Classification and materials of crankshaft castings

As the core part of internal combustion engine, crankshaft plays an important role. Due to the different number and size of crankshaft cranks, and many kinds of cranks, it is applied to all kinds of gasoline and diesel engines. The crankshaft is mostly forged parts and casting parts. The shape of the crankshaft parts is relatively complex, the structure is long and thin, and there are many cranks. When the crankshaft works, it needs to bear large torque and alternating bending stress, so the crankshaft is prone to wear and fatigue fracture. In addition, there are strict requirements for the processing of crankshaft, which requires that the main journal and connecting rod journal have different shafts, but the axes should be parallel. The requirements for the angle between the cranks of different crankshafts may be different. When machining the connecting rod journal, it is also difficult to locate the angle, so it brings many manufacturing difficulties.

As cast crankshaft for automobile, the overall dimension of crankshaft is ∅ 656 * 190. The size of main journal is ∅ 85, the size of connecting rod journal is ∅ 70, the wall thickness of balance block is 25mm, with casting fillet, and the casting quality is 44kg. The three-dimensional model of the crankshaft is as shown in the figure

The crankshaft can be divided into the following two types according to the support mode of crankshaft journal.

(1) Fully supported crankshaft: the main journal is set between adjacent cranks, which is mostly used in diesel engines. Advantages: the crankshaft has excellent rigidity and is not easy to bend. Disadvantages: due to the increase of cylinder center distance, the body needs to be lengthened, otherwise the manufacturing cost will increase.

(2) Non fully supported crankshaft: classified according to the number of main journals not more than the number of connecting rod journals. Commonly used in small and medium power gasoline engines, the advantages and disadvantages of full support crankshaft are opposite.

It can also be divided into integral type and combined type: the integral crankshaft has compact structure, good mechanical properties and wide application; the combined crankshaft has three types: disc type, sleeve type and segmented type. Although the manufacturing of the combined crankshaft is simple, part of the damage can be replaced, but the structure is more complex, and the machining accuracy is required to be higher.

Because the crankshaft needs to bear great torque and continuous bending stress when working, its main failure forms are fatigue fracture and serious wear of journal, so the requirements for crankshaft materials are high, including: good mechanical properties, good fatigue resistance, so as to prevent fatigue fracture and improve service life.

The commonly used materials of crankshaft are nodular cast iron, Quenched and tempered steel and non quenched and tempered steel.

For the crankshaft of gasoline engine, the power requirement is not high. The crankshaft blank can be cast by nodular cast iron. The commonly used ductile iron materials are: QT600-3, QT700-2, qt800-2, QT800-6, qt900-6, etc.

For the crankshaft material of diesel engine, the blank is made of quenched and tempered steel or non quenched and tempered steel. The common materials of quenched and tempered steel are: 40, 45, 40Cr, 42CrMo; for non quenched and tempered steel, 48MnV, c38n2, 38mns6 are used.

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