A clutch is a device that allows a vehicle or other equipment to uncouple and couple two axles while it is in motion. The term also refers to the act of clutching : causing one axis, through a certain mechanism, to be part of the movement of another axis.
automobile clutch system
The best known clutch is that of the automobile . The concept can refer to the system itself, to the action detailed above, or to the pedal that allows the mechanism in question to be actuated.
It can be said that the clutch connects (while the pedal is not pressed) and disconnects (when the pedal is pressed) the engine with the other elements that make the vehicle move. This arrangement is required for the car to begin driving gradually (without jerks or damage to transmission components) and to change gears or speeds. .
When the driver depresses the clutch, it ultimately separates (disengages) the gearbox from the engine. As a result, the gears can travel freely without being pressed by the motor's spin. As the clutch is released, progressively more torque is transmitted.
clutch wear and tear symptoms
The clutch is prone to a lot of wear due to its properties. . To extend its life, mechanical experts suggest not accelerating too much while stepping on it, not resting your foot on the pedal and not leaving the clutch depressed while The vehicle has come to a halt (for example, waiting for the green traffic light).
To know if the time has come to change the clutch, it is necessary to pay attention to certain symptoms that indicate that it is reaching the end of its useful life. To do this we must follow three simple steps:
* activate the parking brake;
* fully depress the clutch and put the car in third gear, although second also works;
* release the clutch suddenly.
If after this procedure we notice that the vehicle takes too long to stop, then it is almost certain that the clutch is worn to the point of requiring replacement. On the contrary, if it stops immediately, we can still continue using it.
Tips for clutch maintenance
As mentioned above, mechanical experts can teach us how to keep our vehicle's clutch in better condition by following some tips . Let's see some of them below.
When we have to wait a few minutes with the vehicle in motion, something very common when waiting for passengers to carry out a short procedure or when giving way to pedestrians at a crossroads, it is recommended to put the neutral gear and release the clutch, since even light pressure can cause wear.
When we start the car, it is necessary to synchronize the movements of pressing the accelerator and releasing the clutch. When changing gear, on the other hand, it is advisable to make smooth and progressive movements , although before moving the gear lever we must have reached the maximum point of clutch pressure.
Before starting the vehicle, it is recommended to depress the clutch fully, so that the engine has less effort . When we face a slope , the best thing is neutral, to save fuel and protect the clutch. It is important to note that these and other tips are often not spread widely enough, as most drivers do exactly the opposite in the above situations.
Cone and multiple disc clutches
The first clutch adopted by Benz in 1885 consisted of a leather belt that was progressively moved from an idler pulley to a pulley attached to the crankshaft. Both pulleys were close together, so as the belt moved, slippage occurred.
A few years later the first cone clutches appeared in 2 versions, right cone or inverted cone. It consisted of 2 cones inserted one into the other, one of which was covered with a layer of leather or suede. One was solidly attached to the crankshaft, while the other was coupled to the change and could move forward or backward along its axis; a spring kept it strongly compressed against the other cone. The force of said spring could be decreased at will with an external lever and, since the spring rotated with the primary shaftof the change, the union between the fork and the spring was made by means of a bearing. It is worth highlighting the fact that, since the first clutches, their position has always been between the engine and the gearbox, but that there is no problem for them to be located between the gearbox and the differential. In this second case, the transmitted torques would be greater and, consequently, the clutch would also have to have larger friction surfaces. Perhaps it was this difficulty, coupled in the early days with the low durability and efficiency of the friction linings, which determined the position of the clutch. Among other advantages, with this solution it is even possible to take advantage of the flywheel of the crankshaft to make one of the clutch cones or discs inside it.
Another characteristic that was already evident in the first automobiles was that the pressure between the friction surfaces was guaranteed by a spring and not by the force exerted on the pedal. In this way, its value is constant and independent of the force that the driver can exert. Some vehicles of the past must be excepted from all this, for example the Ford T, in which there was a lever that when pulled acted as a handbrake and when pushed engaged.
Leather, which for many years was the most used friction material, heated up easily and lost its characteristics; furthermore, it was very sensitive to oil and humidity and never guaranteed constant operation. Burnt clutches were one of the most frequent inconveniences. The cone clutch required a flywheel with a very large diameter and considerable weight, characteristics that did not allow reaching a high number of revolutions. Precisely to reduce the weight of this organ, it was switched to multiple disc clutches, made up of alternating bronze and steel discs.Some are mounted on an outer plate that is integral with the crankshaft and the others on another internal one that is integral with the gearbox. The coefficient of friction was lower, but the number of discs could be increased at will until the desired drag force was obtained. This type of clutch, which was imposed at the beginning of the century, made it possible to have a smaller flywheel, reducing its diameter; however, the weight was quite detrimental, since its effect was noticeable in accelerations.
