It serves to distribute the torque supplied to it between the output shafts and provides the possibility of their rotation with unequal angular velocities.
When a wheeled vehicle is cornering, the inner wheel of each axle travels a shorter distance than its outer wheel, and the wheels of one axle travel different paths compared to the wheels of other axles.
The wheels of the vehicle pass unequal paths when driving over bumps on straight sections and when turning, as well as in the case of straight driving on a flat road when different radii wheel rolling, such as uneven tire pressure and tire wear or uneven load distribution on the vehicle.
If all the wheels rotated at the same speed, this would inevitably lead to their slipping and slipping relative to the supporting surface, which would result in increased wear tires, increased loads in the transmission mechanisms, the cost of engine power for sliding and slipping, increased fuel consumption, as well as the difficulty of turning transport vehicle. Thus, the wheels of the vehicle must be able to rotate at different angular velocities relative to each other. For non-driving wheels, this is ensured by the fact that they are mounted freely on their axles and each of them rotates independently of each other. For the driving wheels, this is ensured by the installation of differentials in their drive.
Basic types of differentials
By location, differentials are divided into:
- interwheel (distributing torque between the drive wheels of one axle)
- interaxle (distributing the moment between the main gears of the two drive axles)
- central (distributing the moment between the group of driving axles)
According to the ratio of torques on the driven shafts, the differentials can be:
- symmetrical (moments on the driven shafts are always equal to each other)
- asymmetric (the ratio of moments on the driven shafts is not equal to one)
There are also differentials:
- non-blocking
- blocked forcibly
- self-locking
By design, differentials are divided into:
- conical
- cylindrical
- cam
- worm
In some cases, mechanisms such as clutches are installed instead of differentials. freewheel.
Currently, on wheeled vehicles, conical symmetrical non-locking differentials are most widely used.
Video: How does a differential work?
Differential schemes
Rice. Schemes of simple differentials with a constant ratio of moments on the driven shafts: a - symmetrical conical; b - symmetrical cylindrical; c - asymmetric cylindrical; g - asymmetric conical; 1, 8 - left and right axle shafts of the differential; 2, 6 - left and right side gears; 3 - satellite; 4 - differential housing; 5 - driven wheel of the main gear; 7 - axis of rotation of the satellites; 9 - sun gear; 10 - epicyclic gear
Rice. Cross-wheel symmetrical conical differential: 1, 8 - differential cups; 2, 7 - support washers for semi-axial gears; 3, 6 - semi-axial gears; 4 - satellite support washer; 5 - satellites; 9 - cross
Rice. Schemes of asymmetric differentials: a - conical; b - cylindrical
Rice. The cam differential of the GAZ-66-11 car (a) and the scheme of its operation (b): 1 - internal sprocket; 2 - separator; 3 - outer sprocket; 4 - differential cup; 5 - cracker
Rice. Lockable cross-axle differential: 1 - clutch; 2 - ring gear
Rice. Center differential of the KamAZ-5320 car: 1 - drive shaft; 2 - sealing cuff; 3 - differential housing; 4, 7 - support washers; 5, 17 - differential cups; 6 - satellite: 8 - blocking sensor; 9 - filler plug; 10 - pneumatic blocking chamber; 11 - fork; 12 - retaining ring; 13 - gear clutch; 14 - blocking clutch; 15 - drain plug; 16 - gear wheel of the middle axle drive; 18 - cross; 19 - gear wheel of the rear axle drive; 20 - a bolt of fastening of cups; 21 - bearing; 22 - bearing cover
Rice. Operation of the cross-axle differential: a - general scheme; b - when moving straight; in - when turning; 1 - differential case; 2, 5 - semi-axial gears; 3 - cross: 4, 6 - satellites; 7 - main gear drive gear; 8, 9 - axle shafts; 10 - driven gear of the main gear
Rice. Center differential Torsen: 1, 3 - right and left side gears; 2 - differential case; 4 - satellite associated with the right side gear; 5, 7 - differential output shafts; 6 - satellite associated with the left side gear
The term “differential lock”, or “self-locking differential” (self-blocking), has been heard by many motorists, but only a few know how this process looks in practice. And if earlier automakers equipped mainly SUVs with such an “option”, now it can also be found on quite a city car. In addition, often the owners of cars not equipped with self-blocks, having understood what benefits they bring, install them on their own.
But before understanding how a self-locking differential works, you need to understand how it functions without locking.
What is a differential
The differential (diff) can rightfully be considered one of the main elements of the design of a car transmission. With its help, the transmission, change, and distribution of the torque produced by the engine between a pair of consumers takes place: wheels located on the same axis of the machine or between its axles. Moreover, the force of the flow of distributed energy, if necessary, can be different, which means that the speed of rotation of the wheels is different.
