Why does the car clutch slip?
Content
The link between the car engine and the gearbox is the clutch. Its task is to transfer torque from the flywheel fixed to the crankshaft to the input shaft of the gearbox. Further, through the transmission, rotation is transmitted to the wheels.
This unit is subject to very significant loads, especially in urban conditions, where you have to change gears and engage the clutch every now and then. Not surprisingly, over time, parts wear out and the clutch starts to fail. The most common problem that motorists have to face is slipping. As a rule, it occurs gradually, at first imperceptibly, but then more and more clearly affecting the behavior of the car.
To understand what it is and why it occurs, you need at least a general understanding of the structure and principle of the clutch.
How the clutch works and works
The main elements of this unit are the driven disc, the driving (pressure) disc, the diaphragm spring, the clutch with the release bearing, the release fork and the drive. Also, a massive flywheel mounted on the crankshaft shank, which, as you know, is driven directly by the engine using a crank mechanism, is directly involved in the work of the clutch.
The driven disc has heat-resistant and wear-resistant friction linings. For their manufacture, special mixtures of resins and rubber are used with the addition of pieces of copper or brass wire, fiberglass, ceramics and other materials. The pads are attached to the disc with rivets or glue. It is this part that is subjected to the greatest loads during operation and, as a rule, is the first to fail. The driven disk is most often one, but there can be two or more of them.
The diaphragm spring is usually structurally integral with the drive disc and is often referred to as a basket. The spring has petals that tightly press the driven disc against the flywheel. In some designs, instead of one diaphragm spring, there may be several spiral ones located around the circumference.
Due to the friction force, the driven disk rotates together with the flywheel. And since the disk is secured to the input shaft of the gearbox through a splined connection, torque is thus transmitted from the gearbox. When in gear, the input shaft transmits rotation to the secondary shaft, and through it to the transmission, which ultimately causes the wheels to spin.
The release drive can be mechanical, hydraulic or pneumatic and is controlled by the clutch pedal. The hydraulics allow for smoother clutch engagement and disengagement on passenger cars. And pneumatics are used only on trucks. When the pedal is not depressed, the clutch is engaged, the clutch plate is firmly pressed against the flywheel by the pressure plate.
The depressed pedal acts on the drive, the main element of which in the mechanical version is a metal cable. When the cable is pulled, the clutch release fork rotates on its axis and presses on the release bearing (release clutch).
The bearing is mounted on the input shaft of the gearbox and can move along its axis. Under the influence of the release fork, the release bearing bends the spring diaphragm in the center, forcing its petals to loosen the pressure at the edges. As a result, the driven disc moves away from the flywheel and free space appears between them. The transmission of torque to the checkpoint is stopped. Now you can change gears without the risk of damaging the gears of the mechanism.
If the drive uses hydraulics, a pusher is connected to the pedal by means of a pivot joint, which presses on the piston of the clutch master cylinder. The master cylinder pumps the working fluid through the pipeline to the working cylinder, which directly acts on the shutdown plug.
How to determine the presence of slippage
When the clutch is slipping, this is manifested primarily by a severe loss of power, which is especially noticeable on an uphill. Accelerating dynamics also suffers. When driving in low gears, the car may jerk.
While the problem has not yet become apparent, it may seem as if the power unit is not pulling. However, symptoms that directly indicate adhesion gradually begin to appear. One of them is the smell arising from the intense friction of the friction layer of the clutch disc against the surface of the flywheel. The smell is reminiscent of burnt rubber and is felt in the cabin.
Difficulty shifting and crunching when shifting gears are common signs of slipping. It becomes more and more difficult to get under way.
In addition, clutching, grinding, or other abnormal sounds indicate clutch problems, especially if they are different when the pedal is depressed and released. Sometimes vibration is observed, the pedal can be pressed tightly or, conversely, fall through, and its free travel may increase.
It also happens that when the clutch pedal is pressed, the discs do not diverge completely, remaining in some contact. In this case, they speak of incomplete disengagement of the clutch. You can check this as follows. With the engine running at low speeds, depress the pedal all the way and try to engage first gear. Difficulty switching on and extraneous sounds will indicate the presence of problems.
Why slip occurs and how to deal with it
Sooner or later, any clutch begins to slip. The inevitability of this problem is determined by the very principle of operation of this device. At the moment the fixed driven disk comes into contact with the surface of the rotating flywheel, a very significant friction occurs. As a result, the friction coating gradually wears out, wears out, and becomes thinner. At some point, the contact becomes not tight enough, and the driven disk begins to slip relative to the flywheel. This is what slippage is.
Although the clutch disc can rightfully be attributed to consumables, you can still slightly extend its resource if you follow some rules. For example, the wear of the clutch is greatly accelerated by the bad habit of some drivers, who, starting from a place, gasp a lot and at the same time abruptly release the clutch pedal.
Fast driving in low gears is no less detrimental to the clutch. In both cases, the driven disc slips for some time and is erased unnecessarily.
Another habit is to keep the clutch pedal depressed at traffic lights or in a traffic jam - although it does not spoil the disc, it contributes to the wear of the spring and release bearing. Getting rid of these bad habits will extend the life of your device and save you money.
The correct way to start driving is to smoothly release the clutch pedal and only then gradually start to put pressure on the gas. And it is better to press the clutch, on the contrary, sharply.
Another possible cause of slippage is grease getting on the clutch disc or flywheel. This sometimes happens if the crankshaft oil seal is worn out. In this case, the mating surfaces of the flywheel and the driven disc can be flushed with a suitable agent such as kerosene. The friction linings should then be lightly cleaned with fine emery paper.
If the clutch has already started to slip, but there is still some reserve of the friction layer (more than 0,2 mm), you can try to adjust the pedal free travel. The corresponding procedure is usually described in the vehicle repair and maintenance manual. Often, this operation allows you to postpone the repair of this unit.
If the pads are worn down almost to the rivets, then you should not pull with the replacement of the disc. When the friction linings are level with the rivets, they will begin to rub against the surface of the flywheel, scratching it. As a result, the flywheel may need to be replaced.
Wear of other clutch parts - release bearing, diaphragm spring, release fork - can also lead to slipping. Their service life is comparable to the disk resource. Therefore, if one of the parts needs to be replaced, and the clutch as a whole has gone about 70...100 thousand kilometers without repair, then it is better to replace the entire assembly. This will save time, effort and money. You can purchase it at reasonable prices in the online store.
And another culprit for slipping may be the clutch actuator. The reasons are different depending on the type of drive. This can be, for example, a damaged lever, a broken or jammed cable. If the drive is hydraulic, it is necessary to check the entire system for leaks of working fluid or pump it through, removing air from it.
In general, clutch repair can be a very difficult and time-consuming process that requires certain skills and experience. Special tools may also be needed. If you are not sure of your own capabilities, it is better to immediately contact a car service.
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