Many motorists talk about the clutch as a technical unit, but few people know how the clutch actually looks and how it works.
The clutch of a car is one of the main automotive components, with the help of which torque is transmitted from the engine to the transmission. In fact, this is a friction clutch that closes the shafts of the power unit and gearbox. There are technical clutches that are installed on manual manual gearboxes, as well as clutches with as many as two clutches for robotic preselective boxes with automatic control, which almost repeat the functionality of hydromechanical machines.
When a beginner gets behind the wheel of a car equipped with a manual transmission, his main problems begin with the clutch. Or rather, not with the technical unit itself, but with its management. A young driver learns to press the pedal correctly in order to close the torque smoothly without jerks and wheel slips, which directly affect the clutch life. It doesn’t happen right away.
The driver may act incorrectly with the accelerator pedal, and the car stalls with an unpleasant twitch when starting. Or, conversely, it presses on the gas so hard that excess torque causes the clutch discs to slip and become hot, which is why the clutch as a whole experiences increased wear. Thus, the clutch can be disabled in just a month, and with proper use, it lasts for many years.
Two types of boxes
Engineers develop new types of mechanical transmissions over time. There are several types of clutch designs. Previously, clutches with a cable, which was attached to the pedal lever and went to the same lever on the box, were widely used. Now they mainly make hydraulic drives. They are electronically controlled, which regulates the pressure in the hydraulic mechanism and helps the driver to close the clutch discs correctly.
At the moment of connecting the clutch discs, the hydraulic mechanism creates exactly the force that is necessary to safely align the rotation of the engine and gearbox shafts. Slip turns are insignificant, because of which the car starts off without risk to equipment. At the same time, the smooth engagement and disengagement of the clutch is ensured due to the slight slippage of the friction discs.
Drivers’ mistakes are now being corrected by an electronic control unit, which in modern cars with a manual gearbox also regulates the gas supply when starting off. It adds a little RPM to the engine to provide the necessary traction to start the car smoothly, which makes the vehicle very easy to operate for beginners. Thus, the driver is now only required to depress the clutch pedal smoothly, even without pressing the accelerator, to start moving. Gas can be added after the clutch discs are completely closed and the machine has started moving in first gear.
The principle of operation of the clutch
The clutch design consists of several main elements:
- The crankcase and casing cover the main mechanism and take on part of the load;
- Inside the basket is enclosed the drive disk together with the flywheel of the crankshaft of the engine;
- The pressure plate with springs is connected through a spline to the gearbox;
- A driven disk with special wear-resistant linings and a vibration damper is connected to the input shaft of the box.
In free operating mode, the clutch is closed very tightly due to the tension of the springs, and the torque from the motor is transmitted from the crankshaft to the flywheel, then to the casing and through the leaf springs to the drive (pressure) disk. When the clutch pedal is depressed, that is, in the operating mode, the driven disk is tightly adjacent to the flywheel and pressure disk.
To open the clutch, the driver must push the pedal, while the pressure plate moves away from the flywheel and releases the driven disc, interrupting the transmission of torque from the engine to the gearbox. This operation takes only a few seconds. The clutch device is not intended for continuous opening.
What happens if you keep the clutch pedal depressed for a long time?
Some drivers, when stopped at a traffic light, do not switch the box to neutral, but wait out the red light, holding the car with the brakes and depressing the clutch pedal. And the first gear remains on. This allows them to save time when starting. However, for technology, such an algorithm is extremely harmful.
When you press the pedal, the force is transmitted through the hydraulic system to the release fork and forces it to move the discs apart. This force is rather big, therefore, with long-term use, the part experiences excess loads. And her margin of safety is not unlimited.
The release bearing presses against the clutch basket leaf spring. Although its petals are designed for frequent operation, they should not hold the load for minutes on end. This leads to wear of parts, loss of elasticity and decrease in elasticity, which subsequently leads to failure of the entire structure. The fork or release bearing, which takes on the main load, may break.
What are the failures of the clutch?
You can find out that the clutch has exhausted its resource by a number of signs. The discs wear out and their engagement becomes loose. The friction linings slip and the discs begin to slip under load. If the driven disk began to slip, first in fourth gear, then in third, then it’s time to go to the service. The driver presses on the gas, and the car reacts lazily, not as actively as before.
Often this malfunction manifests itself after active slipping in mud or snowdrifts. The slipping friction discs rub and burn with the release of a pungent odor. Their burnt surface completely ceases to hold the moment and the clutch can no longer work. When you add gas, the car does not accelerate and, at best, crawls in second gear at a low speed.
It happens that the clutch slips and is not fully engaged due to a small pedal free play, damage to the fork, or due to oil leakage from the engine shaft seal. It happens that active drivers break the springs inside the basket.
If the clutch engages abruptly with a blow, then most likely the cause is damage to the drive mechanism or scoring on the working surfaces of the discs.
What does a clutch look like?
Clutch repair is rarely required, but sometimes you still have to deal with it. In order to remove the clutch basket, you must first hang the car on a lift and dismantle the gearbox. Inside the transmission housing, the so-called clutch basket is immediately visible.
For example, for VAZ 2108−2115 cars, it includes three main parts: a cast-iron pressure plate attached to a metal cast body, the slots of the diaphragm spring are clearly visible inside. This detail is easily recognizable and even a beginner can easily distinguish a technical unit from it. The clamping force that must be developed when the clutch pedal is depressed depends on the shape of the diaphragm spring and the size of its petals.
This force is transmitted through the fork to the release bearing, which takes on a considerable share of the load. A couple of seconds are allotted for gear shifting, after which the disks should close again. However, if you keep the clutch in the open state, the technical unit goes beyond the regulated loads and its increased wear begins. Therefore, at each stop, it is necessary to engage neutral gear and fully release the clutch pedal.
Preselective boxes with two clutches
Robotic transmissions with two clutches are now widely used. They combine the advantages of mechanical and automatic transmissions. They are called preselective, from the English. preselect – “preselect”. That is, the transmission allows you to turn on the next stage of the box even before turning off the running gear.
In their current form, these transmissions were introduced in the mid-80s by designers from Porsche for sports cars. They could provide instant gear shifting under maximum thrust. And for this it was not required to reset the gas pedal. Thanks to the double clutch, the gears shifted without loss of torque.
The heart of a dual clutch gearbox is the double shaft. Inside the outer shaft is the inner one, inserted like a core. Both shafts work alternately.
When the car moves away, the transmission includes two gears at once. The first transmits torque from the engine, and the second is idling. Then, when a certain speed is reached, the automation engages the second shaft with the second stage engaged, and the box pre-activates the third gear, which starts to idle, waiting for its turn. Directly in the process of shifting, both clutches become open for hundredths of a second, due to which the power flow break is almost not felt.
Following sports cars, these boxes switched to ordinary civilian models, and they were not at all designed for a sporty driving rhythm. Volkswagen, BMW , Ford, Audi, Porsche and even Chinese manufacturers use them. The main reason for this step was environmental requirements. Preselective robots, due to the design of their double clutch, provide less energy loss and show high fuel economy, thereby reducing emissions of harmful substances.
But preselective robots also have significant drawbacks. The dual clutch unit has the same resource as a conventional clutch in manual transmissions, and its cost is several times higher. In addition, there are problems with long-term operation of vehicles with preselective robots in difficult urban traffic. In other words, they are afraid of traffic jams due to the fact that during long stops the box does not turn on the neutral, but only keeps the clutch discs open, which results in increased wear.