In the transmission of a car, a diff can be installed: in the gearbox housing and in the transfer case, depending on the drive device (s).
Those diffs that are installed in the bridge or gearbox are called interwheel, and which is located between the axles of the car, respectively - interaxle.
Purpose of the differential
As you know, a car performs various maneuvers while driving: turns, lane changes, overtaking, etc. In addition, the road surface may contain bumps, which means that the wheels of the car, depending on the situation, can at the same time travel different distances. Therefore, for example, when turning, if the speed of rotation of the wheels on the axle is the same, then one of them will inevitably begin to slip, which will lead to accelerated tire wear. But this is not the worst. Much worse is that the vehicle's handling is significantly reduced.
To solve such problems, they came up with a differential - a mechanism that will redistribute the energy coming from the engine between the axles of the car in accordance with the amount of rolling resistance: the smaller it is, the greater the wheel speed will be, and vice versa.
Differential mechanism
To date, there are many varieties of diffs, and their device is quite complex. However, the principle of operation is generally the same, so it will be easier to understand to consider the simplest type - open differential, which consists of the following elements:
- Gears mounted on axle shafts.
- Driven (crown) gear, made in the form of a truncated cone.
- A drive gear mounted on the end of the drive shaft, which, together with the crown, forms the main gear. Since the driven gear is larger than the drive gear, the latter will have to make several revolutions around its axis before the ring gear performs only one. Therefore, it is these two elements of the differential that reduce the amount of energy (speed) that eventually reaches the wheels.
- Satellites that form the player key role in providing the necessary difference in the speed of rotation of the wheels.
- Corps.
How the differential works
During the rectilinear movement of the car, its axle shafts, and hence the wheels, rotate at the same speed as the drive shaft with its helical gear. But during the turn, the load on the wheels becomes different (one of them tries to spin faster), and due to this difference, the satellites are released. Now the energy of the engine passes through them, and since a pair of satellites are two separate, independent gears, a different rotational speed is transmitted to the axle shafts. Thus, the power generated by the engine is distributed between the wheels, but unevenly, and depending on the load acting on them: what moves along the outer radius experiences less rolling resistance, so the diff transfers more energy to it, spinning faster.
There is no difference in how the center differential and the interwheel differential work: the principle of operation is similar, only in the first case the distributed torque is directed to the axles of the vehicle, and in the second - to its wheels located on the same axis.
The need for center diff becomes especially noticeable during the movement of the machine over rough terrain, when its weight presses on the axle that is lower than the other, for example, on an uphill or downhill slope.
Differential problem
Although the differential certainly plays a big role in the design of the car, its operation sometimes creates problems for the driver. Namely: when one of the wheels is on a slippery section of the road (mud, ice or snow), then the other, located on harder ground, begins to experience an increased load, the diff tries to fix this, redirects the engine energy to the sliding wheel. Thus, it turns out that it receives maximum rotation, while the other, which has a tight grip on the ground, simply remains motionless.
It was precisely to solve such problems that a differential lock (disable) was invented.
The principle of blocking and its types
Having understood the principle of the differential, we can conclude that if you block it, then the torque will increase on the wheel or axle that has the best grip. This can be done by connecting its body to one of the two semi-axes or by stopping the rotation of the satellites.
The lock can be complete - when the parts of the differential are connected rigidly. It is carried out, as a rule, with the help of a cam clutch and is controlled by the driver through a special drive from the cab of the car. Or it can be partial, in which case only a limited force is transmitted to the wheels - this is how a self-locking differential works, which does not require human intervention.
How does a self-locking differential work?
The self-locking differential is essentially a compromise between full block and free diff and allows you to reduce the slippage of the wheels of the machine in the event of a difference between them in the coefficient of adhesion to the ground. Thus, cross-country ability, off-road handling, as well as the dynamics of vehicle acceleration are significantly increased, regardless of the quality of the road.
Samoblok excludes complete blocking wheels, which protects the axle shafts from critical loads that can occur on differentials with forced shutdown.
The blocking of the semi-axes is removed automatically if the speed of rotation of the wheels is leveled during rectilinear movement.
The most common types of self-blocks
A disk self-block is a set of friction (rubbing) disks installed between the diff body and the axle gear.
It is not difficult to understand how a differential works with such a block: while the car is driving in a straight line, the diff body and both axle shafts spin together, as soon as there is a difference in rotational speeds (the wheel hits slippery area), friction occurs between the disks, reducing it. That is, a wheel left on hard ground will continue to spin, rather than stop, as in the case of a free differential.
A viscous coupling, or otherwise a viscous coupling, like the previous diff, contains two packs of discs, only this time perforated, installed between each other with a small gap. One part of the discs has a clutch with the body, the other with the drive shaft.
