The work of many products is based on the use, sometimes in unexpected ways, of the most varied properties of the substances we are used to. An example of this is a viscous coupling - a special device designed for selective transmission, depending on external conditions, of torque. For such products, the principle of operation is based on changing the viscosity of the liquid poured into it. This is not to say that they are used extremely widely, for example, as manual transmissions, but it would be wrong to bypass their use.

Viscous coupling - viscous clutch, part of the vehicle transmission, transmission and torque equalization mechanism. In contrast to the fluid coupling and the torque converter, the viscous coupling uses a different principle of operation. In this device, the torque is transmitted not through the dynamic properties of the fluid flow, but using the viscous properties of the fluid filling the internal space of the viscous coupling. It is used as an automatic differential lock mechanism.

History of invention

Viscous coupling was invented in 1917 in the USA by Melvin North, but did not find application at that time. In 1964, the viscous coupling was first installed as an automatic center differential lock on the Interceptor FF of the English company Jensen. Since the mid-60s, viscous couplings have been widely used in self-locking cross-axle differentials on passenger cars with permanent all-wheel drive.

The principle of operation of the viscous coupling

What is a viscous cooling fan? This is a special mechanism that performs the function of selective transmission, which depends on external conditions, torque. The viscous clutch looks like a housing that is completely sealed, with two rows of discs located inside. One of them communicates with the driven shaft, the second with the driving one. These discs are interspersed with each other. Each of them has both holes and protrusions, and the distance between the planes of the discs is minimal. The viscous coupling body is filled inside with a viscous liquid, which is often made on a silicone base. The viscous coupling fluid has specific features that allow it to be used effectively.

These are: An increase in viscosity with increasing mixing intensity. High expansion coefficient during heating. When the car is moving at a constant speed, the viscous discs rotate evenly and the oil does not mix between them. When the speed of the driven and driven shafts is different, the speed of rotation of the discs is also different. As a result, the viscosity of the fluid increases and it affects the transmission of torque to the driven shaft. When the difference in speed of rotation of the discs is very large, the viscous oil becomes so viscous that it becomes blocked. The liquid passes into a state close to solid.

Types of viscous couplings.

Currently, there are 2 types of devices used in cars:

• with a constant volume of dilatant fluid;
• in which the working volume of the substance depends on external conditions.

The first are used in the so-called. self-locking differentials in car transmissions, including for automatic connection of all-wheel drive.

The field of application of the second is engine cooling systems, here they are used to control the fan.

Work of the part in the transmission.

When driving in normal mode on a normal surface, the angular speeds of the wheels of both axles are the same. The disks of the viscous clutch rotate at equal speeds, the transmission of torque from the engine to the driven axle is minimal. In fact, the car becomes fully front or rear wheel drive.

When driving on ice, snow, mud, irregularities, the synchronous rotation of the wheels is disturbed, the viscosity of the liquid in the device increases, increasing the transmission of torque to the second axle. In theory, such power transfer in some emergency modes can be up to 100%, although, as a rule, even partial connection of the second axle is sufficient to safely overcome the problem area.

Important! It should be understood that the viscous clutch is by no means a full-fledged center differential that redistributes the engine torque to both drive axles.

The use of the part is justified on uneven surfaces, in icy conditions in the city or on a wet road. When driving in complete off-road conditions, the delay in the actuation of such a limited-slip differential is unacceptable. In addition, constant loads can lead to overheating of the liquid in the device, the loss of its working properties and complete self-blocking, which is fraught with breakdowns of other vehicle systems.

At present, viscous couplings are operated in "pre-start mode" on vehicles with automatically connected all-wheel drive. It is characterized by a constant transmission of 5% to 15% of the engine power to the driven axle, which significantly reduces the reaction time of the mechanism.

Where are viscous couplings used?

Actually, there are only two main applications, but now only one remains:

• Used to cool the engine. A viscous coupling with a fan was fixed to the rod. It was driven from the car's crankshaft by means of a belt drive. The faster the engine rotated, the more the liquid thickened and the connection with the fan became stiffer. If the rpm dropped, then there was no such strong mixing, which means there were slippages, that is, the fan was rotating, the radiator did not cool so much. Such a system is effective for the cold (winter) period, when the engine does not warm up very much, but it is also cooled. Now the use of such systems on new cars is no longer to be found; it has been replaced by electronic fans (with sensors in the liquid), which are powered by electricity and are in no way connected with the engine crankshaft.

