What Is a Crankshaft?


Nov 19th, 2021

What Is a Crankshaft?

You can't avoid talking about a crankshaft when discussing engine components because it is an integral part of any engine model. 

So, What Exactly Is a Crankshaft?

The crankshaft is the part of the engine that converts the linear motion of the pistons into rotary motion. Pistons move in a repetitive up and down linear movement, and it is the crankshaft’s role to transform this reciprocating motion into rotation.

Connecting rods link the pistons to the shaft, which in turn is driven by a crank mechanism. Without the crankshaft, a vehicle will not move an inch even when the engine is running.

What Are Parts of a Crankshaft?

As the name suggests, the engine crankshaft is a shaft that consists of connecting rods, crank pins, and a set of cranks. Other parts are the main bearing journal, con rod journal, oil bores and counterweights, and a flywheel attached at the end of the crankshaft.

Let’s see how each of these parts contribute to the overall crankshaft function.

  1. Crankpin - aka crank journal, this is where the lower end of the connecting rod is attached. Since most car engines have four or more cylinders, the crankpin consequently serves more than one cylinder. In a V6 engine, for instance, a crankpin works for either one or two cylinders depending on the design. On the other hand, in a radial engine, each crankpin serves the whole cylinder row.
  2. Connecting rod - aka con rod, it joins the piston to the crankshaft. Alongside the crank, the connecting rod converts the back-and-forth motion of the piston into rotation. The rod rotates at both ends and transmits compressional and tensional forces from the piston.
  3. Main bearing journal - in the engine block, the crankshaft is mounted on the bearing journals. The number of main bearing journals in an engine depends on its design.
  4. Conrod journal - it determines the stroke of the engine. In turn, the stroke and number of cylinders determine the engine displacement. You can measure the crank radius by taking the distance from the centre of the conrod journal to the main bearing journal.
  5. Crankshaft counterweights - most engines, especially the V engines, have undesired vibration, and counterweights on the crankshaft are needed to counter the rocking motions. The counterweights balance the weights of the piston and connecting rods, evens the moments, and reduces vibrations.
  6. Oil bores - they supply the connecting rod and main bearing points with oil, while the conrod journal gets oiled from the main bearing journal through an oil channel in the crankshaft.

Functions of a Crankshaft: How Does a Crankshaft Work?

Converts linear motion into rotary motion

In our opening section, we’ve already talked about the primary crankshaft function, and that is - converting the reciprocal motion of the pistons to a rotation motion in a reciprocating engine. 

Here’s how a crankshaft works:

  • Portions of the shaft length are offset to form throws. Connecting rods are attached to these throws. 
  • As the pistons move back and forth, the connecting rods make the crankshaft move around and deliver the resultant rotational motion to the wheels.

Without the crankshaft, the back-and-forth motion of the piston can't be converted and transferred to the driveshaft.

Holds the flywheel in place

Additionally, the crankshaft holds the flywheel attached to it on one end. The flywheel acts as an energy reservoir that smoothens out the pulsing effect in the rotations and helps in maintaining a constant back-and-forth motion of the pistons.

Large engines have several cylinders to minimise the pulsating effect from individual firing strokes.

Drives and links other engine parts

Aside from this, the crankshaft helps drive other components of a vehicle engine including the camshaft and oil and water pumps.

It also acts as the link between the engine and the driveshaft, thereby transmitting power in the form of rotary kinetic energy. In other words, it connects the input and the output bodies of an engine.

As you can see from these functions, the crankshaft is a core component of any engine.

History of the Crankshaft

As the primary rotating part of an engine, the crankshaft’s historied past deserves a bit of highlighting. Or so we think, hence, this section. 

Let's consider the origins of both the crankshaft and the crank mechanism independently. Here they go:

Crank Mechanism

The Han Dynasty between 202 B.C. and 220 A.D. saw the introduction of hand-operated cranks. The cranks were widely used in agriculture and industries, but curiously, it was not used for converting rotational motion into reciprocal motion or vice versa.

Meanwhile, a study of the Roman Empire revealed that cranks taking different forms were developed and used to accomplish various tasks between the 2nd and 6th centuries. In the 5th century, the Celtiberians used a mounted handle of a hand mill, which operated as a crank.

In Medieval Europe, early works dating between the 10th and 13th century depicted cranks used in rotating wheels. In the 15th century, cranked rack-and-pinion devices called cranequins were used in crossbows to exert more force. Around the same time, crank reels were also used in the textile industry to wind skeins of yarn.


In the Book of Ingenious Devices, the Banū Mūsā brothers talked about several hydraulic devices that made use of an automatic crank. Two of these devices had an action that was akin to that of a crankshaft.

The crank, however, did not support full rotation and required modifications for it to fully operate as a crankshaft. Arab Engineer Al Jazari talked about a crank and connecting rod system in two of his water-raising machines, and historian Donald Hill credits him for inventing the crankshaft.

A paddleboat illustration by Italian physician and inventor Guido da Vigevano revealed compound cranks and gear wheels, which American historian Lynn Townsend identified as a crankshaft prototype. Leonardo da Vinci and a Dutch farmer Cornelis Corneliszoon also described it as a crankshaft.

Cornelis came up with a wind-powered sawmill which made use of a crankshaft. The crankshaft converted the windmill's rotation into a reciprocal motion that powered the saw, which is the converse of what a vehicle engine's crankshaft does.

As Europe moved into a period of industrial development, machine designs that made use of crank and connecting rods became popular and widespread.

Before the 1930s, phonographs came with cranks for winding their clockwork motors. Before the invention of electric starters, internal combustion engines relied on hand cranks to start.

That’s it, folks! We hope you learned a bit more about your car engine’s crankshaft today. For other car part articles, feel free to browse our library for references. For car parts you need and have a hard time looking for, you can use our Car Part finder. It’s free to use, so try it now!

By Sam O