The disks are placed in a container filled with an organosilicon liquid, which, when rotated uniformly, remains unchanged. As soon as there is a difference in speed between the packages, the liquid begins to thicken quickly and strongly. There is resistance between the perforated surfaces. An excessively untwisted package is thus slowed down, and the rotation speed is leveled.
Toothed (screw, worm) self-block. Its work is based on the ability of the worm pair to wedge and thereby block the axle shafts when a difference in torque occurs on them.
Cam self-block. To understand how this type of differential works, it is enough to imagine an open differential, in which gear (cam) pairs are installed instead of a planetary gear mechanism. The cams rotate (jump) when the speeds of rotation of the wheels are almost the same, and are rigidly blocked (jammed) as soon as one of them begins to slip.
Differences in how blocking works center differential and interwheel, no - the principle of operation is the same, the differences are only at the end points: in the first case - two axles, in the second - two wheels mounted on the same axle.
Domestic "Niva" and its differentials
In the line of domestic VAZs, the Niva occupies a special place: unlike its “relatives” on the conveyor, this car is equipped with a non-switchable all-wheel drive.
In the transmission of the VAZ SUV, three differentials are installed: interwheel - in each bridge, and interaxle - in the transfer case. Despite such a number, you won’t have to figure out how the differentials work on the Niva again. Everything is exactly the same as described above. That is, during the rectilinear movement of the machine, provided there is no slippage on the wheels, pulling force between them is distributed evenly and has the same value. When one of the wheels begins to slip, then all the energy from the engine, passing through the diffs, is directed to this wheel.
Niva differential lock
Before talking about how the differential lock on the Niva works, one point should be noted, namely, to clarify the purpose of the front (small) handle of the transfer case.
Some drivers believe that with its help the car turns on front-wheel drive- this is not so: both the front and rear drives the "Niva" is always involved, and this handle controls the differential of the transfer case. That is, while it is set in the “forward” position, the diff works in the normal mode, and when it is “back”, it turns off.
And now directly about the lock: when the differential is turned off, the shafts of the transfer case are closed to each other by the clutch, thereby forcibly equalizing the speed of their rotation, that is, the total speed of the front axle wheels is equal to the total speed of the rear axle. The distribution of thrust occurs in the direction of greater resistance. Let's say it slips rear wheel, if you turn on the lock, the traction force will go to the front axle, the wheels of which will stretch the car, but if at the same time as the rear axle it will stall and front wheel, then the Niva will not get out on its own.
To prevent this from happening, motorists install self-blocks in bridges that will help pull out a stuck car. To date, the most popular among Niva owners is the Nesterov differential.
Samoblok Nesterov
It is in how the Nesterov differential works that the secret of its popularity lies.
The design of the differential allows not only to optimally adjust the wheels of the car when making maneuvers, but also in the event of slipping or hanging the wheel, the device gives it a minimum amount of energy from the engine. Moreover, the reaction of the self-block to a change traffic situation almost instantaneous. In addition, the Nesterov differential significantly improves the controllability of the car even on slippery turns, increases exchange rate stability, increases acceleration dynamics (especially in winter), reduces fuel consumption. And the installation of the device does not require any changes in the design of the transmission and is installed in the same way as a classic diff.
The differential has found application not only in automotive technology, it turned out to be very useful on walk-behind tractors, greatly facilitating the life of its owners.
Differential for walk-behind tractor
The walk-behind tractor is a rather heavy unit, and it takes a lot of effort to simply turn it, and with an unregulated angular speed of rotation of the wheels, this becomes even more difficult. Therefore, the owners of these machines, if the diffs were not originally provided for by the design, purchase and install them on their own.
How does a motoblock differential work? In fact, it only provides an easy turn of the car, stopping one of the wheels.
Its other function, in no way connected with the redistribution of power, is an increase in the wheelbase. The design of the differential provides for its use as an axle extension, which makes the walk-behind tractor more maneuverable and resistant to tipping over, especially when cornering.
In a word, the differential is a very useful and irreplaceable thing, and its blocking significantly increases the car's cross-country ability.
In design modern cars there are a number of components and assemblies that are mandatory for all their brands, models, types and types. These include, first of all, the engine, gearbox, brake system. The list also includes a differential.
There is a differential in any car, and in some cars there are several of these nodes. Experienced motorists are well aware of what a differential is in a car, what role it plays and what varieties it can be. For those people who are so far only novice motorists, it will certainly be useful to learn about this.
Bevel differential of the car: 1 - cardan shaft; 2 - axle shaft of the drive wheel;
The differential is a mechanism by which the same torque is transmitted to the wheels of the same axle, rotating at different speeds. In addition, the differential is used to evenly distribute torque between several drive axles.