• Automatic connection of all-wheel drive. It is in this direction that viscous couplings have remained very much in demand. Practically on 70 - 80% of crossovers or SUVs, such systems are now used. True, they are gradually beginning to be replaced by completely electromechanical options, but so far they are more expensive and not so practical.

On the one hand, the viscous coupling is a very simple, cheap, practical and versatile mechanical device, on the other, it has a lot of disadvantages.

INDEPENDENT VISCOMUFT REPAIR

If you notice engine overheating, do not rush to replace the viscous coupling. You may be able to repair the broken part yourself. The most common cause of failure is silicone leakage from the base of the part. To fill in new fluid, you need to: Remove the viscous coupling from the water sediment, and then disassemble it. On the disk of the device itself there is a plate with a spring, under which there is a hole for the silicone fluid.

You need to take the utmost care to remove the pin, and then pour grease with a syringe. Keep in mind that during such a repair, the part is laid horizontally. It is enough to draw fifteen milliliters of oily liquid with a syringe. Slowly pour the silicone inside. Wait a few minutes without removing the syringe from the hole, so that the liquid has time to deeply flow into the viscous coupling. Wipe the surface of the device from excess liquid if necessary. Replace the pin and then install the part.

If you are poorly versed in cars and do not know how certain parts work, it is better not to start repairing yourself. The point here is not the possible breakdown of parts of the vehicle, but the difficulty of putting everything back together.

Also, bearings are a common cause of viscous coupling failure. There is only one symptom of such a malfunction: various kinds of noises in the area of \u200b\u200bthe cooling radiator. To repair the device, the first step is to remove it. To do this, unscrew the three bolts that secure the part. After that, the viscous coupling can be easily removed from the engine compartment. After removing the device, you can start replacing the bearing. Replace them only when you have disassembled the unit and drained the oil fluid. Use a special bearing removal tool - a puller. If you use the tools at hand, you can completely damage the node. Having installed a new bearing, you can start installing the device.

Do not forget to fill in new silicone fluid, which was drained before repairing the viscous coupling. When you notice “misbehavior” in a coupling, you don't need to change the entire part right away, as it can often be repaired. Special skills and abilities for this business will not be required. The only difficulty that can arise is finding a puller to remove the old bearing. The tool is not sold in every car store, which makes it difficult to self-repair the viscous coupling. If you have visited all the car dealerships you know and did not find a puller, ask your friends-drivers. The rest of the details are easy to find.

FEATURES OF VISCOMUFT REPAIR

Not all such devices have a hole for filling oily liquid. If you are a "beginner", do not try to repair the device yourself. Experienced craftsmen make the holes themselves. You can, of course, also try drilling a hole at your own risk. Do not use brute force when handling the disc. If the aluminum on the currency bends, the viscous coupling cannot be repaired - only a complete replacement of the device.

How does the viscous coupling of the cooling fan (VO) work?

Until the engine is warm, the thermostat directs the coolant flow in a small circle. In this mode, the substance in the viscous coupling is squeezed out by a rotating disk into a reserve tank. There is no engagement between the disc and the case, the fan does not rotate.

The flow of coolant into the radiator causes deformation of the bimetallic plate, actuation of valves and accumulation of viscous clutch fluid not in the reserve tank, but in the space between the disc and the housing. Due to viscosity, the unit is blocked, to which power is transmitted from the shaft. The fan rotates, directing air flow to the radiator, reducing the coolant temperature.

When it cools down to the level of the response threshold of the temperature-dependent unit, paths for liquid overflow are opened, the force of viscous friction between the disc and the body decreases, and the fan stops.

Viscous coupling check.

Any suspicion of abnormal operation of a part requires immediate inspection, since its malfunction can lead to serious damage to the main systems.

How to check the transmission?

The most accurate results are obtained by checking the viscous coupling removed from the transfer case.

