Crankshaft Casting: A Comprehensive Guide

Crankshaft casting is a primary method for creating one of the most important components in a combustion engine. The crankshaft, along with a connecting rod, is a part that changes the up-and-down motion of pistons into the rotational energy that powers a vehicle. This transformation of energy from linear to rotational movement happens under great force. Because of this, the design and creation of a cast crankshaft is very important for the engine’s long-term performance.

The specific casting process used has a direct effect on the component’s strength, its balance, and its overall reliability. This guide will provide an overview of the various methods used in crankshaft casting, including sand casting, lost foam casting, shell mold casting, and investment casting.

Materials of Choice for Crankshaft Casting

The materials used for crankshaft casting are very important for how the final product performs. The choice of material affects the crankshaft’s strength, durability, and its ability to handle the forces inside an engine.

The two main materials used for a cast crankshaft are ductile iron and various types of steel. This section will look closely at each of these materials and why they are chosen for crankshaft casting. The choice of different casting methods depends on the size, material and tolerance requirements of the cast crankshaft.

Cast Iron

Cast iron has been a popular choice for crankshafts for many years, especially in high-volume production for cars and light trucks. The main type of cast iron used for this purpose is ductile iron. This material gets its name from the special way its graphite forms inside the metal. Unlike gray cast iron, where the graphite is in thin flakes, the graphite in ductile iron is in small, round spheres.

This spherical shape of the graphite helps stop cracks from forming and spreading. This unique structure, along with its great castability and ease of machining, makes ductile iron a reliable and cost-effective option for many crankshaft applications.

Common grades of ductile iron used for crankshafts are often defined by a series of three numbers, such as 60-40-18 and 80-55-06. These numbers refer to the material’s mechanical properties. The first number is the minimum tensile strength, the second is the minimum yield strength, and the third is the minimum percentage of elongation. For example, a grade of 60-40-18 means it has a tensile strength of 60,000 psi, a yield strength of 40,000 psi, and a minimum elongation of 18%.

Cast Steel

For applications that need higher performance, a cast crankshaft is often made from steel. Steel is an alloy of iron and carbon, but with a much lower carbon content than cast iron. This difference in composition gives steel greater strength and toughness. The process of casting steel can be more complex than casting iron, but it creates a finished part that can withstand very high stress and constant loads inside a powerful engine. There are a few different types of steel used for this purpose, each with its own special properties.

Carbon Steel

Carbon steel is a basic type of steel that contains carbon as its main alloying element. Its strength and hardness increase with its carbon content. Carbon steel casting is a popular method for producing cast crankshafts. It is a good material for a cast crankshaft in many standard and medium-performance engines where a balance of strength and affordability is needed. Carbon steel is a straightforward material that provides solid performance without the higher costs connected to more complex alloys. A common carbon steel casting grade used in casting is AISI 1045 carbon steel. This grade offers good mechanical properties and is a common choice for its balance of cost and function.

Alloy Steel

When a crankshaft must handle more demanding conditions, manufacturers often turn to alloy steel. This type of steel has other elements added to it besides carbon. These elements, which can include chromium, nickel, or molybdenum, are included to give the material specific qualities. For example, chromium can add resistance to corrosion and hardness, while molybdenum can help with strength and heat resistance. These additional elements allow the steel to be heat-treated to a higher hardness, giving the crankshaft more strength and a longer life under stress.

A widely used grade of cast alloy steel for crankshaft casting is 4140 steel. This grade contains chromium and molybdenum. These elements give the steel a good mix of strength, toughness, and resistance to wear, making it a good fit for high-performance engines. Another example is 4340 steel, which contains nickel, chromium, and molybdenum. This combination gives it even more strength and fatigue resistance for extremely demanding applications, such as in racing engines. The choice between these different alloy steels depends on the exact needs of the engine and the forces the crankshaft will face.

Stainless Steel

While not a typical material for a standard cast crankshaft, stainless steel can be used for special situations where a cast crankshaft needs to resist rust and corrosion. The main element in stainless steel is chromium, which forms a thin layer on the surface that protects it from the surrounding environment. This makes it an option for marine engines or other applications where the component is exposed to water or corrosive fluids. However, stainless steel may not have the same overall strength and fatigue resistance as other alloy steels that are specifically designed for the high stress of a crankshaft. A grade such as 316 stainless steel is known for its excellent resistance to corrosion.