(The great advantage of the multiple-disc clutch consisted in the smoothness and progressiveness of the clutch and did not allow, among other things, the deformation of any disc. To adopt this type of clutch to a more powerful engine, it was enough to increase the number of disks.
Around 1920, the construction and spread of asbestos chipboard clutch linings, made it possible to obtain high adherence coefficients (more than 0.30) and to reach high temperatures without damaging the linings themselves. This allowed the definitive success of a type of clutch that had been introduced at the beginning of the century by De Dion Bouton, but had not been successful due to its low resistance to wear. It was the dry single-disc clutch, which reappeared in 1920 (adopted by AC) and became widespread after 1926. The success of the single-disc clutch was made possible by *Ferodo, a British company that had previously built friction linings based on of a conglomerate of asbestos and copper. This material demonstrated its ability to withstand high temperatures and pressures, necessary in a single-disc clutch, and the high temperatures that occur at start-up. Schematically, this type of clutch consists of a steel disc on which the asbestos material attached to the gearbox shaft has been mounted. This disc is located between the faces of 2 other plates, one made up of the inner surface of the engine flywheel and the other by a plate which, while being integral in its rotational movement with the engine flywheel, can slide axially with respect to the engine flywheel. the same. A system of springs, almost always located in the shape of a crown, pushes the second plate against the flywheel, thus compressing the steel disc with the asbestos-based linings between both surfaces. Under these conditions, both the intermediate disk and the end disk are integral with each other.
Both the multiple disc clutch and the single disc clutch can work dry, or submerged in an oil bath. This second system was quite widespread in the past, especially for multi-disc clutches, since it guaranteed a certain cooling of the discs and, although it significantly reduced the coefficient of friction, it allowed higher pressures to be reached. In addition, it had the advantage that it allowed the clutch to be engaged very gradually and progressively. On the other hand, its assembly required a sealed sump and sometimes also a radiator for cooling the oil with a delivery pump. Another drawback was the increased viscosityof the oil (in the morning or in winter), which favored the gluing of the discs. The coefficient of friction of clutches in oil bath is 3-4 times lower than that of dry clutches. In practice, oil-bath clutches last longer, but are also more expensive, which is why they are not used in cars. However, they have a wide application, in the motorcycle sector, in large industrial engines and in machine tools, since there is the possibility of regulating the clearances caused by the wear of the discs.
Evolution of the single-disc clutch
The operation of the dry single-disc clutch proved satisfactory only after about 30 years since its introduction. Indeed, it was in the post-war years that its main flaws were eliminated.
In the first rigid disc clutches, the contact was not completely flat, since a small play in the splined shaft or in the lever system was enough to have a partial contact. In this way the clutch vibrated and the union was produced by blows. To avoid this inconvenience, the harmonic steel disc was built, with an undulating outer circumference, so that it constituted a kind of spring between the 3 friction linings. When the disk was pressed between the flywheel and the thrust plate, both linings were progressively crushed, thus compensating for parallelism defects.
However, this solution was not enough to avoid sudden and dangerous joints caused by unskilled drivers, for this reason a system of springs was introduced in the disc to prevent knocking. The lever that activates the clutch has gradually increased its reduction ratio, until achieving, with a pedal effort of 15 kg and a stroke of 70-80 mm, a disc displacement of approximately 3 mm and a pressure 3-3kg/cm2. In this way the path of the pedal that performs the union is quite wide and adjustable with the foot. In former times, the clutch stroke depended on a movement of a few millimeters of the pedal: therefore, it was practically impossible to carry out progressive starts. Another characteristic drawback of the single-disc clutch was due to the almost general use of 5-6 coil springs. The load of these springs, in fact, was not perfectly equal and produced a higher pressure on one side. In addition, being located on the contour, they were subject to a centrifugal effect, which compressed them at high speed and reduced their efficiency precisely when it should have been greater. Other drawbacks of the peripheral springs were represented by the influence of the temperature of the thrust plate and the large axial dimensions of the clutch group. that at high speed it compressed them and made their efficiency decrease precisely when it should be greater. Other drawbacks of the peripheral springs were represented by the influence of the temperature of the thrust plate and the large axial dimensions of the clutch group. that at high speed it compressed them and made their efficiency decrease precisely when it should be greater. Other drawbacks of the peripheral springs were represented by the influence of the temperature of the thrust plate and the large axial dimensions of the clutch group.
