What is the reason for the cracking of the flywheel housing?


Release time:

2022-08-16

The flywheel housing on the car connects the engine and the transmission, bears part of the weight of the transmission, and protects the clutch and flywheel. It is an important basic component. In use, especially for medium and light trucks with front and rear drive and large and medium passenger cars, the flywheel housing often cracks (especially the Dongfeng series). Crack parts: screw holes for fixing the starter, the parting surface of the upper wall of the flywheel housing, and the connecting screw holes between the flywheel housing and the body.

The flywheel housing on the car connects the engine and the transmission, bears part of the weight of the transmission, and protects the clutch and flywheel. It is an important basic component. In use, especially for medium and light trucks with front and rear drive and large and medium passenger cars, the flywheel housing often cracks (especially the Dongfeng series). Crack parts: screw holes for fixing the starter, the parting surface of the upper wall of the flywheel housing, and the connecting screw holes between the flywheel housing and the body.

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The root cause of the cracking of the flywheel shell is that the flywheel shell is subjected to abnormal vibration or torsion, resulting in stress concentration at the weak point. When the stress exceeds its strength limit or fatigue limit, the flywheel housing will crack. The specific reasons for the cracking of the flywheel housing are complex, which are often the result of the comprehensive influence of the defects of the automobile engine, transmission system, frame and other parts. Common influencing factors are:

1. Fastening bolts of flywheel housing are loose. The flywheel housing fastening bolts are not tightened according to the specified sequence and torque, or the bolt spring washer fails. Under the combined action of the turbulence of the locomotive and the vibration of the engine and the clutch, the bolts are loosened, causing a gap between the engine body and the flywheel housing, resulting in an impact load. The flywheel housing is not uniform under the action of gravity and vibration, resulting in cracks.

2. The axial or radial clearance of the crankshaft is too large. When the clearance of the main bearing of the crankshaft is too large, the high-speed rotation of the crankshaft deviates from the original shaft, and the radial runout is too large, resulting in knocking and vibration; when the thrust plate is damaged, the crankshaft moves axially to produce impact, which can easily cause the flywheel housing to crack.

3. The coaxiality of the crankshaft and the flywheel housing is out of tolerance. For example, the rear end face of the engine block is not perpendicular to the crankshaft centerline, so the end face of the flywheel housing is not perpendicular to the crankshaft shaft. At this time, the distance between the center line of the flywheel and the center line of the electric starter shaft changes. When using the starter, additional force will be generated between the flywheel ring gear and the drive gear of the electric starter, resulting in stress concentration in the flywheel housing. In addition, when the matching size between the crankshaft flange and the flywheel aperture exceeds the allowable error, the flywheel will produce radial circular runout, which will also cause stress concentration in the flywheel housing when the starter is used.

4. The balance of transmission parts is out of tolerance. The crankshaft, flywheel, clutch and drive shaft were subjected to a dynamic balance test before assembly, and a balance weight was installed on the lighter part of the connecting bolt between the clutch housing and the flywheel to make it run smoothly. According to the technical requirements, the imbalance of the crankshaft flywheel group of trucks is not more than 80~150g · cm, and that of light vehicles is not more than 10~50g · cm; the imbalance of the transmission shaft is generally not more than 50~60g · cm. When the dynamic balance of these transmission components is out of tolerance or the technical state is poor, large centrifugal force will be generated during operation, resulting in unstable operation and jitter, resulting in fatigue cracking of the flywheel housing.

5. Improper manufacturing, processing or assembly. If the positioning reference on the flywheel housing is incorrect, deflection will occur after installation; the front and rear end surfaces of the flywheel housing are not parallel. The coaxiality of the transmission shaft and the crankshaft is out of tolerance, which not only easily damages the clutch driven disc, but also generates additional bending moment on the flywheel housing and accelerates the cracking of the flywheel housing. In addition, poor flywheel shell material and casting defects (such as loose, too thin, etc.) are also the reasons for its cracking.

6. Engine bracket failure. The front and rear ends of the engine are supported on the frame by bolts and pads. If the fixing bolts and supporting rivets are loose or broken, vibration will occur between the engine and the frame; if the cushion fails, the engine and the frame will be firmly connected, losing the vibration reduction and buffering effect, causing the flywheel housing to crack.