The basis for the design of any automotive differential working principle planetary gear. Depending on what type of rotary motion transmission is used, there are such types of differentials as:
- Conical;
- Cylindrical;
- Worm.
Between wheels mounted on the same axle, a bevel differential is almost always installed. The cylindrical differential is usually used as a center differential, and the worm gear is distinguished by the versatility of its application. The most widely used differentials of the conical type, which are installed on almost all vehicles as cross-axle. All their main elements are also available in cylindrical and worm differentials.
The housing of the bevel differential (often referred to as a bowl) receives torque from the main gear and transmits it to the gears of the axle shafts through the so-called satellites. They perform the functions of planetary gears, and as for their number, then, depending on the design features of a particular bevel differential, they can be from two to four.
If the car is moving along a straight path, the resistance of each of the wheels to the road is the same. In this case, the rotation of the satellites does not occur, and the rotation of the semi-axial gears is carried out with equal angular velocities. At the moment of turning one of the wheels, what is on inside turn, meets more road resistance, the rotation of its side gear becomes slower, the satellites begin to rotate. As a result of this, the speed of rotation of the outer wheel increases, but the torque remains the same as on the inner wheel.
When driving on a slippery road, when one wheel slips and moves at a slower speed, the situation is similar to the situation with a turn, as a result of which the car often simply cannot move. In order to increase the torque on one or the other wheel, a differential lock is used.
Varieties of automotive differentials
In addition to conical, cylindrical and worm, the following types of differentials exist and are successfully used: full lock differential, Thorsen differential, Quaif differential, viscous coupling.
Full lock differential
This type of differential is most commonly used on trucks and SUVs. Their blocking is turned on and off directly from the passenger compartment using a special key by the driver. They are used to increase the cross-country ability of vehicles.
Differentials Thorsen
The design of Thorsen differentials was developed German company Siemens. In fact, they are combinations of bevel and worm differentials. Thorsen differentials are different high efficiency However, they are quite difficult to manufacture and maintain.
Quaif differentials
A distinctive feature of this type of differentials is that the satellites in them are located parallel to the axis of rotation of the body (bowl), and in two rows. In addition, during the operation of these units, friction forces are formed, which, if necessary, automatically block, increase the cross-country ability and traction force of the car. Most often, Quaif differentials are used for tuning cars and SUVs.
viscous coupling
The operation of this type of differential is based on the same principle as the operation of a torque converter. Most often, viscous couplings are used in vehicles with all-wheel drive and are used to ensure that the front wheels are connected to the rear wheels according to the following principle: if one of them slips, then the torque is transmitted to the others, due to which the problem of slippage is solved. Structurally, a viscous coupling is a cylinder in which there is a package of perforated metal disks immersed in a viscous liquid and connected to the shafts (both driving and driven). Depending on the temperature, the viscosity of the liquid changes, on which the principle of operation of this unit is based.
The use of differentials, their advantages and disadvantages
In those vehicles that have only one leading axle, one differential is installed. Vehicles with two or more driven axles are equipped with differentials installed in each of them. In vehicles with cross-country ability, having two leading axles, three differentials are installed: one for each of the axles and one between them. In the same vehicles, which have more than two driving axles, so-called bogie differentials are used.
When choosing an SUV, many buyers must have come across the term “electronic differential lock” in the description of a particular model. But what it is, and how this very differential works, not all potential owners of cars of this class know. In our today's material, we will tell you in detail what a differential is for, what its varieties are and on which cars it is installed.
In the photo self-locking differentials
History of creation and purpose of the differential
On vehicles equipped with an engine internal combustion, the differential appeared a few years after their invention. The fact is that the first copies of machines powered by an engine had very poor handling. Both wheels on the same axle rotated at the same angular speed during the turn, which led to the slip of the wheel moving along the outer, larger than the inner diameter. The solution to the problem was found simply: the designers of the first cars with internal combustion engines borrowed a differential from steam carts - a mechanism invented in 1828 by French engineer Oliver Pecke-Rom. It was a device consisting of shafts and gears through which the torque from the engine is transmitted to the drive wheels. But after installing a differential on the car, another problem was discovered - wheel slip, which had lost traction.
This usually manifested itself when the car was moving on a road covered with patches of ice. Then the wheel, which fell on the ice, began to rotate from more speed than the one that was on the ground or concrete, which ultimately led to the skidding of the car. Then the designers thought about improving the differential so that when similar conditions both wheels rotated at the same speed and the car did not skid. The first to experiment with the creation of a limited slip differential was Ferdinand Porsche.