For this it is enough:

• Remove the part;
• Turn the differential gear with a wrench with a dynamometer (or measure the force with a spring dynamometer).

The force should not exceed the limits set by the manufacturer (can be found in the technical literature for the device and manuals for accessories).

You can check the part without removing it. To do this, you should:

• hang out the driven axle wheel;
• try to crank it by measuring the force with a dynamometer;
• calculate the moment according to the known wheel diameter

If the obtained value does not exceed the limits set by the manufacturer, the part is working. If the wheel could not be turned at all, the device must be urgently changed.

Video tutorial.

How to check the VO viscous coupling?

Checking the VO viscous clutch is not difficult:

• Start the engine "cold";
• After 30-60 seconds, the fan noise will decrease - it will go into idle mode (at this moment it can be stopped by any obstacle, for example, a rolled newspaper, but safety precautions do not recommend doing this);
• After a while, when the engine warms up (controlled by a temperature sensor), the viscous clutch will begin to increase the speed of the unit;
• In this case, there should be an unambiguous reaction to an increase in engine speed - the engine rotates faster - the fan rotates faster.

You can check the device in motion (more precisely, in the parking lot in the middle of the day). When the temperature rises at idle, we can definitely talk about problems with the viscous clutch.

Using viscous couplings for wheels of one axle.

Another option of using the clutch is possible - for wheels of one axle. In this application, the device is installed between the axle differential and the axle shaft. In this case, when cornering or slipping one of the wheels, the device helps to redistribute the moment between them, ensuring safe maneuvering or passing a problem area.

There are solutions in which a pair of parts is placed on the wheels of one axle. This design makes it possible to abandon the axial (interwheel) differential. At the same time, the use of a pair of viscous couplings makes it possible to provide all modes - both differential rotation and blocking of the axle wheels.

Pros and cons of all-wheel drive viscous couplings.

Like any device, OTs in four-wheel drive vehicles have both advantages and disadvantages.

Advantages:

• Automatic connection of all-wheel drive or interlocking of axle wheels, which significantly increases cross-country ability and traffic safety on difficult surfaces;
• The simplicity of the technical solution and, accordingly, its cheapness and reliability - manufacturers, most often, make devices maintenance-free, counting on a service life equal to the life of the car.

In practice, the mileage of 90-100 thousand km (in difficult conditions and less) becomes limiting due to the loss of its properties by the liquid and malfunctions in other vehicle systems. With this mileage, it is recommended to replace the device.

Disadvantages:

• The response delay and non-linearity do not allow for absolute control over the four-wheel drive;
• In most cases, on vehicles equipped with viscous couplings, manual on / off of the second drive axle is not provided;
• Limited dimensions impose restrictions on the transmission of torque, as well as on the possibility of long-term operation in a loaded state (overheating and failure are possible);
• The joint work of the air intake and electronic systems such as ABS, ESP and similar is difficult, since they solve opposite problems.

Crossovers are now very popular in the automotive market. They have both full and mono drive. It is connected using a device such as a viscous coupling. The principle of operation of the unit is further in our article.

Characteristic

So what is this element? Viscous coupling is an automatic mechanism for transmitting torque by means of special fluids. It is worth noting that the principle of operation of the all-wheel drive viscous coupling and the fan is the same.

Thus, the torque on both elements is transmitted using the working fluid. Below we will consider what it is.

What's inside?

A silicone-based fluid is used inside the coupling housing. It has special properties. If it is not rotated or heated, then it remains in a liquid state. As soon as torque energy arrives, it expands and becomes very dense. As the temperature rises, it looks like cured glue. As soon as the temperature drops, the substance turns into a liquid. By the way, it is flooded for the entire service life.

How does it work?

What is the principle of operation of a product called "viscous coupling"? According to the algorithm of actions, it is similar to the hydraulic transformer of an automatic box. Here, too, torque is transmitted by fluid (but only by transmission oil). There are two types of viscous couplings. Below we will look at them.

First type: impeller

It includes a closed metal housing. The principle of operation of the viscous coupling (including the cooling fan) is based on the action of two turbine wheels. They are located opposite each other. One is located on the drive shaft, the other on the driven shaft. The body is filled with a silicone-based liquid.