The Casting Processes of Cast Crankshaft

Just as the material choice is important for casting crankshafts, the method used to cast a crankshaft is equally important for its final quality and performance. Each casting process has unique characteristics that affect everything from the part’s dimensional accuracy to its surface finish and internal structure. The process chosen depends on the required strength, the number of parts needed, the budget, and the specific design of the crankshaft. This section will explain in details the most common method for casting crankshafts, beginning with sand casting.

Sand Casting

Sand casting is one of the oldest and most widely used methods for casting metal parts. It is a very versatile process, and it is a popular choice for casting crankshafts for several reasons. The process starts with creating a pattern, which is a replica of the finished part. This pattern is then pressed into a special mixture of sand and a binding agent to form a mold. The mold is split into two halves (cope and drag), and the pattern is removed.

Once the two halves are put back together, a cavity in the shape of the crankshaft is left inside. Molten metal is then poured into this cavity. After the metal cools and solidifies, the sand mold is broken apart to reveal the finished crankshaft.

Sand casting is especially well-suited for producing large, complex parts like crankshafts because it is a very cost-effective process for high-volume manufacturing. The materials used, like sand, are cheap and can be reused. It is also an adaptable method that allows for complex shapes to be made without high tooling costs. However, there are some trade-offs. The surface finish on a sand-cast part is often rough and requires more machining and finishing work to meet the exact dimensions needed for an engine. Also, the lower cooling rate of sand can sometimes result in less precise grain structures and internal defects compared to other methods.

• Pros of Sand Casting: Sand casting is an especially attractive method for its cost-effectiveness, as it has low tooling and material costs that are ideal for large-scale production. This process is also highly versatile, capable of producing a wide range of complex shapes and sizes. Furthermore, it is a great method for creating very large parts that might be difficult to produce using other casting techniques.
• Cons of Sand Casting: Despite its benefits, sand casting does have some drawbacks. One major issue is the poor surface finish of the finished product, which often requires significant post-processing and machining to achieve the necessary specifications. The process also suffers from lower dimensional accuracy compared to other methods, as sand molds are less precise and can lead to a part that isn’t perfectly to size. Finally, there is a higher potential for internal defects, such as blowholes or sand inclusions, which can compromise the structural integrity of the final crankshaft.

Lost Foam Casting

Lost foam casting, also known as evaporative pattern casting, is a newer and more advanced method than traditional sand casting. For crankshaft casting, this process involves using a pattern made from a foam, typically expanded polystyrene (EPS). The foam pattern is a precise replica of the final crankshaft. This pattern is then coated with a special refractory ceramic slurry that is able to withstand high temperatures.

Once the slurry is dry, the foam pattern is placed into a flask and surrounded by unbonded sand. When molten metal is poured into the flask, it instantly vaporizes the foam pattern, and the molten metal flows into the space that was previously occupied by the foam. This process results in a part that is an exact replica of the original foam pattern.

The main advantage of lost foam casting for crankshaft casting is the high dimensional accuracy and the excellent surface finish of the final part, which can significantly reduce the need for later machining. Since the mold is formed around a single, solid foam pattern, there are no mold halves to join together, which eliminates the risks of misaligned parts and flash. The process is also great for making complex shapes with intricate internal details that would be very difficult to achieve with other methods. However, the process is more complex and the foam patterns themselves are not as durable as traditional patterns, making it a more costly and specialized method.

• Pros of Lost Foam Casting: Lost foam casting is highly valued for its ability to produce parts with exceptional dimensional accuracy and a superior surface finish. Since the foam pattern is a solid piece, it eliminates the need for separate mold halves and a core, which greatly simplifies the casting process and allows for very complex shapes to be made with a single pour. This also reduces the need for extensive post-casting machining and finishing, saving both time and money in the production cycle.
• Cons of Lost Foam Casting: Despite its advantages, lost foam casting can be a more costly process due to the expense of creating the foam patterns and the specialized equipment needed. The foam patterns themselves are also fragile and can be easily damaged, which means extra care must be taken during handling. This method is also primarily suited for high-volume production to justify the initial costs, and it may not be the best choice for all types of metal alloys.