It took him three years to develop, test and market the so-called cam differential - the first limited-slip mechanism that was installed on the first models. Volkswagen brands. Subsequently, engineers developed different kinds differentials, which will be discussed below.
In a car, the differential performs three functions: 1) transmits from the engine to the drive wheels, 2) sets the wheels to different angular velocities, 3) is used in combination with the final drive.
Differential device
Improved automotive designers differential is arranged in the form planetary gear, where the torque from the engine is transmitted through the cardan shaft and conical gear train to the differential housing. That, in turn, directs the torque to two gears, and they already distribute the moment between the axle shafts. The clutch between the satellite gears and the axle shafts has two degrees of freedom, which allows them to rotate at different angular speeds.
Thus, the differential provides a different speed of rotation of the wheels located on the same axle, which also prevents slipping when turning. After it was invented, the car had two, and later three (with center) differentials, which distributed torque between the drive axles.
It is already clear that not a single car can do without a differential. In front- and rear-wheel drive vehicles, it is located on the drive axle. If the car has a dual drive axle, then two differentials are used in the transmission design - one for each axle. IN all-wheel drive vehicles there are two differentials (for models with all-wheel drive, one for each axle) or three (for models with permanent all-wheel drive, one for each axle, plus a center differential that distributes torque between the axles). In addition to the number of mechanisms installed on cars with different types drives, differentials are distinguished by the type of blocking.
Varieties of differentials
According to the type of blocking, differentials are divided into two - manual and electronic blocking. Manual, as the name implies, is done manually by the driver using a button or toggle switch. In this case, the satellite gears of the mechanism are blocked, the drive wheels move at the same speed. Typically, a manual differential lock is provided on SUVs.
Electronic or automatic differential lock is carried out using electronic block management, which, analyzing the state pavement(using information from sensors and traction control system), blocks the satellite gears by itself.
Rear differential With electronic control range rover Sport
According to the degree of blocking, this device is divided into a differential with full blocking and a differential with partial blocking of satellite gears.
Full differential lock implies a 100% stop of the rotation of the satellite gears, at which the mechanism itself begins to perform the function of a conventional clutch, transmitting an equivalent torque to both axle shafts. As a result, both wheels rotate at the same angular speed. If one of the wheels loses traction, all the torque is transferred to the wheel with better grip, which will allow you to overcome impassability. Such a differential device is used on SUVs, and others.
Partial differential lock involves an incomplete stop of the rotation of the satellite gears, that is, with slipping. This effect is achieved due to the so-called self-locking differentials. Depending on how this mechanism works, they are divided into two types: Speed sensitive (function with a difference in the angular speeds of rotation of the semi-axes) and Torque sensitive (function with a decrease in torque on one of the semi-axes). Such a differential device is used on Mitsubishi SUVs Pajero, Audi s, BMW s X-Drive system and so on.
Differentials belonging to the Speed sensitive group have a different design. There is a mechanism in which the viscous coupling plays the role of a differential. It is a reservoir located between the axle shaft and the rotor cardan shaft, filled with a special viscous liquid, in which, in turn, the disks are immersed, articulated with the axle shaft and the rotor. When the angular speed of rotation of the wheels differs (one wheel rotates faster than the other), the disks in the tank also begin to rotate with different speeds, but the viscous liquid gradually equalizes their speed, and, accordingly, the torque. As soon as the angular speeds of both wheels are equal, the viscous coupling is turned off. According to its characteristics, a viscous coupling is less reliable than a friction differential, therefore it is installed on vehicles designed to overcome off-road conditions of medium or sports modifications cars.
Another differential mechanism belonging to the Speed sensitive group is the gerotor differential. Here, the role of blocking, in contrast to the viscous coupling, is played by oil pump and friction plates, which are mounted between the differential housing and the satellite gear of the axle shafts. But the principle of operation is in many ways similar to that of a viscous coupling: when a difference occurs in the angular speeds of the drive wheels, the pump pumps oil onto the friction plates, which, under pressure, block the differential housing and the axle gear until the wheel speeds are equal. As soon as this happens, the pump stops working and the blockage is disabled.
Differentials belonging to the Torque sensitive group also have a different design. For example, there is a mechanism that uses a friction differential. Its feature is the difference in the angular speeds of rotation of the wheels when the car is moving in a straight line and in a turn. When driving on a straight road, the angular velocity of both wheels is the same, and when cornering, its value is different for each wheel. This is achieved by installing a friction clutch between the differential housing and the satellite gear, which helps to improve the transmission of torque to the wheel that has lost traction.
Another type of differentials is with hypoid (worm or screw) and helical gearing. They are conditionally divided into three groups.