When these shafts rotate at the same frequency, no mixing of the composition occurs. But as soon as slip appears, the temperature inside the case rises. The liquid becomes thicker. Thus, the driving turbine wheel engages with the axle. Connects As soon as the vehicle is off-road, the impeller speed is restored. As the temperature drops, the density of the liquid decreases. All-wheel drive is disabled in the car.

Second type: disk

There is also a closed building here. However, unlike the first type, there is a group of flat discs on the driving and driven shafts. What is the principle of this viscous coupling? The discs rotate in silicone fluid. As soon as the temperature rises, it expands and presses these elements.

The clutch begins to transmit torque to the second axle. This only happens when the car stalls and there is a different wheel speed (while some are standing, the others are skidding). Both types do not use automatic electronic systems. The device is powered by rotational energy. Therefore, the viscous coupling of the fan and all-wheel drive has a long service life.

Where is it used?

First, let's note the element that is used in the engine cooling system. The principle of operation of the viscous fan coupling is based on the operation of the crankshaft. The clutch itself is attached to the rod and has The higher the crankshaft revolutions, the more the fluid in the clutch warmed up. Thus, the connection became stiffer, and the fan element began to rotate, cooling the engine and radiator.

With a drop in speed and a decrease in fluid temperature, the clutch stops working. It should be noted that the viscous fan clutch is no longer used. Modern engines use electronic impellers with a coolant temperature sensor. They are no longer associated with the crankshaft and operate separately from it.

Four-wheel drive and viscous coupling

Its principle of operation is the same as that of the fan. However, the part is placed not in the engine compartment, but under the bottom of the car. And, unlike the first type, the viscous all-wheel drive does not lose its popularity.

Now it is installed on many crossovers and SUVs with a disconnect drive. Some use electromechanical counterparts. But they are much more expensive and less practical. Among worthy competitors, it should be noted only the mechanical blocking, which is on the "Niva" and "UAZ". But due to urbanization, manufacturers abandoned the real lock, which rigidly connects both axles and increases the vehicle's cross-country ability. The driver can choose when he needs four-wheel drive. If the SUV needs to overcome the off-road, it will quickly get stuck and after slipping, the rear axle will start working. But it won't help him get out of the heavy mud.

Benefits

Let's look at the positive aspects of viscous coupling:

• Simplicity of design. Only a few impellers or discs are used internally. And all of this is powered without electronics, by the physical expansion of the fluid.
• Cheapness. Due to the simple design of the viscous coupling, it practically does not affect the cost of the car (if it concerns the "four-wheel drive" option).
• Reliability. The coupling has a robust body that can withstand pressures up to 20 kilograms per square centimeter. It is installed for the whole service life and does not require periodic replacement of the working fluid.
• It can work in any road conditions. It prevents slipping in mud or when driving on snow. The external temperature is not important for heating the working fluid.

disadvantages

It is worth noting the lack of maintainability. The viscous coupling is installed permanently.

And if it is out of order (for example, due to mechanical deformations), then it changes entirely. Also, motorists complain about the inability to connect a four-wheel drive on their own. The clutch engages the second axle only when the car is already "buried". This does not allow the car to easily overcome mud or snow obstacles. The next disadvantage is low ground clearance. The node requires a large body. And if you use a small viscous coupling, it will not transmit the required torque. And the last drawback is the fear of overheating.

You cannot skid for a long time on all-wheel drive. Otherwise, there is a risk of disabling the viscous coupling. Therefore, this type of "dishonest" drive is not welcomed by off-road enthusiasts. With prolonged loads, the knot simply wedges.

Conclusion

So, we found out how the viscous coupling of the all-wheel drive and fan works. As you can see, the device, thanks to a special fluid, can transmit torque at the right time without the involvement of additional sensors and systems. This is very

Consider the principle of viscous coupling. Viscous coupling is a device found in four-wheel drive cars that can transfer and equalize torque between the axles without any smart electronics.

That is, the viscous coupling performs work similar to that of a differential lock, only in automatic mode.

What is viscous coupling? If you decipher the name of the viscous coupling, it turns out that it is based on the phrase "viscous coupling".