Shell Mold Casting

Shell mold casting, AKA shell molding is a casting process that uses a thin, heat-resistant shell as the mold. This method is often used to produce a cast crankshaft because it offers a great balance of dimensional accuracy and a high-quality surface finish. The process starts by creating a metal pattern which is heated and then coated with a mix of special resin-coated sand. The heat causes the resin in the sand to harden around the pattern, forming a thin, rigid shell.

This process is repeated until a shell of the desired thickness is formed. The shell is then stripped from the pattern, and two halves are joined together to create the mold cavity. Molten metal is then poured into the cavity. Because the shell is much stronger and more rigid than a traditional sand mold, it can produce a part with very accurate dimensions and a smooth surface.

• Pros of Shell Molding: Shell molding is known for its ability to produce castings with a superior surface finish and excellent dimensional accuracy. This reduces the amount of post-casting machining and finishing required, which can lower production costs and speed up the manufacturing process. The process also allows for complex shapes and intricate details to be cast with high precision, making it a good choice for parts like crankshafts.
• Cons of Shell Molding: While shell molding offers many advantages, it does have some drawbacks. The cost of the tooling and the resin-coated sand can be higher than with traditional sand casting, which makes it less cost-effective for low-volume production. The process is also limited in the size of the parts it can produce, as creating large, thick shells can be difficult. Additionally, the heat generated during the process can create fumes from the resin, which requires proper ventilation and safety measures.

Investment Casting

Investment casting, also known as lost wax casting, is a very precise casting method that is used to cast crankshafts with a high degree of detail and dimensional accuracy. The investment casting process begins with creating a detailed pattern from wax. This wax pattern is then coated with a fine ceramic slurry, and this process is repeated several times until a thick, rigid shell is formed around the pattern.

The mold is then heated, which causes the wax pattern to melt and drain out, leaving a hollow ceramic shell. Molten metal is then poured into this hollow shell. After the metal solidifies, the ceramic shell is broken away to reveal the finished part. This method is particularly well-suited for producing complex, intricate, and thin-walled crankshafts (those used in robots or specialized machineries) that require minimal post-casting work.

• Pros of Investment Casting: Investment casting offers a number of significant advantages, most notably its exceptional dimensional accuracy and a very smooth, high-quality surface finish that often eliminates the need for machining. The process is also capable of producing extremely complex and detailed shapes that would be impossible to create with other casting methods. This level of precision makes it ideal for high-performance crankshafts where exact dimensions are critical.
• Cons of Investment Casting: The main drawback of investment casting is its high cost. The process is complex, time-consuming, and involves expensive materials for the wax patterns and ceramic shells. This makes it a less viable option for mass production of standard parts and is typically reserved for high-value components or specialty applications. The time required for creating the wax patterns and the multiple layering steps also means that the overall production cycle is longer compared to other methods.

Quality Control and Inspection for Cast Crankshafts

Quality control and inspection are critical steps in the production of a cast crankshaft to ensure that the finished part meets the necessary standards for performance, safety, and durability. After casting, crankshafts undergo a series of rigorous tests to identify any potential defects, dimensional inaccuracies, or material flaws. These inspections guarantee that the final component can withstand the extreme forces it will experience within an engine.

Visual Inspection

The first and most basic step in quality control is a visual inspection. Trained technicians carefully examine the surface of the crankshaft for visible defects. This includes checking for cracks, surface imperfections, or any signs of a flaw that might have occurred during the casting process. A thorough visual check is often the quickest way to spot major problems and is a necessary starting point before moving to more advanced testing methods.

Dimensional Inspection

Once a crankshaft passes a visual inspection, it moves on to dimensional inspection. This step is about checking the dimensional accuracy of the part to make sure it matches the engineering drawings and specifications precisely. Using advanced tools such as coordinate-measuring machines (CMMs) and micrometers, technicians measure key areas like the main bearing journals, rod journals, and overall length. This process is crucial to ensure that the crankshaft will fit and function correctly within the engine block and connect to other components accurately.

