What is a Crank Shaft ?


Oct 08th, 2019

What is a Crank Shaft ?

You can't fail to talk about a crankshaft when discussing engine components. The crankshaft is an integral part of any engine model. But what exactly is a crankshaft?

The crankshaft is the part of the engine which converts the linear motion of the pistons into a rotary motion. It is the primary rotating component of the engine. Pistons move in a repetitive up and down linear movement, also referred to as reciprocating motion.

The connecting rod links the pistons to the shaft. This shaft is in turn driven by a crank mechanism. Without the crankshaft, a vehicle wouldn't move an inch even when the engine is running.

History of the Crankshaft

Let's consider the origins of both the crankshaft and the crank mechanism independently.

Crank Mechanism

The Han Dynasty between 202 BC and 220 AD saw the introduction of hand-operated cranks. These cranks had different applications ranging from agricultural to industrial. One significant absent purpose of these cranks, however, was the conversion of rotational motion into reciprocal motion or vice versa.

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.

From early works dating back between the 10th and 13th century in Medieval Europe, cranks are depicted to have been used to rotate wheels. In the 15th century, cranked rack-and-pinion devices called cranequins were utilized 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. Thus, it required modifications for it to operate as a crankshaft fully. Al Jazari, an Arab engineer, 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.

Parts of the Crankshaft

The engine crankshaft is a shaft that consists of connecting rods, crank pins and a set of cranks depending on the model and configuration of the engine. Other parts are the main bearing journal, con rod journal, oil bores and counterweights. The flywheel attached at the end of the crankshaft makes the operation of the crankshaft complete.

Directly or indirectly, each of these constituents contributes differently to the working of the crankshaft.

1. Crankpin

Also known as the crank journal, this is where the lower end of the connecting rod is attached.

Since most car engines have four or more cylinders, the crank journal 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

Sometimes referred to as the con rod, it joins the piston to the crankshaft.

Alongside the crank, the connecting rod converts the back and forth movement of the piston into a rotary motion. 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

The conrod journal 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, still have undesired vibration. Counterweights on the crankshaft are therefore necessitated to compensate for the rocking motions.

The counterweights balance the weights of the piston and connecting rods, which in turn, evens the moments and reduces vibration amplitudes.

6. Oil bores

The oil bores supply the connecting rod and main bearing points with oil. The conrod journal gets its oil supply from the main bearing journal through an oil channel in the crankshaft.

Functions & Operation of the Crankshaft

Converting the reciprocal motion of the pistons to a rotation motion in a reciprocating engine is the primary function of the crankshaft. 

Portions of the shaft length are offset to form throws. The connecting rods are attached to these throws. As the piston moves 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.

Additionally, the crankshaft holds the flywheel attached to it on one end. The flywheel acts as an energy reservoir that smooths 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 minimize the pulsating effect from individual firing strokes.

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.

Sam O