Limited differential: how does it work?

2024 Author: Erin Ralphs | [email protected]. Last modified: 2024-02-19 11:41

The term “differential lock”, or “limited 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 such an “option” mainly with SUVs, 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 rightly be considered one of the main elements of the car transmission design. 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 strength of the flow of distributed energy, if necessary, can be different, which means that the speed of rotation of the wheels is different.

Bcar transmission diff can be installed: in the rear axle housing, gearbox and 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.

Differential Assignment

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, in one and the same time can cover 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, 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 value of rolling resistance: the lower it is, the greater the wheel speed will be, and vice versa.

Differential mechanism

Today, 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 the simplest type - an open differential, which consists of the following elements:

1. Gears mounted on axle shafts.
2. Driven (crown) gear, made in the form of a truncated cone.
3. 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.
4. The satellites that form the planetary gear, which plays a key role in providing the necessary difference in the speed of rotation of the wheels.
5. Cases.

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, spinningfaster.

There is no difference in how the center differential and the wheel differential work: the principle of operation is similar, only in the first case the distributed torque is directed to the vehicle's axles, and in the second - to its wheels located on the same axle.

The need for center diff becomes especially noticeable when driving a car over rough terrain, when its weight presses on the axle that is lower than the other, for example, on an uphill or downhill.

Differential problem

Despite the fact that 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 stationary.

It was precisely to solve such problems that the locking (disabling) of the differential 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 axle shafts or by stopping the satellites from rotating.

Blockcan 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 limited force is transmitted to the wheels - this is how a self-locking differential works, which does not require human intervention.

How a self-locking differential works

The self-locking differential is essentially a compromise between a full block and a free diff, and allows you to reduce the slippage of the car's wheels in the event of a difference in traction coefficient between them. 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.

Self-locking eliminates full wheel lock, which protects the axle shafts from critical loads that can occur on forced-off differentials.

Axle locks are released automatically if the wheel speeds equalize during straight-line driving.

The most common types of self-blocks

Disk self-block is a set of friction (rubbing) discs 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 rotate together, as soon as there is a difference in rotation speeds (the wheel has hit a slippery area), friction occurs between the disks, reducing it. That is, the wheelremaining on hard ground will continue to rotate, rather than stop, as in the case of a free differential.

Viscous coupling, or otherwise viscous coupling, as well as the previous diff, contains two packs of disks, only this time perforated, installed between themselves with a small gap. One part of the discs has a clutch with the body, the other with the drive shaft.

The discs 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 wedged 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 wheel speeds are almost the same, and are hard locked (jammed) as soon as one of them begins to slip.

There is no difference in how the locking of the center differential and the interwheel differential works - the principle of operation is the same, the differencesonly 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

The Niva occupies a special place in the line of domestic VAZs: unlike its "relatives" on the assembly line, this car is equipped with non-switchable all-wheel drive.

In the transmission of the VAZ SUV, three differentials are installed: interwheel - in each axle, 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, the traction 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 front-wheel drive is turned on in the car - this is not so: both the front and rear-wheel drives of the Niva are always engaged, and this handle controls the transfer case differential. That is, while it is set in the “forward” position, the diff works normally, and when"back" - off.

And now directly about the lock: when the differential is turned off, the transfer case shafts are closed by a clutch, thereby forcibly equalizing their rotation speeds, 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 the rear wheel is slipping, 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 the front wheel also slips along with the rear wheel, then the Niva will not get out on its own.

To prevent this from happening, motorists install self-blocks in bridges to help pull out a stuck car. To date, the most popular among Niva owners is the Nesterov differential.

Nesterov's self-block

The secret of its popularity lies in the way the Nesterov differential works.

The design of the differential allows not only to optimally adjust the angular speed of 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 in the traffic situation is almost instantaneous. In addition, the Nesterov differential significantly improves the car's handling even on slippery turns, increases directional stability, increases acceleration dynamics (especially in winter), and reduces fuel consumption. And the installation of the device does not require anychanges in the transmission design and is installed in the same way as the classic diff.

The differential has found application not only in automotive technology, it turned out to be very useful on motoblocks, greatly facilitating the life of its owners.

Differential for motoblock

Motoblock - the unit is quite heavy, 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 walk-behind tractor differential work? In fact, it only provides an easy turn of the car, stopping one of the wheels.

Its other function, not related to the redistribution of power, is to increase 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.