Non-Destructive Testing (NDT)

Non-destructive testing (NDT) is a group of techniques used to inspect a cast crankshaft for internal and external flaws without causing damage to the part. This is an essential part of the inspection process for high-performance applications. One common NDT method is magnetic particle inspection, where a magnetic field is applied to the part and fine iron particles are used to reveal surface cracks. Another method is ultrasonic testing, which uses high-frequency sound waves to find internal voids, porosity, or other flaws within the metal that are not visible from the outside.

Metallurgical Analysis

The final step in quality control is metallurgical analysis, which is used to verify the material properties of the cast crankshaft. This analysis involves taking a small sample from the finished part to examine its microstructure and chemical composition. Tests are performed to ensure the material has the correct hardness, tensile strength, and other mechanical properties required by the design. For ductile iron crankshafts, a metallurgical analysis will confirm that the graphite is in the correct spherical form, which is vital for the part’s strength and ductility.

Industries That Use Cast Crankshafts

Cast crankshafts are a foundational component in a wide range of industries that rely on internal combustion engines for power. The versatility and cost-effectiveness of casting methods make them a popular choice for manufacturers seeking to balance performance with production efficiency.

Automotive Industry

The automotive sector is the largest consumer of cast crankshafts. The vast majority of mass-produced cars and light trucks use cast ductile iron crankshafts. This material is strong enough to meet the demands of everyday driving while being affordable to manufacture in large quantities. The ability to produce complex designs efficiently is a major benefit for this industry, where high-volume production is key.

Marine Industry

Marine engines, from small outboard motors to large ship engines, also use cast crankshafts. In this industry, the choice of material is crucial for resisting corrosion in harsh, saltwater environments. Cast steel, especially stainless steel, is often preferred for its durability and resistance to rust, ensuring the engine can withstand the elements and provide a long service life at sea.

Industrial Machinery

Industrial machinery, such as generators, pumps, and compressors, frequently uses cast crankshafts. These applications require robust, reliable parts that can handle continuous operation under heavy loads for extended periods. Cast ductile iron and alloy steel are common choices because they provide the necessary strength and resistance to fatigue, ensuring a long service life for the machinery and minimizing downtime.

Agricultural Machinery

Tractors and other agricultural equipment rely on powerful internal combustion engines to perform demanding tasks in tough environments. The crankshafts in these engines must withstand constant stress and significant forces. Cast ductile iron is a popular choice for agricultural machinery because of its combination of strength, durability, and cost-effectiveness, making it a reliable component for everything from plowing to harvesting.

Power Generation

Large-scale power generators, whether for backup power or primary energy production, rely on massive internal combustion engines. The crankshafts in these engines are often cast from alloy steel to withstand the immense forces and high rotational speeds, providing the reliability needed for a continuous and stable power supply.

Conclusion

In conclusion, the production of a high-quality cast crankshaft is a complex process that relies on careful material selection, precise casting methods, and rigorous quality control. Whether a manufacturer chooses ductile iron for its cost-effectiveness or cast steel for its superior strength, the goal is always to create a reliable component for its specific application. The various casting processes each offer different advantages, and the final part must undergo a comprehensive inspection program. This includes checking for defects, verifying dimensional accuracy, and using advanced methods like non-destructive testing to ensure the crankshaft can withstand the extreme forces within an engine.

A crankshaft can be manufactured through both casting and forging, but the two methods can result in significant differences in terms of performance, cost, production lead times, and other factors. If you are interested in how both processes compare for the production of crankshafts, check out our comparison article: ‘Cast Vs Forged Crankshaft: Which Should You Choose?‘.

Taiyuan SIMIS Investment Casting Co., Ltd. is a professional casting manufacturer in China, offering high-quality casting services for producing crankshafts. At SIMIS, we pride ourselves on our robust casting capabilities, which can accommodate various crankshaft casting needs, including different sizes, weights, designs, and strict tolerance and machining requirements.

We are equipped with not only advanced casting equipment, personnel, and facilities, but we also offer a complete range of value-added services such as tooling, machining, surface treatments, heat treatments, and comprehensive testing and inspection. This ensures we deliver the highest possible quality and defect-free cast crankshafts.

Contact us today for more information.