The first is with hypoid gearing, in which each axle shaft has its own satellite gears. They are combined with each other by means of spur gearing, and the gear axis is perpendicular to the semi-axis. If there is a difference in the angular speeds of the driving wheels, the gears of the semi-axes are wedged, friction is formed between the differential housing and the gears. There is a partial blocking of the differential and the torque is transmitted to the axle, the angular velocity of which is less. As soon as the angular speeds of the wheels align, the lock is deactivated.
The second is with helical gearing, in which each axle shaft also has its own satellite gears (they are helical), but their axes are parallel to the axle shafts. And these units are combined with each other with the help of helical gearing. The satellites in this mechanism are installed in special niches on the differential case. When the angular speed of rotation of the wheels differs, the gears wedging occurs, and they, mating with the gears in the niches of the differential housing, partially block it. In this case, the torque is directed to the axle shaft, the rotation speed of which is lower.
The third - with helical gears of the axle shafts and helical gears of satellites, which are parallel to each other. This type is used in the design of the center differential. Thanks to the planetary design of the differential, it is possible, by means of partial blocking, to shift the torque to that axle, the angular speed of rotation of the wheels of which is lower. The range of such a shift is very wide - from 65/35 to 35/65. When establishing an equivalent angular speed of rotation of the wheels of the front and rear axle the differential is unlocked.
These groups of differentials have been widely used in the automotive industry: they are installed both on “civilian” models and on sports ones.
What is a differential
A differential is a device that distributes the flow of power from the engine to other transmission elements. In a car with a drive on one axle, only one differential is used, an interwheel, in an all-wheel drive there are three of them - two interwheels and an interaxle.
Consider, for example, a classical differential (unlike blockable ones, it is called “open” or “free”). It is installed in the main gear housing and receives torque from its driven gear. The differential box contains satellite bevel gears. They engage with gears mounted on the axle shafts, and those, in turn, rotate the drive wheels. When driving on a flat and straight road, the angular speeds of the wheels are the same, and the satellites do not rotate around their axis. When cornering or driving over bumps, when the starboard and portside wheels pass different way, satellites begin to rotate and redistribute torque.
The main gear of the rear axle VAZ-2101: 1 - flange of the cardan shaft; 2 - stuffing box; 3 - oil flinger ring; 4 - front bearing drive gear; 5 - rear bearing drive gear; 6 - adjusting ring; 7 – a basic ring of a gear wheel of a semiaxis; 8 – half shaft gear; 9 - satellite; 10 - pin satellites; 11 - driven gear of the main gear; 12 - differential box; 13 – a bolt of fastening of a stopper of an adjusting nut; 14 - adjusting nut stopper; 15 - differential box bearing; 16 - adjusting nut of the driven gear; 17 – a bolt of fastening of a conducted gear wheel to a flange of a differential box; 18 - main gear drive; 19 - main gear housing; 20 - spacer sleeve; 21 - washer; 22 – rear axle drive gear nut.
There is a simple formula that reflects the relationship between the speeds of the differential box and side gears. If through a1 And a2 designate the rotational speeds of the side gears, and through A- the frequency of rotation of the differential box, then: a \u003d (a1 + a2) / 2. The formula shows that if one of the wheels of the car is stationary, then the other wheel rotates with double frequency. If one of the two drive wheels hits a slippery road surface ( wet asphalt, oil stains, ice), the resistance to its rotation drops sharply, the adhesion to the road also decreases, which means that the wheel is not able to have the necessary traction force. Such a wheel will begin to rotate faster and slip. The other drive wheel, which has sufficient traction, will be supplied with the same torque as the slipping one. Having the ability to generate a large traction force, the second wheel will not be able to do this because the differential will only give it half the torque from the final drive.
If the vehicle's resistance to motion exceeds the traction force of the non-spinning wheel, the vehicle will not be able to move. The speed of the spinning wheel will increase sharply, and the second wheel will stop. There will be a car slip. An attempt by the driver to increase the traction force on the wheels by increasing the fuel supply will only increase the speed of one of the wheels. In such a situation, it appears significant disadvantage a conventional differential that reduces the vehicle's cross-country ability both on slippery roads, and on soils that provide great resistance to rolling wheels (sand, snow, slush).
Forced blocking
On vehicles designed for off-road driving, it is necessary to install differentials of special designs. Locks Often apply differentials with forced locking. In them, the driver, using a special drive (most often pneumatic), stops the rotation of the satellites for a while, and the wheels of the car begin to rotate at the same speed. It should be noted that a car with a locked differential on a winding road consumes more fuel and it has intensive tire wear, which means that the cost of operating the car increases.
As soon as the mutual rotation of the wheels on a common axle with a locked differential is greater than the elastic deformation of the tires allows, wheel slip will occur, continuing until any wheel breaks off the road on an uneven surface. This suggests that the driver should not forget to turn off the differential lock after overcoming a difficult section. In a number of designs, it is provided automatic unlock or limiting the ability to enable speed lock.
