Differential - a mechanical device that transmits torque from one source to two separate consumer in such a way that the angular velocity of rotation of both the source and the consumer may be different from each other. The transfer time is possible due to the use of so-called planetary mechanism. In the automotive industry, the differential is one of the key transmission parts. Firstly, it serves to transfer the moment of the gearbox to the wheels axle. Why is it requires a differential? In any turn, the road wheel axle, moving the short (inner) radius, smaller than the path of another wheel on the same axis, which passes through the long (outer) radius. As a result, the angular velocity of rotation of inner wheel should be less than the angular velocity of rotation of the outer wheel. In the case of no drive axle, to comply with this condition is quite easy, since both wheels can not be connected with each other and to rotate independently. But if the bridge leading, it is necessary to transmit the torque to both wheels at the same time (if you pass the time only one wheel, then the possibility of driving on today's standards would be very bad). In the same tight connection wheel axle and transmission time on a single axis of both wheels, the car could not properly rotate as the wheel, having an equal angular velocity, would try to take the same path in the rotation. The differential makes it possible to solve this problem: it sends torque to the twin axis of both wheels (half), through its planetary gear with any ratio of angular velocity of rotation axes. As a result, the car can normally move and managed as in the direct path, and in turn. Scheme of the differential and planetary gear in the image on the right. Animated scheme of work can be seen on the site Howstuffworks. However, in view of device physics, the planetary gear is a very bad feature: it seeks to convey the resulting torque is there, much easier. For example, if both wheels of the bridge have the same grip and the force required for unwinding of each of the wheels of the same, the differential will distribute torque evenly between the wheels. But as soon as feasible, the difference in adhesion to the road (for example, one wheel fell on the ice, while the other remained on the asphalt) as a differential immediately begin to redeploy the time at the wheel, the pressure for promotion is the smallest (ie the fact that is on the ice). As a result, the wheel, located on the asphalt stops to get the torque and stops, while the wheel is located on the ice will take over the entire time and will rotate with an increased angular velocity, and the planetary gear reducer will play a role that enhances the speed of rotation of the wheel. Naturally, this phenomenon strongly affects the permeability and handling. After all, logically, to consider the situation when it is desirable to transmit the wheel, located on the asphalt, so the car could proceed.

In all-wheel drive vehicles are usually equipped with a differential two bridges, and often differential can be found also between the bridges (inter-axle differential). Thus, we get the transmission scheme, in which there are three differential: two bridges and an inter-axle. The latter is required for continuous motion with all-wheel drive and the transfer time to all four wheels. After all, turning the steering wheel axle (usually the front) have very different angular velocity, rather than wheel rear axle. Center differential is designed to transmit torque from the gearbox to the two bridges leading to the different ratio of angular velocities. Such a scheme with three differentials is one of the most common schemes for all wheel drive (Full time 4WD). However, this is the subject of another section. In this section we are interested in differential and its properties. Returning to the above described property to the problematic planetary mechanism, it is interesting to consider the situation when the four-wheel drive vehicle with differential one of the four wheels came on the same ice (or in the slippery pit). What will happen then? Differential axle, the wheel which is on the ice, give up all the resulting torque on the wheel. Center differential, in turn, is also seeking to transfer torque to where easier. Naturally, interaxable differential is easier to give time to the bridge with the scrolling wheel on the ice, rather than on the bridge, the wheels that have good grip and can move the car. As a result, all the torque from the engine and gearbox will go on spinning a single wheel located on the ice. The other three wheels will stop and will not receive any torque on the differentials. Outcome: out of four driving wheels have only one, which slips on the ice - four-wheel drive car is stuck. How to get the differentials transfer the torque to the wheels with a better road grip? For this we have developed different methods of partial and complete, manual and automatic locking differentials, which will be discussed below.
The main purpose of the differential lock is to transfer the necessary torque to both its customers (the half-axes or cardan joint). There are fundamentally different methods of solving this problem.

1. Total (100%-I) manual override.



In this type of locking differential is virtually ceases to perform its function and becomes a simple sleeve, rigidly connecting the half (or cardans) between itself and transmits the same torque with the same angular velocity. In order to completely block the classical differential is enough to block any possibility of rotation of the satellite, or rigidly interconnect cup differential with one of the axes. This lock is usually implemented using pneumatic, electric or hydraulic actuator operated by the driver of the car. Suitable for bridge, and for interaxle differentials. The picture shows a block diagram for the bridge company ARB differential, in which the satellites are blocked.

