In the world of automotive engines, the crankshaft plays a crucial role. It acts like the heart of the engine, converting reciprocating motion into rotational motion, providing power for the car. However, not all crankshafts are the same; they differ significantly in material and manufacturing process. Today, we will take an in-depth look at the differences between cast and forged crankshafts, as well as their respective advantages and applications.
The crankshaft is one of the most important components in an engine. It bears the forces transmitted by the connecting rods and converts them into torque, which is output to drive other engine accessories. Imagine, without a crankshaft, the engine cannot convert the pistons' up-and-down motion into rotational motion, and the car cannot move.
During operation, the crankshaft is subjected to centrifugal forces from rotational mass, cyclic gas inertia forces, and reciprocating inertia forces, resulting in enormous bending and torsional loads. Therefore, the crankshaft must have sufficient strength and rigidity, with wear-resistant bearing surfaces, uniform operation, and good balance. Only in this way can the engine operate stably and efficiently.
Currently, automotive engine crankshafts are mainly made of ductile cast iron and steel. In China, more than 85% of crankshafts for small- and medium-power diesel engines use ductile cast iron. Ductile cast iron has good casting performance and certain strength, suitable for mass production. However, as market demands for engine quality increase, the blanks for some medium- and light-duty automotive engine crankshafts have gradually shifted to forged forms.
Forging is a process in which metal is deformed under pressure. The production process of a forged crankshaft usually includes heating the metal blank, then forming it under high pressure using a die. This method adjusts the metal's grain structure, thereby enhancing the crankshaft's strength and fatigue resistance. The forged crankshaft is then machined to achieve precise dimensions and surface finish required for optimal engine performance. Forged crankshafts have full internal metal flow lines, which can improve fatigue strength by more than 20%. This makes forged crankshafts highly favored in high-performance engine applications.
Casting is another common method for producing crankshafts. In the casting process, molten metal (such as cast iron) is poured into a mold shaped like the desired crankshaft. Casting allows relatively easy production of complex shapes and is more cost-effective for mass production. However, compared to forging, cast crankshafts may have slightly lower strength performance. To improve the performance of cast crankshafts, techniques such as heat treatment and surface modification are commonly used. For example, molten iron is melted in a cupola furnace, desulfurized outside the furnace, then heated and composition-adjusted in an induction furnace. In China, molten iron composition is commonly tested using vacuum direct-reading spectrometers, which helps improve the quality of cast crankshafts.
Forged crankshafts are considered a necessity rather than a luxury in high-performance engine applications. Their outstanding qualities include durability, the ability to withstand harsh conditions, enhanced strength, uniformity, and overall quality. Compared to rough-machined bar stock, these qualities make forged steel crankshafts the best choice for important applications.
The strength of forged crankshafts comes from the increased ductility and durability provided by the forging process. These characteristics make forged cranks stronger than cast crankshafts. The enhanced tensile strength and excellent fatigue performance of forged crankshafts allow them to withstand higher stress without failure, which contributes to improved engine performance. For example, in large marine diesel engines, forged crankshafts ensure the crankshaft can withstand the tremendous forces generated by the engine.
Producing forged crankshafts requires high-precision equipment and technology. Hot forging presses, due to their high guiding accuracy and ejection mechanisms, are the preferred equipment for manufacturing high-precision crankshafts. The die forging method shapes metal bars or ingots into crankshaft blanks through a series of forging dies. This method offers high productivity and material utilization, good metal flow lines, and more accurate crankshaft shape and dimensions. Compared to free forging, it greatly reduces machining time.
The production of forged crankshafts using automatic lines with hot forging presses or electro-hydraulic hammers is the development direction. These lines commonly adopt advanced processes such as precision shearing, roll forging (wedge cross-rolling), medium-frequency induction heating, and hydraulic press finishing, equipped with auxiliary devices such as robots, conveyors, and die-changing devices with turntables, forming flexible manufacturing systems (FMS). This advanced production method not only improves production efficiency but also ensures the quality and performance of the crankshafts.
Cast crankshafts also have unique advantages. First, cast crankshafts are low in manufacturing cost, suitable for mass production. For example, ordinary family cars generally use cast crankshafts, balancing economy and practicality. Cast crankshafts can be designed with more complex structures, such as hollow crankshafts, achieving weight reduction. This is particularly important for vehicles with strict weight requirements.
Cast crankshafts have short development cycles, high metal utilization, simple equipment, and superior product performance. For instance, airflow impact molding is significantly better than clay sand molding, producing high-precision crankshaft castings. Sand molds made by this process have minimal rebound deformation, which is especially important for multi-crank crankshafts. The use of electroslag remelting in crankshaft production allows cast crankshaft performance to potentially rival that of forged crankshafts.
However, cast crankshafts have limitations. Their strength is relatively low, and long-term operation under high load may lead to cracks or deformation. They have poorer fatigue resistance and are not suitable for extreme working conditions. Therefore, in high-performance engines (such as turbocharged or racing engines), forged crankshafts are preferred to ensure reliability and durability.
Whether forged or cast, crankshafts require extensive machining after initial forming to achieve precise dimensions and surface tolerances. These operations include turning, milling, drilling, and grinding. Main journals and crank pins, which contact bearings, must be machined with extreme precision. Surface finish in these areas is critical, as it affects friction and wear characteristics. Smooth surface finish reduces friction, improving engine efficiency and extending the service life of the crankshaft and bearings.
In addition, surface hardening processes, such as induction hardening, are often used to enhance wear resistance in critical areas. These post-processing steps ensure the crankshaft's performance and longevity in practical use.
Selecting a crankshaft requires consideration of the product's intended use. For high-performance engines, such as turbocharged or racing engines, forged crankshafts are the best choice. Their high strength and excellent fatigue resistance meet the demands of high-speed, high-power operation. The durability and reliability of forged crankshafts make them ideal for high-performance applications.
For ordinary family cars, cast crankshafts are usually a more economical choice. They have lower manufacturing costs and are suitable for mass production. Although cast crankshafts have lower strength and fatigue resistance than forged ones, they are sufficient for daily use in ordinary cars. In addition, cast crankshafts can be designed with more complex structures, achieving weight reduction, which helps improve fuel efficiency and performance.
The crankshaft is one of the most important components in an engine, converting reciprocating motion into rotational motion and providing power for the car. When choosing a crankshaft, factors such as engine application, performance requirements, and cost must be considered. Forged crankshafts, with high strength, excellent fatigue resistance, and durability, are favored in high-performance engines, but their production cost is high, and machining is complex and expensive. Cast crankshafts, with lower manufacturing costs, suitability for mass production, and ability to achieve complex structures, are widely used in ordinary family cars. Although their strength and fatigue resistance are lower than forged crankshafts, they are sufficient for daily use in ordinary cars.
In summary, both forged and cast crankshafts have their own advantages and suitable applications. Choosing a crankshaft requires making a wise decision based on specific needs and budget. Only in this way can engine performance and reliability be ensured, allowing vehicles to operate safely and efficiently on the road.