Self-locking differentials
To simplify the control process, so-called self-locking differentials are used. Currently, four types of locks are mainly used: disk (friction, increased friction, LSD), viscous (viscous couplings) and screw (worm). The most modern developments use electronic systems wheel slip control based on the use of rotation sensors and the use regular brakes(As a rule, these systems are combined with anti-lock and anti-slip).
disk lock
There are two most characteristic designs of differentials with friction clutches. In the first, one is used, in the second - two couplings. In the first case, the friction disc clutch 1 is inserted between one of the axle shafts and the differential box. The bronze discs are installed in the splines of the sleeve 2, connected to the differential box, the steel discs sit on the splines of the axle shaft 3. The discs are pressed against each other by springs 4. When both wheels experience the same resistance, the entire differential rotates as one and there is no friction in the clutch 1.
The second design is a limited slip differential with double friction clutches, which is widely used on american cars. In this design, the crosspiece is replaced by two separate axes 5 of satellites 6 intersecting at right angles. Axes 5 have the ability to move one relative to the other both in the axial and in the angular direction, for which their ends have bevels A and B, respectively, with which they rest on box 9 differential. In addition, intermediate cups 7 are introduced into the differential, as well as side gears put on the splines of the axle shafts.
With non-rotating satellites, the force to the axle shafts is transmitted as in a simple differential. During the rotation of the satellites, the latter will shift the end bevels of the axles 5 so that the force on the friction clutch 8, transmitted through the cup 7, will increase for the lagging axle shaft and decrease for the faster rotating axle. In this case, the magnitude of the braking torque will not be constant, as in a differential with one disc clutch, but will be proportional to the moment transmitted by the wheels.
For normal operation such a differential requires the use of a special gear oil for LSD or appropriate conventional oil additives. In addition, over time, it becomes necessary to adjust due to wear on the discs.
Viscous blockage
The principle of its operation is the same as that of the disk. The hydraulic clutch consists of a large number of discs with sticky working surfaces. Due to the properties of a special viscous silicone-based fluid to harden when heated, the discs transmit torque depending on the difference in the speeds of the input and output shafts. Heating occurs when one half shaft begins to rotate faster than the other.
A characteristic feature of the design is that in the case of long-term slipping of the wheels, the blocking clutch with a viscous fluid works softly at first, and then there is a significant increase in the blocking efficiency. In the hardened silicone, the discs are firmly engaged and the axle shafts are blocked. Viscous couplings do not require maintenance and are considered very reliable, however, for their long-term operation, it is necessary to maintain the complete tightness of the device.
screw lock
The principle of its operation is as follows: in normal mode, the screws (or worms, as they are called because of their characteristic shape) freely roll around the central gear. In the event of a change in torque, the screws slip into extreme position and are fixed in eccentric grooves. When the moment equalizes, the screws return to their original position. The actuation moment of the screw locks is determined by the profile of the screws. Such differentials are little subject to wear (the service life is comparable to the life of a box or a classic differential), and ordinary transmission oil is used.
Cam lock
This lock is triggered when there is a difference in the speed of rotation of the wheels. Consider an example of a differential implementation from Tractech. Rotary cams are installed in the differential housing between pairs of crown gears. Under normal conditions, they do not participate in the work, but as soon as one of their wheels begins to slip (i.e., rotate significantly faster than the other), the cams turn and the pairs of gears engage, thereby ensuring complete blocking. The lock is released when the slipping wheel stops slipping. This type of differential is also quite durable and does not require special oils.
Control Features
Driving a car equipped with a self-locking cross-axle differential has some features. In particular, a car in a turn on a slippery surface may oversteer, if the acceleration is too intense on mixed surfaces, it may drift away from the intended trajectory, etc. This is especially true for developments offered as additional equipment third firms. However, the competent use of the properties of such differentials allows you to confidently move in difficult road conditions, and significantly increases off-road patency.
M axle differential and its locks
In the absence of an inter-axle power split (an inter-axle differential or a disengaging mechanism), the front axle must be disengaged so that the rotation of the front and rear wheels at different angular speeds becomes possible. According to the traffic conditions, it is required that the wheels of both the front and rear axles, and the wheels of one bridge could rotate at different frequencies and pass different ways. This is especially true when cornering: the front wheels travel a greater distance when turning than the rear wheels.
Various factors influence the change in the path of the wheels: tire slip, their slip angles, air pressure, wheel load, suspension kinematics. At the same time, it is obvious that the ratio between the paths traversed by the wheels of the front and rear axles also changes during movement. This circumstance excludes the possibility of using different gear ratios in the main gears of bridges to compensate for the difference in passable paths.
