Crankshaft

Definition and Function

A crankshaft is a key component in piston engines, converting the up-and-down (reciprocating) motion of pistons into rotational motion. This rotation drives the vehicle's wheels or other machinery, such as generators. It is typically located within the engine block and connected to pistons via connecting rods.

Materials and Manufacturing

Crankshafts are commonly made from steel or cast iron, chosen for their strength and durability. Manufacturing processes include forging (heating and shaping metal), casting (pouring molten metal into molds), and machining (cutting and shaping from a solid block). Forging, especially, is often used for high-performance applications due to its strength.

Introduction to Crankshafts

A crankshaft is a mechanical component integral to the operation of piston engines, particularly in internal combustion engines found in vehicles, boats, and generators. Its primary function is to transform the linear, reciprocating motion of pistons—driven by fuel combustion—into rotational motion. This rotational energy is then transmitted through systems like the flywheel, transmission, and drivetrain to propel the vehicle or power machinery. The crankshaft's role is essential, as it directly influences engine performance, efficiency, and longevity.

Function and Operation

The crankshaft operates by connecting to the pistons via connecting rods, which transfer the piston's up-and-down movement to the crankshaft's crankpins. These crankpins, offset from the crankshaft's main axis, create a rotational force as the pistons move. This process is often described as converting linear motion into rotary motion, a fundamental mechanism in reciprocating engines. For example, in a car engine, the crankshaft's rotation turns the clutch and gearbox, ultimately driving the wheels.

The distance between the center of the main journal and the crankpin, known as the crank radius or throw, determines the engine's stroke length—the distance the piston travels from bottom dead center to top dead center. This stroke length is critical, as it affects the engine's torque and power output. Modern engines are often "over square" or short-stroke, where the stroke is less than the cylinder bore diameter, optimizing for higher RPMs. Conversely, increasing the stroke, known as "stroking," can enhance low-RPM torque, though it may increase vibration at high speeds due to higher piston velocities.

A flywheel is typically attached to one end of the crankshaft to smooth out power delivery and reduce vibrations, especially in four-stroke engines where power pulses can be uneven. Additionally, torsional or vibrational dampers may be used to mitigate stresses caused by cylinders farthest from the output end acting on the crankshaft's elasticity.

Materials and Design

Crankshafts are subjected to enormous stresses, with forces up to 8.6 tonnes (19,000 pounds) per cylinder, necessitating robust materials and precise design. Common materials include:

1. Steel: Preferred for high-performance and durability, often used in forged crankshafts. Vanadium micro-alloyed steels are popular due to their air-cooling properties post-forging, reducing the need for additional heat treatment except for bearing surface hardening.
2. Cast Iron: Used in cheaper production engines, particularly ductile iron, which offers good shock absorption and wear resistance. Since the invention of nodular cast iron in 1947, its tensile strength has increased to 600–900 MPa, approaching carbon steel levels.

The design varies based on the engine type. Single-cylinder engines have simpler crankshafts, while multi-cylinder engines, especially V8s, require more complex designs. For instance, most production V8 engines, like the Ford Modular and General Motors LS engines, use a cross-plane crank with throws spaced 90 degrees apart, while high-performance engines like the Ferrari 488 use a flat-plane crank with 180-degree spacing, resembling two inline-four engines sharing a crankcase.

Manufacturing Processes

The manufacturing of crankshafts involves several methods, each with advantages depending on the application:

1. Forging: This process involves heating a steel ingot to about 2,200°F, making it malleable, and then pressing it into shape using dies. Forging can be done by hammering or pressing, with options for twisted or non-twisted methods. Twisted forging involves forging one crank throw at a time and twisting the crank, while non-twisted forging shapes all throws simultaneously, reducing internal stresses and preserving grain structure. Forged crankshafts are known for higher metal density, greater strength, and resistance to flexing, making them ideal for high-stress conditions. The process includes subsequent machining for precision, such as shaping counterweights and drilling lubrication passages.
2. Casting: Involves pouring molten metal, typically ductile iron, into a mold to form the crankshaft. This method is simpler and cost-effective, suitable for lower-load engines. Cast crankshafts require less finish machining but may lack the strength of forged counterparts, especially under high stress.
3. Machining from Billet: Starts with a solid bar of high-quality vacuum-remelted steel, fully machined to shape. This method offers design flexibility and is used for custom or competition racing crankshafts, such as in motorsport. It involves demanding operations like rifle-drilling journals and shaping counterweights, enabled by advanced CNC machining. Billet crankshafts can cost a minimum of $3,000, reserved for applications requiring precise balance and performance.

The choice of process depends on factors like initial investment, manufacturing cost, and intended use. Forging is preferred for high-performance engines due to its strength, while casting is common in budget-friendly production engines. Modern advancements, such as multi-axis CNC machining and 3D-CAD design, have made billet machining more cost-effective, enhancing precision and reducing the need for subsequent balancing.

Historical Context

The concept of the crankshaft has ancient roots, with early evidence of crank mechanisms in Roman sawmills from the 3rd century AD, such as the Hierapolis sawmill. The crank-and-connecting rod system evolved significantly in the Islamic Golden Age, with Islamic scholar Al-Jazari credited with inventing the crankshaft in 1206. His work included incorporating the crank in water-raising machines, marking a pivotal advancement in mechanical engineering. By the Renaissance, figures like Leonardo da Vinci and Cornelis Corneliszoon van Uitgeest further developed crankshaft applications, such as in wind-powered sawmills, laying the groundwork for the Industrial Revolution.

Historically, crankshaft design has evolved to handle increasing engine speeds and compression ratios. For example, the shift from straight-eight engines in the 1950s was partly due to crankshaft flexing under high RPMs and compression, leading to the adoption of more rigid designs like V8s with cross-plane cranks.

Comparative Analysis: Forged vs. Cast Crankshafts

To illustrate the differences, consider the following table comparing forged and cast crankshafts:

This table highlights why forged crankshafts are preferred for demanding applications, while cast crankshafts suffice for less stressful environments.