Include this kind of lock can only be fully stopped car. Use them should be extremely careful, because the efforts of the motor is enough to "disrupt" the locking mechanism or break the axis. Use these locks preferably only at low speed for driving on rough terrain, as when they are used in bridges (especially in the steering), the car is very much lost in handling. Typically, hard locks and bridge interaxle differentials fitted full frame SUVs such as the Toyota Land Cruiser, 4Runner (Hilux Surf), Mercedes G-Class i.t.p.
Limited Slip Differentials - differentials with limited "slippage" (one half on the other).

2. Automatic blocking of using viscous coupling as a "Slip Limiter".



In this case, is used to lock one of the axes with a cup of differential. Viscous coupling mounted coaxially to the half so that one of her drive is rigidly fixed to the cup of the differential, and the other - to the half. In the normal movement of the angular velocity of rotation of the cups and half-identical or slightly different (in turn). Accordingly, the working plane viscous coupling have the same small difference in angular velocities and clutch remains open. As soon as one of the axes begin to receive significantly more time and higher angular velocity relative to the other, the viscous coupling there is friction and she begins to be blocked. The greater the difference in speed, the greater the friction inside the viscous coupling and the degree of blockage. As the degree of viscous coupling locking and alignment of angular velocity and the half cup, friction inside the viscous coupling begins to fall, leading to a smooth opening and unlock the viscous coupling. This scheme is used to interaxle differentials, since its design is too massive to be installed on the pavement gear. (How in the picture) This locking mechanism is well suited for operation in conditions of poor road surface, but in this roadless his ability is not outstanding: viscous coupling can not cope with constant change of states of adhesion bridges with soil, lag when you turn on, overheated and out of system. This type of locking center differential can be found on the "parquet" off-roader: Toyota Rav4, Lexus RX300 i.t.p.

3. Cam and gear automatic lock.



Principle of operation of these locks is quite simple. Instead of the classical planetary gear mechanism using the cam or gear pair, which, when a small difference in angular velocity axes are able to mutually rotate (jump), while slips to bind and block the half-line with each other. It is easy to imagine what happens with the car when it triggers a blockage in the corner. Some specimens simply disable one of the axes at the time that the small difference in speed. Therefore, staff are equipped with such locking differentials only military and special equipment (APCs so on) The pictures shows (left to right): cam locks national production (BTR 1960), Detroit Locker and Detroit EZ Locker. (Company Tractech

4. Self-locking differentials.

According to the principle of work, self-locking differentials can be divided into two types: speed sensitive, that is triggered by the difference in angular velocity axes, and torque sensitive - triggered by the difference be sent on a half-torque.

Speed sensitive differentials

The device of such differentials is quite simple and essentially no different from conventional open differential unit. Between the axes and add a cup of differential sets of blocks of friction plates (which are marked on the image on the right red dots). That is why such differentials are often called "friction based LSD". When the differential is trying to redistribute torque to one of the axes and is beginning to emerge is the difference in angular velocity axes and cups, plates under the action of friction holding back the emergence of this difference. Of course, when the torque value exceeds the force of friction plates, all the rotation is transmitted to more easily rotating axis. These locks are working in a relatively small range of relationship points. Quite often, friction blocks podpruzhinivayut. Such differentials are nominally set in the rear axle of many SUVs - Toyota 4Runner (Hilux Surf), Nissan Terrano, Kia Sportage i.t.p. American company ASHA Corp. went further by providing friction clutch pack LSD differential locking device, consisting of a pump with a piston (gerotor differential). If you have any difference in angular velocities of the half cups and pump pumps oil (liquid) on the piston and compresses the clutch pack, thereby blocking differential. This design is called Gerodisk (Hydra-Lock) and nominally set at off-Chrysler (the picture left). Almost all friction based differentials should use the special oil, which contains additives that ensure the normal operation of the friction blocks

Torque sensitive differentials

This is one of the most interesting, efficient, technologically and practically applicable forms of locking differentials. The principle of operation is based on the property hypoid pair "wedging." In this regard, the main (or all) links in such hypoid differentials (worm, or the common people - screw). Design format is not so much - there are three main types.
производит компания Zexel Torsen. The first type of manufacturing company Zexel Torsen. (T-1) hypoid gear pairs are the leading axes and satellites. In addition, each floor has its own satellites, which are pairwise connected to the satellite the other half conventional spur gears. It should be noted that the axis of the satellite is perpendicular to the axis. In the normal movement and equality transmitted on a half points, hypoid pair "Satellite / drive gear" or stopped, or pulled off, providing the difference of angular velocity axes in turn. Once the differential is trying to give time to one of the axes, the hypoid pair of half-starts and wedging block with a cup of the differential, which leads to partial differential lock. This construction works in the very large range of relations of torque - 2.5 / 1 to 5.0 / 1, then there is the most powerful in the series. Detection range is governed by the angle of inclination of teeth worm.

