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1、附 录Crankshaft design requirements and working conditionsCrankshaft is in constant cyclical changes in the gas pressure, reciprocating and rotating motion of the inertial force and the quality of their work under the joint action of the moment, so that both the torsion and bending the crankshaft, res
2、ulting in fatigue, stress state; internal imbalance of the engine crankshaft also withstand bending moment and shear force; not taken measures to make the crankshaft torsional vibration damping effect may also be a large amplitude torsional elastic torque. These loads are cross degeneration, may cau
3、se fatigue failure of the crankshaft. Practice shows that the bending has a decisive role in bending fatigue failure is the main failure modes. Therefore, the structural strength of the crankshaft bending fatigue strength is the focus, the crankshaft is designed to be committed to improving the fati
4、gue strength of the crankshaft. Crankshaft complex shape, stress concentration is very serious, especially in the connecting rod journal and the crank arm of the fillet and lubricants at the stress concentration near the exit hole is particularly prominent. Common crankshaft fracture, fatigue crack
5、begins with fillet and the hole place. Figure 7-1 shows the crankshaft bending fatigue and fatigue failure of the reverse situation. Root bending fatigue cracks in the surface of the fillet from the journal at the development of the crank, the crank is basically broken into 450; torsion fatigue dama
6、ge is usually bad from the machining start hole edge, about 450 cut into the crank pin. Therefore, in the design of the crankshaft, pay special attention to finding ways to ease stress concentration, strengthen the stress concentration. Crankshaft journal at a very high ratio of pressure to a large
7、relative velocity of sliding friction in the bearings in place. The bearings in the actual operation conditions changed conditions does not always guarantee a liquid friction, especially when the oil is not clean, the journal was a strong abrasive wear surface, making the actual life of the cranksha
8、ft greatly reduced. Therefore, the design, to wear to the friction surface, and the appropriate material bearing a good match. Crank in the crankshaft is the central link, the stiffness is very important. If the crankshaft bending stiffness, then the possible occurrence of more severe bending, the p
9、iston rod and bearing deterioration in working conditions greatly affect the reliability of these parts work and durability, even the crankcase is too large and the local stress cracking. Crankshafts torsional stiffness is poor, the working speed range may be a strong torsional vibration. Ranging fr
10、om noise, such as transmission gear on the crank to accelerate the wear; while in the crankshaft fracture. Therefore, the design should ensure it has the highest possible bending stiffness and torsional stiffness. As the crankshaft by the power complex, geometric cross-section shape is rather specia
11、l, in the design, has yet to reflect the objective reality of a theoretical formula for Universal. Therefore, the current design of the crankshaft design relies mainly on experience.曲轴的工作条件和设计要求曲轴是在不断周期性变化的气体压力、往复和旋转运动质量的惯性力以及它们的力矩共同作用下工作的，从而使曲轴既扭转又弯曲，产生疲劳应力状态；对内不平衡的发动机曲轴还承受内弯矩和剪力；未采取扭转振动减振措施使曲轴还可能作
12、用着幅值较大的扭转振动弹性力矩。这些载荷都是交变性的，可能引起曲轴疲劳失效。实践表明，弯曲载荷具有决定性作用，弯曲疲劳失效是主要破坏形式。因此曲轴结构强度的研究重点是弯曲疲劳强度，曲轴设计上要致力于提高曲轴的疲劳强度。曲轴形状复杂，应力集中现象相当严重，特别在连杆轴颈与曲柄臂的过渡圆角处和润滑油孔出口附近的应力集中尤为突出。通常的曲轴断裂、疲劳裂纹都始于过渡圆角和油孔处。图7-1表明了曲轴弯曲疲劳破坏和扭转疲劳破坏的情况。弯曲疲劳裂缝从轴颈根部表面的圆角处发展到曲柄上，基本上成450折断曲柄；扭转疲劳破坏通常是从机械加工不良的油孔边缘开始，约成450剪断曲柄销。所以，在设计曲轴时，要特别注意设
13、法缓和应力集中现象，强化应力集中部位。曲轴各轴颈在很高的比压下，以很大的相对速度在轴承中发生滑动摩擦。这些轴承在实际变工况运转条件下并不总能保证为液体摩擦，尤其当润滑油不洁净时，轴颈表面遭到强烈的磨料磨损，使得曲轴的实际使用寿命大大降低。所以，设计时，要使其各摩擦表面耐磨，并匹配好适当材料的轴瓦。曲轴是曲柄连杆机构中的中心环节，其刚度亦很重要。如果曲轴弯曲刚度不足，则可能发生较剧烈的弯曲振动，使活塞连杆和轴承的工作条件大为恶化，影响这些零件的工作可靠性和耐久性，甚至使曲轴箱局部应力过大而开裂。曲轴的扭转刚度差，则可能在工作转速范围内产生强烈的扭转振动。轻则引起噪音，加速曲柄上齿轮等传动件的磨损；重则使曲轴断裂。所以，设计时，应保证它有尽可能高的弯曲刚度和扭转刚度。由于曲轴受力复杂，几何断面形状比较特殊，在设计时，至今还没有一个能反映客观实际的理论公式可供通用。因此，目前曲轴的设计主要是依靠经验来设计。
