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曲軸折斷主要是受彎曲和扭轉作用而引起的，多發生在曲軸臂的中部、曲軸臂與連桿軸頸端部，或曲軸臂與曲軸軸頸的接合處。

　　(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曲軸受扭力、推力和沖擊載荷的交變作用，在有缺陷的部位產生異常的應力，使曲軸折斷。

　　(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連桿軸頸小于曲軸軸頸，因此，彎曲應力又以連桿軸頸側面為大，故裂紋和折斷多發生在該處。

　　(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光磨軸頸時，沒有使軸頸與曲軸臂連接處保持一定的過渡內圓角。當內圓角磨損，引起應力集中，造成疲勞損傷而折斷。

　　(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氣缸體變形，曲軸軸承座孔不正、不圓，修配曲軸軸承時各軸承不在同一軸線上。

　　_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軸承徑向間隙過小，軸瓦燒損，合金脫落，引起沖擊載荷增大。軸瓦燒損后，仍繼續行駛，促使曲軸裂斷。

　　_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機油道不通，曲軸處于干摩擦狀態，溫度過高，導致曲軸折斷。

　　_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曲軸經常處在臨界（危險）轉速運轉。

　　_Crankshaft is often running at critical (dangerous) speed.

　　⑨曲軸或飛輪的平衡遭到破壞，曲軸受很大的慣性力沖擊，使曲軸疲勞而折斷。

　　_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曲軸軸線不正，使飛輪偏擺，跳動量超過規定，在慣性力作用下，使曲軸產生疲勞而折斷。

　　_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曲軸后端與飛輪連接螺栓（母）松動，運轉時曲軸發生抖動，使曲軸在尾端折斷。

　　_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曲軸材質不佳，或在制造時留有缺陷。或在曲軸軸頸修理時，超過縮小允許量。

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

　　⑩柴油機發生“飛車”搗缸事故引起曲軸折斷。

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