What causes the clutch to sink and the difficulty of shifting gears after a traffic jam?

2023-08-10

Have you ever encountered such a situation when driving a manual transmission car:
1. The clutch suddenly becomes very heavy, not only is it very difficult to step on, but also the joint and separation points of the clutch are particularly blurred;
2. It is easy to have difficulty in shifting gears after a traffic jam or long-term semi-linkage.

If it is, then it is likely that the Clutch Pressure Plate has "decayed". At this time, the only way to solve the problem is to replace the clutch pressure plate.
What is "clutch pressure plate failure"?
Need to start from the structure of the clutch
The pressure plate is an important part of the manual transmission clutch. It has a diaphragm spring on it, which is responsible for the separation and combination of the clutch.

Clutch Cover

The above is a schematic diagram of the structure of the manual clutch, and the red parts are the flywheel and the pressure plate. The flywheel is connected to the crankshaft of the engine, and the pressure plate is fixed on the flywheel with screws. When the engine is started, the flywheel and the pressure plate rotate accordingly. The green part is the Clutch Plate and the gearbox input shaft. The flywheel and the pressure plate clamp the clutch plate in the middle, and the power is transmitted to the clutch plate by friction, and the clutch plate transmits the power to the gearbox through the gearbox input shaft.

As shown in the figure above, it is a diaphragm spring. Diaphragm springs provide elastic force so that the pressure plate presses the clutch plate tightly against the flywheel. When we step on the clutch, the diaphragm spring springs up, the pressure plate releases the pressure on the clutch plate, and the power transmission is interrupted.

The decline of the pressure plate is actually the decline of the diaphragm spring. The diaphragm spring of the pressure plate is made of steel. When the clutch is stepped on, the diaphragm spring forms a lever structure with the fixed pin as the fulcrum. The clutch fork pushes the release bearing to press down one end of the diaphragm spring. At this time, the other end of the diaphragm spring starts to lift under the action of the lever force, and the pressing force of the friction plate on the clutch plate is reduced. Due to the longer moment arm on the side of the release bearing, the clutch pedal will be particularly light.

In addition, the diaphragm spring is a whole stamped and formed by a whole steel sheet. After being installed on the pressure plate, there is a large internal stress between the diaphragm springs. When the clutch is stepped on to a certain extent, the internal stress of the diaphragm spring and the separation of the bearing It pops fully up very easily under pressure, at which point the clutch disengages completely. This is why we have an obvious sense of separation and integration when we step on the clutch.

If the clutch is used normally, this characteristic of the pressure plate diaphragm spring will remain well until the clutch plate wears to the limit, and the clutch pedal will not become significantly heavier.

But in reality, there may be various problems that can cause the clutch to overheat, and the most heat-resistant clutch is the clutch plate and diaphragm spring. The overheating of the clutch plate may burn out, and the overheating of the diaphragm spring will cause the burning loss of rigidity.

Clutch Disc

After the diaphragm spring burns out, its hardness and rigidity will decrease. At this time, when the clutch is stepped on, the diaphragm spring will be deformed due to its lack of rigidity, and the other end cannot be lifted normally to release the pressure. When the diaphragm spring bends, the moment arm of the lever becomes shorter, and it takes more force to pop the other end of the diaphragm spring, so the pedal becomes heavier. And because the deformation of the diaphragm spring acts as a buffer, the action of clutch disengagement also becomes particularly blurred.

After the diaphragm spring is burned out, the clutch can barely work, because the diaphragm spring still has a certain degree of rigidity when the car is cold. At this time, the clutch pedal is heavier, but as long as you step on it hard, the clutch can still be completely disengaged. So at this time, it's just heavy to step on, but the separation is no problem.

When the clutch is in a traffic jam for a long time, due to frequent semi-linkage, the temperature of the clutch rises rapidly. At this time, the temperature of the diaphragm spring rises and the stiffness further decreases, which is prone to incomplete separation. The specific manifestation is that the gear is engaged. difficulty. At this time, even if the clutch is pressed to the bottom, the diaphragm spring is not rigid enough to completely disengage the pressure plate.

The only solution when this happens is to replace the clutch pressure plate. Moreover, the annealing of the pressure plate is generally caused by the poor rotation of the release bearing, so if you want to change it, you must at least replace the release bearing together, otherwise the new pressure plate will be damaged again within a few days.

Next: What are the symptoms of severe wear of the clutch pressure plate?