Vehicle differential. Varieties and features of functioning
Content
free (without blocking);
with manual override;
with auto-lock.
A differential is a mechanism that transmits torque from one source to two consumers. Its key feature is the ability to redistribute power and provide different angular speeds of rotation of consumers. With regard to a road vehicle, this means that the wheels can receive different power and rotate at different speeds through the differential.
The differential is an important element of an automobile transmission. Let's try to figure out why.
Why you can't do without a differential
Strictly speaking, you can do without a differential. But only as long as the car is moving along a flawless track, without turning anywhere, and its tires are the same and evenly inflated. In other words, as long as all the wheels travel the same distance and rotate at the same speed.
But when the car enters a turn, the wheels have to cover a different distance. Obviously, the outer curve is longer than the inner curve, so the wheels on it have to turn faster than the wheels on the inner curve. When the axle is not leading, and the wheels do not depend on one another, then there is no problem.
Another thing is the leading bridge. For normal control, the rotation is transmitted to both wheels. With their rigid connection, they would have the same angular velocity and would tend to cover the same distance in a turn. Turning would be difficult and would result in slippage, increased tire wear and excessive stress on the . Part of the engine power would go to slip, which means that fuel would be wasted. Something similar, although not as obvious, occurs in other situations - when driving on rough roads, uneven wheel loads, uneven tire pressures, varying degrees of tire wear.
This is where it comes to the rescue. It transmits rotation to both axle shafts, but the ratio of the angular speeds of rotation of the wheels can be arbitrary and change quickly depending on the specific situation without driver intervention.
Types of differentials
Differentials are symmetrical and asymmetrical. Symmetrical devices transmit the same torque to both driven shafts, when using asymmetric devices, the transmitted torques are different.
Functionally, differentials can be used as inter-wheel and inter-axle differentials. Interwheel transmits torque to the wheels of one axle. In a front-wheel drive car, it is located in the gearbox, in a rear-wheel drive car, in the rear axle housing.
In an all-wheel drive car, the mechanisms are located in the crankcases of both axles. If the all-wheel drive is permanent, a center differential is also mounted in the transfer case. It transmits rotation from the gearbox to both drive axles.
The axle differential is always symmetrical, but the axle differential is usually asymmetrical, the typical percentage of torque between the front and rear axles is 40/60, although it may be different.
The possibility and method of blocking determines another classification of differentials:
Blocking can be either complete or partial.
How the differential works and why to block it
In fact, the differential is a planetary type mechanism. In the simplest symmetrical cross-axle differential, there are four bevel gears - two semi-axial (1) plus two satellites (4). The circuit works with one satellite, but the second is added to make the device more powerful. In trucks and SUVs, two pairs of satellites are installed.
The cup (body) (5) acts as a carrier for satellites. A large driven gear (2) is rigidly fixed in it. It receives torque from the gearbox through the final drive gear (3).
On a straight road, the wheels, and therefore their wheels, rotate at the same angular velocity. The satellites rotate around the wheel axles, but do not rotate around their own axes. Thus, they rotate the side gears, giving them the same angular speed.
In a corner, a wheel on the inner (smaller) arc has more rolling resistance and therefore slows it down. Since the corresponding side gear also starts to rotate more slowly, it causes the satellites to rotate. Their rotation around their own axis leads to an increase in gear revolutions on the axle shaft of the outer wheel.
A similar situation can arise in cases where the tires have insufficient grip on the road. For example, the wheel hits the ice and starts to slip. An ordinary free differential will transfer rotation to where there is less resistance. As a result, the slipping wheel will spin even faster, while the opposite wheel will practically stop. As a result, the car will not be able to continue moving. Moreover, the picture will not fundamentally change in the case of all-wheel drive, since the center differential will also transfer all the power to where it encounters less resistance, that is, to the axle with a slipper wheel. As a result, even a four-wheel drive car can get stuck if only one wheel slips.
This phenomenon seriously impairs the patency of any car and is completely unacceptable for off-road vehicles. You can fix the situation by blocking the differential.
Types of locks
Full forced blocking
You can achieve complete manual blocking by jamming the satellites so as to deprive them of the ability to spin around their own axis. Another way is to enter the differential cup into rigid engagement with the axle shaft. Both wheels will spin at the same angular speed.
To enable this mode, you just need to press a button on the dashboard. The drive unit can be mechanical, hydraulic, pneumatic or electric. This scheme is suitable for both interwheel and center differentials. You can turn it on when the car is stationary, and you should use it only at low speed when driving over rough terrain. Having left on a normal road, the lock must be turned off, otherwise the handling will noticeably worsen. Abuse of this mode can cause damage to the axle shaft or related parts.
Of greater interest are self-locking differentials. They do not require driver intervention and work automatically when the need arises. Since the blocking in such devices is incomplete, the probability of damage to the axle shafts is low.
Disc (friction) lock
This is the simplest version of a self-locking differential. The mechanism is supplemented with a set of friction discs. They fit tightly to each other and through one are rigidly fixed on one of the axle shafts and in the cup.
The whole structure rotates as a whole until the speed of rotation of the wheels becomes different. Then friction appears between the disks, which limits the growth of the speed difference.
Viscous coupling
A viscous coupling (viscous coupling) has a similar principle of operation. Only here the discs with perforations applied to them are placed in a sealed box, all the free space of which is filled with silicone fluid. Its distinguishing feature is the change in viscosity during mixing. As the discs spin at different speeds, the liquid is agitated, and the more intense the agitation, the more viscous the liquid becomes, reaching almost a solid state. When the rotational speed levels off, the viscosity of the fluid drops rapidly and the differential unlocks.
The viscous coupling has rather large dimensions, therefore it is used more often as an addition to the center differential, and sometimes instead of it, in this case acting as a pseudo-differential.
The viscous coupling has a number of disadvantages that significantly limit its use. These are inertia, significant heating and poor compatibility with ABS.
Thorsen
The name comes from Torque Sensing, that is, “perceiving torque”. It is considered one of the most effective self-locking differentials. The mechanism uses a worm gear. The design also has friction elements that additionally transmit torque when slippage occurs.
There are three types of this mechanism. Under normal road traction, the T-1 and T-2 varieties function as symmetrical type differentials.
When one of the wheels loses traction, the T-1 is able to redistribute torque at a ratio of 2,5 to 1 to 6 to 1 and even more. That is, the wheel with the best grip will receive more torque than the slipping wheel, in the specified proportion. In the T-2 variety, this figure is lower - from 1,2 to 1 to 3 to 1, but there is less backlash, vibration and noise.
Torsen T-3 was originally developed as an asymmetric differential with a blocking rate of 20 ... 30%.
QUAIFE
The Quife differential is named after the English engineer who developed this device. By design, it belongs to the worm type, like Thorsen. It differs from it in the number of satellites and their placement. Quaife is quite popular among car tuning enthusiasts.