The load on the coil springs increases as the clutch is pushed in, while it decreases as the clutch is released, contrary to what would be desirable. Also, the wear of the disc makes the springs work with lower pressures, which facilitates the slippage of the clutch.
In the sixties there was a clear advance towards eliminating all these drawbacks, with the use of diaphragm springs and cup springs of the Belleville type. With this system, the spring is unique and is made up of a large slightly conical steel plate. The main advantage is that with it an evenly distributed pressure and a shorter stroke of the drive forks can be achieved. On the other hand, cup-shaped or diaphragm springs make it possible to eliminate the 3 levers that actuate the thrust plate and, consequently, obtain a lighter clutch that lacks components that can wear out over time and give rise to mechanical play. . The characteristic of this type of spring is that, even with a great disc wear, the effort that must be exerted on the pedal is constant. In fact, while helical springs have a load that increases linearly with deformation, diaphragm springs have a curve with return, which guarantees constant pressure over a considerable period of time and does not show the effects of eventual wear. of the disk.
In addition, they are not influenced by centrifugal force or clutch heating. The assembly can be balanced more easily, is more compact and is less subject to vibrations and noise. In practice, more than 90% of new vehicles are equipped with diaphragm or Belleville-type springs.
The operation of the diaphragm spring clutch is similar to that of the coil springs. However, the group is very simple, since the control levers are not necessary, since the same spring works as a rocker arm.
Clutch proportions, control bodies and drawbacks
When designing a clutch, it is necessary to establish the value of the maximum torque that is to be transmitted. In general, a value of 1.3-1.4 times the maximum torque of the engine is considered: it is not advisable to use higher values and build a stronger clutch, since in this case it is very difficult to achieve a progressive union and soft. On the other hand, if lower values are adopted, prolonged slippage at starting or gear changes may easily occur. This is more likely when the clutch is hot, since the coefficient of friction decreases with increasing temperature, which decreases the value of the transmittable torque. In addition, it is necessary to take into account the ingress of dust and the ability to dissipate heat. Thus, the surface of the disc and the contact pressure must be related to the type of clutch lining used. Generally, this calculation is carried out by clutch manufacturers who, based on their experience, make tables in which each type of transmissible torque corresponds to a disc with a specific diameter and material, depending on its application. In general, the coefficients of friction vary from 0.25 to 0.35, while the temperatures that can be tolerated for a short time at start-up are between 300 and 400 °C. The pressure required in the springs must be of the order of 3-6 kg/cm2. It should be noted that larger torques could be transmitted at higher pressures; however, the clutch also has the function of a safety joint, that is, it slips in the event that a torque higher than the one for which it is designed is transmitted. This is not possible in the motor -<• wheels direction (unless a more powerful motor is mounted), but it is -and convenient- in the opposite direction; for example, to avoid dangerous over-revving when the wrong gear is engaged.
The lever that actuates the clutch is controlled mechanically by the pedal. In the 1960s, hydraulic controls were spread, made up of a small pump, similar to the one used for brakes, and a drive cylinder. With this system play and noise are eliminated and a smoother control is obtained. A very important component of the controls is constituted by the thrust regulating bearing. Since the clutch is a rotating member, while the lever is stopped, the connection between these two members requires the interposition of a bearing whose inner race rotates at the same time as the clutch. The most widespread types of thrust regulator bearings are graphite and ball bearings. In the past, the type of graphite was applied universally, consisting of aself-lubricating carbon washer . This solution is inexpensive, but of very limited duration. For somewhat difficult operations, it is preferable to use radial-type ball bearings, with a lateral thrust plane and provided with an internal reserve of sealed grease. Theoretically they have an unlimited duration, but due to poor lubrication and construction they can become noisy.
Accurate centering of the control yoke is very important to correct operation of the thrust adjusting bearing (or thrust bearing). While the graphite bearing can work even slightly eccentrically (and this occurs during the actuation stroke, since the lever describes an arc of a circle), the ball bearing must be mounted on a sleeve.
