The flywheel housing on the car is connected to the engine and transmission and takes part of the weight of the transmission while protecting the clutch and the flywheel. It is an important basic part. In use, flywheel shell cracking often occurs in heavy and medium-duty trucks and large and medium-sized passenger cars driven by front and rear drives. The cracked parts are: the screw hole of the fixed starter, the parting surface of the upper wall of the flywheel housing, and the screw hole of the flywheel housing and the body. After the flywheel shell cracks, it will destroy the coaxiality of the crankshaft and the first axis of the transmission, causing the clutch and starter to work abnormally.
Causes of cracks in the flywheel housing
The root cause of the flywheel shell cracking is that the flywheel shell is subject to abnormal vibration or torsion, and stress concentration occurs at the weak point. When the stress exceeds its strength limit or fatigue limit, the flywheel shell is cracked. The specific cause of cracks in the flywheel housing is relatively complex. It is often caused by the combined effects of defects in various parts of the car engine, drive train, and frame. Common influencing factors are:
1. The flywheel shell fastening bolts are loose. The flywheel shell fastening bolts are not tightened according to the specified sequence and torque, or the bolt spring washers fail. Under the combined action of the locomotive bumps and the engine and clutch vibrations, the bolts are loosened, so that a clearance is formed between the body and the flywheel shell and the impact load is generated. The shell is cracked by uneven force under the action of gravity and vibration force.
2, the crankshaft axial or radial clearance is too large. When the crankshaft main bearing clearance is too large, the high-speed rotating crankshaft deviates from the original axis and the radial runout is too large, resulting in knocking and vibration. When the thrust washer is damaged, the crankshaft axially oscillates to generate an impact, which may easily lead to cracks in the flywheel housing. .
3, crankshaft and flywheel shell coaxial degree of tolerance. If the rear end surface of the machine body is not perpendicular to the crankshaft centerline, the end face of the flywheel housing is not perpendicular to the axis of the crankshaft, etc. At this time, the distance between the flywheel centerline and the axis line of the electric starter changes. When the starter is used, additional force is generated between the flywheel ring gear and the drive gear of the electric starter, resulting in stress concentration of the flywheel housing. In addition, when the fitting size of the crankshaft flange and the flywheel aperture exceeds the allowable error, the flywheel generates a radial runout, which also causes stress concentration of the flywheel housing when the starter is used.
4, the transmission module balance is poor. The crankshaft flywheel, clutch and transmission shaft have undergone dynamic balance test before assembly, and balance blocks are mounted on the lighter parts of the clutch housing and the connecting bolts of the flywheel to make it run smoothly. According to the technical requirements, the unbalanced amount of the crankshaft flywheel set is less than 80 to 150g•cm for the truck, 10~50g•cm for the light truck, and less than 50~60g•cm for the drive shaft. When these drive components are out of balance or in poor technical condition, large centrifugal forces will be generated during operation, resulting in unstable operation and vibration, resulting in fatigue cracking of the flywheel housing.
5, improper manufacturing or processing. For example, if the positioning reference on the flywheel housing is incorrect, it will be deflected after installation; the front and rear end faces of the flywheel housing will not be parallel. The coaxial misalignment of the first shaft of the transmission and the crankshaft is not only easy to damage the clutch driven plate, but also generates an additional bending moment on the flywheel housing to accelerate the crack of the flywheel housing. In addition, poor flywheel shell material and casting defects (such as loose, too thin, etc.) are also the cause of cracking.
6, engine support failure. The front and rear ends of the engine are supported on the frame with bolts and cushions. If the fixing bolts or bracket rivets loosen or break off, vibration will occur between the engine and the frame. If the cushion fails, the engine and the frame will be rigidly connected, and vibration damping and cushioning will be lost, resulting in cracking of the flywheel housing.
7, frame deformation. If the car is hit by collision, overload impact, etc., the frame bending and twisting deformation, changing the relative position of the engine and the transmission, the flywheel housing will suffer additional bending stress and cracking.
8, improper use. Such as overloading, high-speed driving on bad roads, slippery slope start, loosening clutch pedals to overcome obstacles, etc., will accelerate the formation and development of flywheel shell cracks.
In summary, the causes of cracks in the flywheel housing are manifold. However, in practice, flywheel housing damage caused by crankshaft, clutch assembly overbalanced, flywheel housing shape position tolerance, transmission shaft imbalance, and frame deformation are the most common.
Flywheel shell crack prevention
In order to prevent cracks in the flywheel housing, attention should be paid to the following points in use and maintenance:
1. During the maintenance, the static balance of the crankshaft and the flywheel should be checked according to the technical requirements, and the balance between the flywheel and the clutch pressure plate assembly must be taken care not to miss the missing, missing or misplaced balance weights.
2. Install the flywheel, clutch and flywheel housing in strict accordance with the technical requirements to ensure that the coaxiality of the flywheel centerline, the crankshaft centerline, the flywheel housing centerline, and the transmission first axis is within the tolerance range.
3, when installing the flywheel housing should be aligned with the positioning pin, the flywheel shell and the body surface should be flat, the flywheel shell fixing bolt made of alloy steel, bolt head with a casting ring logo, can not be wrongly loaded, and the bolt should be in accordance with the order and torque Twist alternately.
4. When the crankshaft bearing clearance and the crankshaft axial movement are too large, the main bearing and the thrust washer should be checked or replaced and adjusted.
5. When assembling the drive shaft, pay attention to its arrow mark, do not install dislocation, so as to avoid jitter; the bending of the drive shaft should be adjusted, and the bearings and cross shafts should be replaced when they are worn loose.
6. The engine support should be firm and reliable. If the connecting bolt is loose, it should be tightened. If the support cushion is cracked or deformed or the elasticity is weakened, it should be replaced.
7. Regularly check the tightening of the bolts in the drive train and the flywheel housing. If any looseness, tighten it in time.
8, strictly in accordance with operating instructions and driving operations, to avoid speeding overload, pay attention to traffic safety.
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