In principle, it explains the whole essence of the viscous coupling - a special viscous fluid filling the unit is the very link that transfers torque from one shaft to another, but they themselves are not mechanically connected.

This fluid has one interesting property - it begins to thicken when it is actively mixed, due to which there is a change in the transmission of torque between the shafts.

Viscous coupling began to be actively used by automotive engineers to create automatic inter-axle locks for all-wheel drive vehicles. We will consider the design and principle of operation of the viscous coupling in more detail later, but for now let's look into the past.

History reference

It should be noted that the viscous coupling is not a new invention. This principle was known back in 1917 in the United States. It was there that its creator, the talented engineer Melvin Severn, lived.

Unfortunately, in those days, the principle of fluid viscosity in a transmission was not appreciated at its true worth, and there was no particular need for it. So the viscous coupling would have sunk into oblivion, but unexpectedly in 1964 it reappeared on the arena of the world automotive industry in the transmission of the British sports car Jensen Interceptor FF.

It was the debut of the viscous coupling in the production car and since then it has been actively used and used by various automakers.

Let's look inside the device

Let's get into in detail the device and the principle of operation of the viscous coupling of the all-wheel drive, because it is in such systems that it is most often used.

So, in general terms, we have already described this principle - there is a viscous coupling, as a rule, between the front and rear axle of the car and connects two shafts - one going from the transfer case, and the other to the rear axle.

Sometimes this clutch is mounted directly in the rear axle of the car, but its essence and principle of operation does not change from this. The main elements of the device are:

• sealed housing;
• filler made of a special viscous liquid (usually based on silicone);
• drive shaft disc pack;
• drive shaft disc pack.

The all-wheel drive viscous coupling functions as follows.

At the moment of uniform and calm movement, both shafts, as well as the rear and front wheels, rotate at the same speed - synchronously.

Under such conditions, the fluid in the coupling has a minimum density, and the torque from the drive shaft to the driven shaft is practically not transmitted.

As soon as there is a difference in the speed of rotation of the shaft, and therefore the disks inside, the liquid begins to actively mix (mixer effect) and, due to its unique physical properties, thicken.

This causes a gradual inter-axle blocking and more torque begins to flow to the driven shaft. The front or rear axle, depending on the design of the vehicle, begins to be put into operation.

Thus, the viscous coupling works in automatic mode, and without any electronics or intervention from the driver.

It seems that at first glance everything looks almost perfect, it would seem that everyone should have a viscous coupling, but this is not so.

Moreover, this device is practically not used in the modern automobile industry. Why?

Pros, there are also disadvantages of viscous coupling

Consider the positive and negative sides of all-wheel drive viscous couplings, and also answer the question: why have they become a thing of the past and why are automakers abandoning them?

The advantages of viscous couplings are unambiguous design simplicity. These devices do not require any routine maintenance and are extremely reliable. This is where the pros end.

I must say that the disadvantages of viscous coupling are very tangible. The most serious ones are:

• the inertia of a viscous fluid - it "thickens" not immediately, but gradually, which is very impractical, and sometimes even dangerous in constantly changing road conditions. It is also difficult to predict how quickly it will work and center blocking will occur;
• the dependence of the efficiency of the clutch on the size - to create an adequately working mechanism, large dimensions of the body and impressive diameters of the disc packs are needed, and this negatively affects the vehicle's clearance.

In general, the above predetermined the fate of viscous couplings. Despite their interesting properties, electronic blockers, for example, Haldex couplings, are already more popular in the modern automotive industry.

I think you figured out this simple mechanism and can explain the principle of the viscous coupling. Write, if you have any thoughts on this, in the comments, subscribe to the blog and explore the cars with us.

Implemented by viscous coupling. Most often, such a unit is found on Toyota cars of the mid-2000s, and on such asphalt machines, many, buying an all-wheel drive car, for example, the RAV4 of the first and second generation, are very much surprised that in fact they are buying a mono-drive station wagon, due to the building viscous couplings. Toyota calls this all-wheel drive system V-flex.

In the photo - a viscous coupling

Today I propose, using this all-wheel drive system as an example, to make out in more detail what a viscous coupling is, how it works, and whether it is worth buying a car with an all-wheel drive system built around it.

