" in the transmission.
Increases the torque on the drive wheels to the extent necessary to overcome the rolling resistance of the wheels when driving on bad roads and off-road, as well as on steep climbs(see demultiplier).
Provides stable vehicle movement low speed when the engine is running at maximum torque.
Transfer case types
There are several types of transfer boxes, which differ depending on the purpose of the vehicle on which they are installed. It is customary to distinguish three main types of transfer cases: according to the location of the drive axle drive shafts - with coaxial and non-coaxial shafts; on the drive of leading axles - with a blocked and differential drive; - according to the number of gears - one-, two- or three-stage.
Vehicles equipped with an all-wheel drive system must be equipped with a transfer case.
The device of the transfer case and the purpose of its parts
Despite some differences, it is possible to single out what is common in the device of all handouts.
The box consists of:
- drive shaft;
- center differential;
- center differential lock mechanism;
- rear axle drive shaft;
- chain (or gear) transmission;
- downshift;
- front axle drive shaft.
Torque is transmitted by the drive shaft from the gearbox to the transfer case. The function of the center differential includes the distribution of torque between the axles. This gives them the ability to rotate at different angular velocities.
The symmetrical center differential equally distributes torque between the axles. An asymmetric axle differential distributes the same moment in a certain ratio. The center differential is not used in transfer cases installed in all-wheel drive systems with automatic and manual connection.
The center differential lock mechanism provides for its complete or partial disabling, which ensures a rigid connection of the front and rear axles between themselves and complete implementation all-wheel drive capabilities of the car. The differential lock can be operated manually or automatically.
The rear axle drive shaft is mounted coaxially to the drive shaft. The function of a chain, or gear, transmission is to ensure that torque is delivered to the front axle.
The chain drive consists of driving and driven wheels and a drive chain. Along with the chain, a spur gear can be used in the transfer case.
Downshifting serves to increase torque when driving off-road. It is made in the form of a planetary gearbox and is installed in the transfer boxes of off-road vehicles.
Transfer cases for vehicles with a manually connected all-wheel drive system provide for the possibility of using the front axle. Transfer boxes of permanent four-wheel drive include the function of disabling the front shaft (axle).
The design of the transfer case determines the modes of its operation. It could be:
- inclusion rear axle;
- inclusion of two bridges;
- the inclusion of two bridges with a locking center differential;
- the inclusion of two bridges in a reduction gear with a locking center differential;
- the inclusion of two bridges with automatic locking of the center differential.
You can switch the transfer case operating modes using the control lever, rotary switch or instrument panel buttons.
Typical damage to the transfer case
Transfer case - complex technical node car, so there can be many reasons for its failure.
Signs of a breakdown in the transfer case are a howling sound when the car is moving on high speed, vibration of the floor of the body at the time of start and acceleration. The noise that appears when turning or slipping the wheels also indicates problems in the transfer case.
Enough frequent breakdowns transfer case are differential lock and spontaneous shutdown gears.
In all similar cases you should immediately pay attention to this and contact the experts.
