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What is a Crankshaft?

  • Author :
  • TATA AIG Team
  • Published on :
  • 07/05/2024

A crankshaft is a fundamental component of an internal combustion engine. Its primary role is to transform the pistons' up-and-down movement into a circular motion. This is crucial because wheels rotate, and the engine needs to convert this type of motion to make the vehicle move.

Let’s discuss the crankshaft’s meaning through its types, components, and mechanism.

Types of Crankshaft

Before we discuss the crankshaft function in the engine, let’s discuss its types:

1. Fully Built Shaft

A 'Fully Built Shaft' crankshaft has crankpins, main journals, and webs as main components. These parts are usually made using steel casting or forging and are joined together using shrink fitting. This type of crankshaft was common in older engine models but has seen reduced usage due to issues with grain flow and weakness at the joints.

2. Welded Shaft

This kind of shaft is created by joining multiple parts through welding techniques such as friction welding. In this, parts are pressed together with high-speed rotation to create friction that heats the parts to a plastic-like state and allows them to be forged together.

3. Forged Crankshaft

They are made by compressing heated steel billets into shape with forging dies. This process creates a uniform grain structure, producing a more robust and durable crankshaft.

You can find this crankshaft in high-performance engines due to their exceptional strength and capability to handle higher loads.

4. Solid Single Piece Crankshaft

They are formed from a single piece of material through forging or casting. Their design can endure the load during firing and cyclic tension, making them suitable for both high-speed and low-speed engines.

5. Billet Crankshaft

Billets are machined from a single solid piece of premium alloy steel. They offer unmatched toughness and are custom-made to match specific project requirements. They are known for their low drag coefficient and are often used in high-performance applications.

6. Cast Crankshaft

Cast crankshafts are produced by pouring molten material, typically cast iron, into a mould to create a raw casting. Due to their less precise grain structure, cast crankshafts may not be suitable for high-performance applications.

7. Semi-Built Shaft

This crankshaft is not fully assembled from many parts; crank webs are fully forged and then shrink-fitted to the bearing journal. This process results in a continuous grain structure over the crank web to offer better fatigue strength. You can find them in large marine engines. They are also preferable in modern engines due to their construction advantages.

Components in Crankshaft in Car

The key components of crankshaft are:

Main Journals: These are the primary supports for the crankshaft, allowing it to rotate within the engine block.

Flywheel Mounting Flange: The flywheel is a heavy wheel that stores rotational energy. The purpose of the crankshaft is to help smooth out the engine's power delivery.

Counterweights: Attached to the crankshaft, counterweights are used to balance the assembly. They offset the weight of the crankpins and connecting rods to reduce the vibration and stress on the crankshaft during operation.

Crank Pin: Also known as the rod bearing journal, the crank pin is where the big end of the connecting rod attaches. It transfers the piston's reciprocating motion into rotational motion at the crankshaft.

Crank Web: The crank web connects the main journals and crank pins. It forms the "arm" part of the crankshaft and is subjected to twisting forces during operation.

Thrust Washers: They control the crankshaft's axial movement or end play. They are located on either side of the main journals and help maintain the crankshaft's correct position within the engine block.

Oil Seals: They prevent oil from leaking out of the engine block and keep contaminants from entering. They are located at the ends of the crankshaft, where it protrudes from the engine block.

Understanding Crank Shaft Mechanism

Here is a Step-by-Step breakdown of how a crankshaft functions:

Step 1: Piston Movement: The process begins with the pistons moving up and down within the engine's cylinders due to the combustion of the air-fuel mixture.

Step 2: Connecting Rods: A connecting rod connects each piston to a crankshaft. As the pistons move, they push and pull on these rods.

Step 3: Crankpins and Journals: The connecting rods are attached to the crankshaft at points called crankpins. The crankshaft also has leading journals that rest on bearings, allowing it to rotate smoothly within the engine block.

Step 4: Conversion of Motion: The crankpins are offset from the axis of rotation of the car engine crankshaft. This offset causes the up-and-down motion of the pistons to be converted into a circular motion as the crankshaft turns.

Step 5: Power Stroke: During the power stroke, when the pistons are pushed down with great force, this motion is transferred to the crankshaft, causing it to rotate with significant power.

Step 6: Flywheel: A flywheel is attached at one end of the car engine crankshaft. This heavy wheel helps smooth out the engine's power delivery and maintain momentum.

Step 7: Rotational Motion: The crankshaft's motion is then transferred to the gearbox, which adjusts the speed and torque as needed before sending it to the vehicle's wheels.

Step 8: Engine Cycles: The crankshaft completes two full rotations for every power cycle in a four-stroke engine. On the other hand, a two-stroke engine completes one rotation per power cycle.

Step 9: Balancing: Counterweights are often added to the crankshaft to balance the assembly and reduce vibrations caused by the pistons' rotation and reciprocating motion.

Step 10: Lubrication: Oil is used to lubricate the crankshaft and its bearings, reducing friction and wear and ensuring smooth operation and a longer life for the engine components.

Step 11: Vibration Damping: A harmonic balancer or vibration damper may be attached to the crankshaft to reduce the torsional vibrations and increase the engine's reliability.

Step 12: Engine Output: Finally, the crankshaft's rotational motion ultimately drives the vehicle forward, making it an indispensable part of the engine's operation.


The crankshaft is an engineering marvel. It efficiently converts linear motion into rotational motion to enable vehicles to move. This component is important to understand about the engine’s operation, to diagnose any issue, optimise performance and for the vehicle’s safety and reliability.

Similarly, to ensure your vehicle’s, self and third-party safety, it is important to purchase a four-wheeler insurance policy. Car insurance offers financial coverage for damages sustained due to floods, fires, riots, or road mishaps.

Moreover, a comprehensive car insurance policy also covers injury to self and third-party liabilities in case of an unfortunate event caused due to your insured car.

If you are wondering about the best car insurance policy in India, you may consider Tata AIG.

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