A viscous coupling is a mechanical device that transmits or stabilizes torque through a viscous fluid. The viscous coupling device is as follows: in a closed sealed case, in which a pack of flat round disks with perforations is installed. The discs are formed in such a way that driven and driving discs move and are at a very small distance from each other. The silicone-based fluid filling the coupling body tends to change its viscosity with intense stirring and heating, due to which the gap between the discs is literally glued together by a viscous fluid, due to its expansion.

In other words, the operation of the viscous coupling depends on the difference in the rotation speed of the driven and driving discs, due to which the physical properties of silicon, an organic viscous substance, change.

Such a system is far from perfect due to the fact that there is no adjustment of the disc closure, and roughly speaking, everything is left to chance, in the literal sense of these words, and depends on the properties of a viscous fluid. Also, the disadvantages include a significant amount of time required for the clutch to operate. This is not so scary when the viscous coupling is used to turn on the drive of the radiator cooling fan, but when it takes several minutes to turn on the all-wheel drive, they can become fatal. Also, it is impossible to forcibly turn on the all-wheel drive system, and even more so to block it.

If we talk about the node, then the viscous coupling is quite simple, and as a result, an inexpensive device that cannot be repaired, and usually runs the entire service life of the car. It can fail only because of long work, and most often it just goes crazy. Which again, speaks of the shortcomings of such an all-wheel drive, because if you get stuck somewhere, you will have to slip a lot, and the first thing that will let you down is the viscous coupling.

Viscous coupling is not repairable, and it is easier to buy a new one, or contract, than to perform an autopsy. In addition, not every master will undertake such work, and even more so, only a few will be able to successfully repair it.

To date, all automakers have already abandoned viscous couplings in all-wheel drive systems, focusing on hydromechanical or electromagnetic couplings, since their work is much easier to regulate, and in modern cars stuffed with electronics, they are more appropriate, since viscous couplings are almost impossible to use in cars equipped and.

In no case would I recommend buying a car with an all-wheel drive system, which is based on a viscous clutch. Frankly, this is the last century. The market is full of crossovers and just all-wheel drive sedans and station wagons, which are equipped with plug-in all-wheel drive, based on which Haldex clutches (Volkswagen Tiguan, Opel Mokka, Ford Kuga), Dynamax (all-wheel drive Kia or Hyundai models) or generally the Torsen differential (mostly expensive models Audi equipped with an all-wheel drive system). These are modern AWD systems that are capable of providing both high cross-country ability and safety in motion, both for people in the car and outside it.

Best regards, Andrey Chervyakov.

5th generation Haldex coupling device

Now a very large number of so-called crossovers have a not entirely honest four-wheel drive. It is not permanent, and even connected for a very short time (I would like to mark connected automatically) - is it good or bad we will definitely talk in another article, today I want to talk about "automatic connection" using "viscous coupling" - and what is it you know? After all, this unit is now very much in demand, but unfortunately, many simply do not understand the principle of its operation, although this name is on everyone's lips. Well, as usual, I figured out the topic and I will try to tell you in detail what it is and how everything actually works, there will be a detailed video at the end, so we read - we look ...

In fairness, I would like to note that viscous couplings are used not only in all-wheel drive systems, but also in car cooling systems and not only. To begin with, as usual the definition.

Viscous coupling (or viscous coupling) Is an automatic device for transmitting torque by means of the viscous properties of special fluids.

Simply put, the torque is transmitted by changing the viscosity of a special fluid in the viscous clutch housing.

About the liquid inside

At the very beginning, I want to tell you about the fluid that is inside the viscous coupling, what it is and what properties it has.

To begin with, I would like to say that they pour inside - a dilatant liquid, which is based on silicone. Its properties are very interesting, if it is not heated and stirred too much, it remains liquid. BUT it is worth mixing it strongly and heating a little, it thickens and expands very much, it becomes more like frozen glue. After mixing again becomes insignificant, it again acquires its initial state of aggregation, that is, it becomes liquid.

It should be noted that the liquid is filled for the entire service life of this unit and is not subject to replacement.