является англичанин Rod Quaife. The author of the second type is an Englishman Rod Quaife. In this case, the axis parallel to the satellite axis. Satellites are located in the original pockets cup differential. This pair of satellites are not spur gears, and form an another hypoid pair that rasklinivayas, also participates in the process blocking (in the picture left). This device has a differential and True Trac companies Tractech. Even here in Russia appeared similar to the production of differentials for domestic cars UAZ etc. But the company Zexel Torsen differential in his T-2 offered a slightly different layout essentially the same device (the image on the right). Because of its unusual design, paired satellites are interconnected with the outside of the sun gear. Compared with the first type, these differentials have a smaller range of locks, but they are more sensitive to the difference in the transmitted torque and triggered earlier (from 1.4 / 1). The company recently released roadway Tractech torque sensitive differential Electrac, equipped with electric forced lock.

производится компанией Zexel Torsen (Т-3) и используется в основном для межосевых дифференциалов. The third type is manufactured by Zexel Torsen (T-3) and is used mainly for interaxle differentials. Planetary structure design allows slip nominal distribution date in favor of one of the axes. For example, used to 4Rannere 4-th generation of the differential T-3 has a nominal distribution date 40/60 in favor of the rear axle. Accordingly, the displaced and the entire range of partial block: from (front / rear) 53/47 to 29/71. In general, the shift of the nominal allocation of time between the axes may be in the range from 65/35 to 35/65. Firing the partial blocking occurs when 20-30% of the difference in transmitted on the axis points. Just such a structure of differential makes it compact, which in turn, simplifies design and improves the layout of the transfer case.
The above torque sensitive differentials are very popular in motorsports. Moreover, many manufacturers install such differentials on their models, normally, both as interaxle and cross-axle differentials. For example, Toyota sets such as the differentials in passenger cars (Supra, Celica, Rav4, Lexus IS300, RX300 etc.), and on SUVs (4Runner (Hilux Surf), Land-Cruiser, Mega-Cruiser, Lexus GX470) and bus (Coaster Mini-Bus). These differentials did not require the use of special additives to the oil (as opposed to friction-based differentials), but it is better to use quality oil for loaded hypoid gears.

5. Manage the work of the differentials with electronic braking force (Traction Control so on)



In the modern automobile is used more and more electronic systems controlling the movement of the car. Already rare cars not equipped with the system ABS (not giving the wheels lock up during braking). Moreover, since the end of the 80-ies leading steel producers complete their flagship model, traction control system and traction wheel - Traction Control. For example, Toyota has established a system of Traction Control in the Lexus LS400 in 1989 (90) year. The principle of such a system is simple: universal (as well served by ABS) sensors of rotation established by the controlled wheels, fix the beginning of a skidding wheel axis relative to the other and the system automatically slows down the stalled wheel, thus increasing the burden on him and forcing the differential pay point on the wheel with good adhesion. При сильной пробуксовке, система так же может ограничивать подачу топлива в цилиндры. Работа такой системы очень эффективна, особенно на заднеприводных автомобилях. Как правило, при желании такую систему можно принудительно деактивировать кнопкой на приборной панели. Со временем, электронная система контроля тормозных усилий совершенствовалась и к ней добавлялись всё новые функции, работающие наряду с ABS и TRAC. (например управление разностью разблокировки рулевых колёс для более успешного прохождения поворотов). У всех производителей эти функции назывались по разному, однако смысл при этом оставался одинаковым. И вот, данные системы стали устанавливаться на полноприводные автомобили и внедорожники, причем в некоторых случаях они являются единственным средством контроля тяги и перераспределения крутящего момента между осями и колёсами (Mercedes ML, BMW X5). В случае, если внедорожник оснащен более серьёзными средствами распределения крутящего момента (жесткими блокировками и/или самоблокирующимися дифференциалами), то электронная система контроля тормозных усилий очень удачно дополняет эти средства. Хороший пример тому - великолепная управляемость и проходимость последнего поколения Тойотовских внедорожников 4Runner (Hilux Surf), Prado, Lexus GX470. Являясь представителями одной платформы, они обладают межосевым дифференциалом Torsen T-3 с возможностью жесткой блокировки, а так же электронной системой контроля тормозных усилий и тяги со множеством функций, помогающих водителю управлять автомобилем