Crankshaft fracture due to crack is a serious accident of engine. When the crankshaft will break, the engine will vibrate greatly, there will be heavy and dull abnormal sound, the lower crankcase will echo very loudly, and then the engine will stop running, the crankshaft has been completely broken. The reasons are as follows:

　　①曲轴折断主要是受弯曲和扭转作用而引起的，多发生在曲轴臂的中部、曲轴臂与连杆轴颈端部，或曲轴臂与曲轴轴颈的接合处。

　　(1) Crankshaft fracture is mainly caused by bending and torsion, which mostly occurs in the middle of the crankshaft arm, at the end of the crankshaft arm and the connecting rod journal, or at the joint of the crankshaft arm and the crankshaft journal.

　　②曲轴受扭力、推力和冲击载荷的交变作用，在有缺陷的部位产生异常的应力，使曲轴折断。

　　(2) The crankshaft is subjected to the alternating action of torsion, thrust and impact load, which produces abnormal stress in the defective part and breaks the crankshaft.

　　③连杆轴颈小于曲轴轴颈，因此，弯曲应力又以连杆轴颈侧面为大，故裂纹和折断多发生在该处。

　　(3) The connecting rod journal is smaller than the crankshaft journal, therefore, the bending stress is the largest on the side of the connecting rod journal, so cracks and breaks occur mostly in this place.

　　④光磨轴颈时，没有使轴颈与曲轴臂连接处保持一定的过渡内圆角。当内圆角磨损，引起应力集中，造成疲劳损伤而折断。

　　(4) When polishing the journal, there is no interior fillet between the journal and the crankshaft arm. When the inner corner is worn, the stress concentration will be caused, which will cause fatigue damage and fracture.

　　⑤气缸体变形，曲轴轴承座孔不正、不圆，修配曲轴轴承时各轴承不在同一轴线上。

　　_Cylinder block deformation, crankshaft bearing seat hole is not right, not round, when repairing crankshaft bearings, the bearings are not on the same axis.

　　⑥轴承径向间隙过小，轴瓦烧损，合金脱落，引起冲击载荷增大。轴瓦烧损后，仍继续行驶，促使曲轴裂断。

　　_The bearing radial clearance is too small, the bearing bush is burned out, and the alloy falls off, which causes the impact load to increase. After the Bush is burnt out, it still keeps driving, which causes the crankshaft to crack.

　　⑦机油道不通，曲轴处于干摩擦状态，温度过高，导致曲轴折断。

　　_The oil passage of the engine is blocked, the crankshaft is in the state of dry friction, and the temperature is too high, which causes the crankshaft to break.

　　⑧曲轴经常处在临界（危险）转速运转。

　　_Crankshaft is often running at critical (dangerous) speed.

　　⑨曲轴或飞轮的平衡遭到破坏，曲轴受很大的惯性力冲击，使曲轴疲劳而折断。

　　_The balance of crankshaft or flywheel is destroyed, and the crankshaft is impacted by great inertia force, which makes the crankshaft fatigue and break.

　　⑩曲轴轴线不正，使飞轮偏摆，跳动量超过规定，在惯性力作用下，使曲轴产生疲劳而折断。

　　_The crankshaft axis is not correct, which makes the flywheel sway and jump more than specified. Under the action of inertia force, the crankshaft will be fatigued and broken.

　　⑩曲轴后端与飞轮连接螺栓（母）松动，运转时曲轴发生抖动，使曲轴在尾端折断。

　　_The connecting bolt (mother) between the rear end of the crankshaft and the flywheel is loosened, and the crankshaft fluctuates during operation, which breaks the crankshaft at the rear end.

　　⑩曲轴材质不佳，或在制造时留有缺陷。或在曲轴轴颈修理时，超过缩小允许量。

　　_Crankshaft material is not good, or there are defects in manufacturing. Or when the crankshaft journal is repaired, the reduction allowance is exceeded.

　　⑩柴油机发生“飞车”捣缸事故引起曲轴折断。

　　_The crankshaft was broken due to the "flying car" cylinder tamping accident of diesel engine.