The wheels of different axles of the car, kinematically rigidly connected to one another, have the same angular speeds during rotation. On a hard road surface, when a vehicle is moving with all-wheel drive (in the absence of an interaxle differential), conditions may arise under which the wheels of different axles will try to move at different linear speeds, and a rigid mechanical connection between them will become an obstacle to achieving this. In rectilinear motion, the described phenomenon can be caused, for example, by the difference in the rolling radii of interconnected wheels. The rolling of the wheels in this case should be accompanied by a relative movement of the points of the tire contact area along the road surface (with slipping or slipping).
The same is possible with the same rolling radii, but when driving on a road with an uneven surface or when cornering. The sliding or slipping of tires that occurs under these conditions is accompanied by increased wear, wear of transmission mechanisms and unproductive expenditure of engine energy for driving a car. In order for the wheels to roll without harmful accompanying phenomena in the transmission, in addition to interwheel differentials, interaxle differentials are installed.
However, in off-road traffic, the car may lose mobility at the moment when the wheels of one of the axles lose traction and begin to slip. In such a situation, a conventional type differential will not be able to transfer the amount of torque required for movement to the rear wheels resting on hard ground. To avoid this, cross-axle differentials with forced locking are installed on SUVs. An example of such constructive solution can serve as "Niva" VA3-2121, equipped with transfer case with forced locking center differential.
The blocking is used by the driver of the car to overcome the difficult section of the road. When returning to the highway, the center differential must be unlocked. In modern designs, in addition to mechanical, other drives are also used (pneumatic, hydraulic, electric), while the switching process itself is reduced to simply pressing a button on the panel.
The next step was the emergence of self-locking center differentials. The principles of their work are similar to interwheel ones, but the conditions and tasks are somewhat different. So, when turning the car, the shaft that transmits the moment to steered axle, which is determined by the kinematics of the turn of the machine with a 4x4 wheel arrangement. Based on this, when running drive shaft of a controlled axle, it is desirable to have a low blocking coefficient, and when running (slipping) of an uncontrolled axle, it is desirable to have a slightly larger one. Such a differential is called self-locking with asymmetric blocking properties.
Currently on passenger SUVs center differentials with automatic locking by means of a viscous fluid hydraulic clutch are widely used. They provide optimal strength traction in all driving conditions, and therefore there is no need for forced blocking. They also have other advantages. This assembly protects the transmission from overload, which may occur, for example, in the event of a sudden impact of a wheel.
Differential, automatically locking hydraulic clutch with a viscous fluid, sensitive to the condition of the road surface and provides a more uniform speed of the car, and also reduces the likelihood of getting stuck. When braking, this type of center differential prevents the wheel of one axle from locking relative to the wheel of another, leading to loss of stability. In addition, the redistribution of excess braking force from one pair of wheels to another significantly reduces braking distances and retains full control over the machine.
Let's consider how the GKN automatic locking center differential with a hydraulic clutch works. The change in the friction moment in it is calculated so that when maneuvering on a surface with good grip properties (asphalt, concrete, etc.), there is a small friction moment between the output shafts. With an increase in the difference in the frequencies of their rotation, the friction between the coupling links increases significantly. Locking with a viscous fluid clutch takes place precisely in accordance with the distribution of torque in the center differential.
Tests have confirmed that the distribution of moments between the front and rear wheels provides almost neutral steering of the car. Ease of driving and safety four-wheel drive vehicles with such a drive are superior even to front-wheel drive cars. However, with all the advantages of this kind of blocking, it should be noted that the actual inclusion of the blocking after the start of wheel slip, which is characteristic of a viscous coupling, significantly reduces the chances of successfully overcoming serious off-road obstacles in the form of soft soil, mud or snow, since a slipping wheel can quickly burrow. As a result, the capabilities of a car, even with a locked center differential, may not be enough for independent exit.
Pluggable front axle
A lot of SUV manufacturers use a plug-in front axle scheme (the so-called part time 4WD). In this case, the center differential, as a rule, is absent, and in the mode all-wheel drive a rigid kinematic connection is established between the bridges. Manufacturers recommend connecting the front axle only in difficult road conditions, when the wheels are prone to slip.
Prolonged driving in this mode on hard surface roads causes increased tire and transmission wear (in particular, the chain is overloaded in transfer cases with a chain drive), increased fuel consumption, and also worsens controllability on high speeds. To avoid these negative consequences, many designs provide not only shutdown front axle, but also detaching the front wheels from the axle shafts.
For this, wheel hubs (freewheels) are used, which can be automatic and manual, disconnection of the axle shafts using an electric or pneumatic drive, etc.
About the author: admin