It is necessary to make a stroke limiter and a clearance adjusting screw on the clutch lever. Due to the wear of the linings, the idle run tends to decrease, since the plane on which the bearing must rest progressively moves towards the change. By means of the screw a value of 30-30 mm is maintained; Said free run corresponds to a clearance of about 2 mm in the bearing, which serves to avoid continuous friction of the bearing, wearing down its surface for no reason. The stroke limiter serves to prevent the actuation stroke from exceeding a certain value, causing serious damage to the internal parts of the clutch. In fact, the pressure exerted by the foot is considerable, since the lever multiplies its value by 10. To avoid this, a stop is necessary, which stops the bearing, at least 2 mm before the limit of the separation stroke. In this regard, it must be remembered that when the clutch pedal is depressed, the effort is transmitted to the flywheel and from there to the crankshaft, which rests on the support rings themselves. For this reason, when the engine is started, the clutch pedal must not be depressed, otherwise the bearing rings of the crankshaft, which have not been lubricated yet, wear out quickly. On the other hand, pressing the pedal frees the change, which, when the oil is cold, opposes a lot of resistance to turning. Depending on the case, it may be convenient to start the engine by depressing or releasing the clutch pedal. In this regard, it must be remembered that when the clutch pedal is depressed, the effort is transmitted to the flywheel and from there to the crankshaft, which rests on the support rings themselves. For this reason, when the engine is started, the clutch pedal must not be depressed, otherwise the bearing rings of the crankshaft, which have not been lubricated yet, wear out quickly. On the other hand, pressing the pedal frees the change, which, when the oil is cold, opposes a lot of resistance to turning. Depending on the case, it may be convenient to start the engine by depressing or releasing the clutch pedal. In this regard, it must be remembered that when the clutch pedal is depressed, the effort is transmitted to the flywheel and from there to the crankshaft, which rests on the support rings themselves. For this reason, when the engine is started, the clutch pedal must not be depressed, otherwise the bearing rings of the crankshaft, which have not been lubricated yet, wear out quickly. On the other hand, pressing the pedal frees the change, which, when the oil is cold, opposes a lot of resistance to turning. Depending on the case, it may be convenient to start the engine by depressing or releasing the clutch pedal. that have not been lubricated yet, wear out quickly. On the other hand, pressing the pedal frees the change, which, when the oil is cold, opposes a lot of resistance to turning. Depending on the case, it may be convenient to start the engine by depressing or releasing the clutch pedal. that have not been lubricated yet, wear out quickly. On the other hand, pressing the pedal frees the change, which, when the oil is cold, opposes a lot of resistance to turning. Depending on the case, it may be convenient to start the engine by depressing or releasing the clutch pedal.
Some clutch defects depend on the assembly, while others are a consequence of the wear of various components. Incorrect alignment (lack of coaxiality) of the rotating parts (flywheel, clutch disc and thrust plate) are almost always attributable to vibrations, premature wear, disc breakage and damage to the gearbox bearings. The presence of oil on the disc due to the infiltration of the crankshaft tow or, more frequently, to the oil seal of the bearing on the gearbox side, are due to clutch slippage or the joining of blows during starting. Sometimes the operation of the clutch is noisy: in this case, if the noise is noticed when lowering the pedal, the defect consists in the wear of the thrust bearing; if it is perceived when leaving the pedal,
To the different types described, automatic clutches must be added, which, although they are similar in their constitution, have special controls and different constructions, in addition to performing a certain automation in a less expensive way than the hydraulic system. One of the simplest automatic clutches is the one fitted to the Citroen 2 CV . In this car, in addition to the normal pedal-actuated clutch, there is another clutch in series with the first, which is actuated by centrifugal force; when the engine revolutions increase, this clutch is automatically blocked, so that the normal pedal clutch remains for the change.
Another type is the Ferodo Gravina, in which the centrifugal force serves to actuate the main clutch, while an electromagnet actuated by a button on the flywheel activates an auxiliary clutch which, by stopping the rotation of the centrifugal masses, annuls their effects by unlocking the main clutch, allowing the gear change to be carried out. The Perodo Perlec clutch that was mounted on the Renault 4 CV at the express request of the client also belongs to this category. In this clutch the action of the springs that pushed the discs against each other was replaced by an electromagnet; it was enough to cut the current for the separation of the clutch to take place. The Citroen DS 19 clutch is actuated by a pistonmoved by the central hydraulic system and linked to the gear lever.
Finally, we must mention the Kupplomat automatic control for clutches, built in Austria, which with a coupling of electrical circuits and depression in the intake achieves a good automation of the manoeuvre. In this case, as in that of the Citroen DS 19, one cannot speak of a special clutch but of servo controls that are more or less automated and synchronized with the gear change.
What is the average life of a car clutch?
It is a question that has no concrete answer. Both the manufacturer and the quality of the clutch as well as the use and type of driving that we have have a great influence. In other words, if we use the car in the city with many stops and starts, the clutch will suffer more and will wear out more quickly . Likewise, we must be very careful with vices such as resting our foot on the clutch pedal, which also greatly accelerates wear.
