What Is a Crankshaft? | How Crankshaft Works?

What is Crankshaft?

A crankshaft is a shaft driven by a crank mechanism, consisting of a series of cranks and crankpins to which the connecting rods of an engine are attached. It is a mechanical part able to perform a conversion between reciprocating motion and rotational motion.

The main purpose of this connecting rod is to absorb the reciprocating motion of the piston and pass it on to the crankshaft. When the crankshaft is moved by the connecting rod, it converts that movement into rotary motion and turns the flywheel, which continues to move the vehicle wheels.

Without a crank, a reciprocating piston engine is not able to transmit the piston reciprocating movement to the drive shaft. In simple terms, a reciprocating engine cannot move a vehicle without a crankshaft.

Various engines go through a power cycle with different numbers of crankshaft revolutions. For example, a 2-stroke engine completes a power cycle after one revolution of the crankshaft, while a 4-stroke engine completes a power cycle after the completion of two revolutions of the crankshaft.

Crankshafts can be welded, semi-integrated, or one-piece. This component of the engine connects the output section of ​​the engine to the input section.

The crank acts as a link that supplies the output power in the form of rotational kinetic energy – the piston is connected to the center of the crank via a connecting rod. The crank arm allows the piston to rotate the crankshaft to generate force to move the vehicle.

How crankshaft works?

Basically, the crankshaft performs a simple task: translate the linear motion of the pistons into rotation. It’s doing the same job as the crank arm of a bicycle, which turns the more-or-less up and down motion of your legs into rotation.

Although the principle is simple, there are complications aplenty when it comes to high-performance engines. The combustion of fuel shoots the piston straight through the cylinder, it is the job of the crankshaft to convert this linear motion into the rotation – basically by rocking the piston back and forth in the cylinder.

The terminology of a crankshaft is quite specific, so let’s start by naming a few parts. A journal is the part of a shaft that rotates in a bearing. As can be seen above, there are two types of journals on a crankshaft – the main journal journals form the axis of rotation of the crankshaft and the connecting rod journals are attached to the ends of the connecting rods that run-up to the pistons.

For additional confusion, the connecting rod journals are abbreviated as connecting rod journals and are also generally referred to as crank journals or connecting rod journals. The rod journals are connected to the main journals by webs.


The distance between the center of the main bearing journal and the center of the crankshaft journal is referred to as the crank radius, also known as the crank stroke. This measurement determines the stroke range of the piston when the crankshaft rotates – this distance from top to bottom is called the stroke. The stroke of the piston is twice the crank radius.

The rear end of the crankshaft extends outside the crankcase and terminates with a flywheel flange. This precision-machined flange is bolted to the flywheel, the heavy mass of which helps smooth out the pulsation of the pistons at different times. The flywheel transfers the rotation to the wheels via the gearbox and the final drive.

In an automatic, the crankshaft is bolted to the ring gear that carries the torque converter and transfers the drive to the automatic transmission. This is basically the power of the engine – and the energy is directed where it is needed: propellers for boats and airplanes, induction coils for generators, and to the road wheels in a vehicle.

The front end of the crankshaft, sometimes called the nose, is a shaft that extends beyond the crankcase. This shaft connects to a gear that drives the valve train through a toothed belt or chain [or gear sets in high-tech applications] and a pulley that uses a drive belt to power accessories such as the alternator and water pump.

Parts of Crankshaft

Following are the main parts of the crankshaft with its diagram:

  • Crankpin
  • Main journals
  • Crank web
  • Counterweights
  • Thrust washers
  • Oil passage and oil seals
  • Flywheel mounting flange
Parts of Crankshaft

1. Crank Pin

The crankpin is a mechanical part of an engine. This allows the connecting rod to be attached very firmly to the crankshaft.

The surface of the crankpin is cylindrical in order to transmit the torque to the large end of the connecting rod. These are also known as connecting rod bearings.

2. Main Journals

Journals are attached to the engine block. These bearings hold the crankshaft and keep it rotating in the engine block. This bearing is, for example, a plain bearing or journal bearing. The main bearings vary from engine to engine, often according to the forces exerted by the engine. 

3. Crank Web

The Crank web is the most essential part of the crankshaft. The Crank web connects the crankshaft to the main bearing journals.

4. Counterweights

The counterweights are a type of weight that applies an opposing force that gives balance and stability to the crankshaft. These are mounted on the crank web.

The reason for adding counterweights to the crankshaft is so that they can eliminate the reaction caused by the rotation. And it is very helpful to get the higher RPM and keep the engine running easily.

5. Thrust Washers

At some points, two or more thrust washers are provided to stop the crankshaft from moving lengthways. These thrust washers assemble among the machined surfaces in the web and the crankshaft saddle.