Device and principle of operation

It's a lot like an automatic transmission torque converter, if you like, where torque is transmitted using oil pressure. Here, too, the transmission of torque occurs due to the fluid, however, there are global differences in the principle of operation.

There are only two main viscous coupling devices:

• There is a closed sealed casing in which two turbine wheels with impellers (sometimes more) rotate opposite each other, one is installed on the drive shaft, the other on the driven one. Of course they rotate in our dilatant fluid. As long as the shafts rotate synchronously, there is practically no mixing of the liquid. BUT it is necessary for one axis to stand up and the other to rotate very quickly (wheel slip), then the liquid inside begins to mix and heat very quickly, which means it thickens. Thus, the first driving impeller meshes with the driven one and torque is transmitted to the second axle. After the vehicle has finished off-road, the agitation is stopped and the rear axle is automatically disengaged.

• The second design also has a closed body. There are several groups of flat discs only on the drive and driven shafts. Part on the slave, part on the leader. They also rotate in a special fluid. While the rotation is uniform, the mixing of the liquid is minimal and it is liquid, but after one axis rises, the second starts to slip, the mixing is huge! It not only thickens, but also expands. Thus - very tightly pressing the discs against each other. As a result, the transmission of torque - the second axis also begins to rotate.

The viscous coupling is a fairly simple and effective mechanical device, with proper use it can walk without any problems for a very long time.

Where are viscous couplings used?

Actually, there are only two main applications, but now only one remains:

• Used to cool the engine. A viscous coupling with a fan was fixed to the rod. It was driven from the car's crankshaft by means of a belt drive. The faster the engine rotated, the more the liquid thickened and the connection with the fan became stiffer. If the rpm dropped, then there was no such strong mixing, which means there were slippages, that is, the fan was rotating, the radiator did not cool so much. Such a system is effective for the cold (winter) period, when the engine does not warm up very much, but it is also cooled. Now the use of such systems on new cars is no longer to be found; it has been replaced by electronic fans (with sensors in the liquid), which are powered by electricity and are in no way connected with the engine crankshaft.

• Automatic connection of all-wheel drive. It is in this direction that viscous couplings have remained very much in demand. Practically on 70 - 80% of crossovers or SUVs, such systems are now used. True, they are gradually beginning to be replaced by completely electromechanical options, but so far they are more expensive and not so practical.

On the one hand, the viscous coupling is a very simple, cheap, practical and versatile mechanical device, on the other, it has a lot of disadvantages.

Pros and cons of viscous coupling

To begin with, I propose to talk about the advantages of this node:

• Simple construction. Indeed, the design is very commonplace, there is nothing more complicated in it.
• Cheap. Due to its simplicity, it is not expensive at all
• Durable. The viscous coupling housing can withstand a pressure of 15 - 20 atmospheres, it all depends on the design. If initially there were no breakdowns, then this means that it can take a very, very long time.
• Practical. FOR PROPER USE. Installed for the entire service life of the car, does not require any attention.
• On a dirt road or asphalt, it can also work. If you say abruptly "started" from a place or is slipping on ice or dust. Then the rear axle is automatically connected. This gives the advantage of handling even in the city.

Despite the advantages of the design, it is worth noting about its shortcomings, because there are also many of them.

• Maintainability. As a rule, it is not repaired, that is, disposable, it is not profitable to repair it and it is very difficult for a simple man in the street. They almost always change to a new one.
• Connectivity. There is no linear dependence of the all-wheel drive connection, it is practically impossible to guess when the discs inside will slow down! Therefore, there is no four-wheel drive control.
• You cannot manually connect the drive yourself.
• Low efficiency of four-wheel drive. The maximum torque will be transmitted only when the front wheels are slipping very hard.
• Large viscous couplings are not used. Because it needs a large body, and since it hangs from below, it really greatly reduces the vehicle's clearance. The use of small housings, that is, small viscous couplings, leads to limited transmission of torque to the rear axle, because there are fewer discs and a small volume of special fluid
• The viscous coupling cannot work for a long time. This is highly undesirable! It is not designed for long-term loads, otherwise it will simply fail, it will completely jam. That is, it tells us that it is impossible to meddle on serious off-road! Use as soon as possible for snowy yards and a little dirt in the country, that's all.