With the help of thrust washers, it can be easily maintained the gap and helps to reduce the lateral movement of the crankshaft. In many engines, these are made as part of the main bearings, usually, older types, use separate washers.

6. Oil Passage and Oil Seals

The crankshaft oil passage passes oil from the main bearing journals to the large end journals. Usually, the hole is drilled in the crank web. When the crankpin is in an up position and combustion forces push the connecting rod down, oil can penetrate between the journal and the bearing.

The crankshaft extends slightly beyond the crankcase at both ends. This will cause oil to leak from these ends. Oil seals are provided to keep oil out of these openings. There are two main oil seals connected at the front end and the rear end.

  • Front-end oil seals: They are very similar to the rear-end oil seals. However, their failure is less destructive and more accessible. The front oil seal is installed behind the pulley and timing gear.
  • Rear end oil seals: They are placed in the main journals and flywheels. This is pushed into a hole between the engine block and the oil pan. The oil seal has a molded lip that is kept tightly into the crankshaft by a spring called a garter spring.

7. Flywheel Mounting Flange

In most cases, the crankshaft attaches to the flywheel through the flanges. The diameter of the crankshaft wheel end is larger than the other end. This gives a flange face to mount the flywheel.

Construction of Crankshaft

The following materials were used to build the crankshaft:

  • Cast Iron
  • Carbon Steel
  • Vanadium Micro-Alloyed Steel
  • Forged Steel

The cranks can be assembled from different parts or made in one piece (monolithic).

The monolithic version is the most popular crank worldwide. However, some large and small internal combustion engines have assembled crankshafts.

These shafts can also be cast from malleable cast iron, modular or ductile steel. Welded assemblies are cast in steel. This inexpensive method is suitable for inexpensive production engines with acceptable loads. The forging process has excellent strength. Hence, forging is known as the preferred method for building crankshafts.

What is Crankshaft Sensors?

The Crankshaft Position Sensor is attached to the engine block facing the timing rotor on the engine crankshaft. The sensor detects signals used by the engine ECU to calculate the crankshaft position and the engine rotational speed.

A crankshaft sensor is an electronic device used in an internal combustion engine, both gasoline and diesel, to monitor the position or speed of the crankshaft. This information is used by engine management systems to control fuel injection or ignition system timing and other engine parameters.

Before there were electronic crank sensors, the distributor on gasoline engines had to be manually set to a timing mark.

The crankshaft sensor can be used in combination with a similar camshaft position sensor to monitor the relationship between pistons and valves in the engine, which is particularly important in variable valve timing engines.

This method is also used to “synchronize” a four-stroke engine at start-up so that the management system knows when to inject fuel. It is also widely used as the primary source for measuring engine speed in revolutions per minute.

Types Of the Crankshaft Position Sensors

There are 2 types of crankshaft position sensors.

  • MPU type
  • MRE type

MPU type

34 teeth placed at every 10 ° crank angle (CA) plus two missing teeth for top dead center (TDC) detection are arranged around the outside diameter of the timing rotor. Therefore, 34 alternating current waves are output from the sensor for every revolution of the crankshaft.

These AC waves are converted into rectangular waveforms by the waveform shaping circuit within the engine ECU and used to calculate the crankshaft position, TDC, and engine speed.

MRE type

Due to the rotation of the timer rotor, the direction of the magnetic field (magnetic vector) emitted from the sensor magnet changes according to the position of the detection tooth during the time the detection tooth attached to the timer rotor approaches the camshaft position sensor and then moves away from the camshaft position sensor.

As a result, the MRE resistance value also changes. Voltage from the engine-ECU is applied to the camshaft position sensor, and the change in MRE resistance value is output as a change in voltage.

The waveforms of the outputs of the two MREs are differentially amplified and shaped into a rectangular waveform by the amplification/waveform shaping circuit within the sensor. The MRE outputs are then sent to the engine-ECU.


Another type of crank sensor is used on bicycles to monitor the position of the crankset, usually for the cadence readout of a cyclo-computer. These are usually reed switches mounted on the bicycle frame with a corresponding magnet attached to one of the pedal’s crankset arms.

Common Crankshaft & Crankshaft Sensor Failure Symptoms

  • Check engine light comes on
  • Engine not starting
  • Poor performance
  • Engine stalling
  • Increased fuel consumption
  • Inconsistent acceleration
  • Sudden drops in RPM

Common Causes of Failure

Over time, any sensor will fail either from accidents, power issues, or normal wear. Due to crankshaft or camshaft position sensor failure, an engine might cut, die while driving or refuse to start.

A faulty sensor may lead to catastrophic engine failure.