Crankshaft

The crankshaft is a moving part of the internal combustion engine (ICE). It’s main function is to transform the linear motion of the piston into rotational motion. The pistons are connected to the crankshaft through the connecting rods. The crankshaft is mounted within the engine block.

Engine Crank Mechanism

Image: Engine Crank Mechanism (source: Rheinmetall)

  1. Pistons
  2. Connecting rods
  3. Flywheel
  4. Crankshaft

The pistons, connecting rods and crankshaft together form the crank mechanism.

The secondary function of the crankshaft is to transmit power to other engine systems:

  • valve timing
  • oil pump
  • cooling (water) pump
  • air conditioning compressor
  • alternator, etc.
ICE Crankshaft with Forged Counterweights

Image: ICE Crankshaft with Forged Counterweights

The crankshaft is fitted into the engine block through it’s main journals. The connecting rods are fixed on the conrod journals of the crankshaft. On opposite sides of the conrod journals the crankshaft has counterweights which compensates outer moments, minimises internal moments and thus reduces vibration amplitudes and bearing stresses.. At one end of the crankshaft the flywheel is connected and on the other end the valve timing gearing.

Engine Crankshaft Description

Image: Engine Crankshaft Description (source: Rheinmetall)

  1. Control side or drive end
  2. Counterweights
  3. Main bearing journal
  4. Conrod journal
  5. Flywheel side/force transfer
  6. Oil bore

The number of main journals and conrod journals depends on the number of cylinders and the type of the engine (V-type, straight, etc.). On both main journal and conrod journals, the crankshaft has lubrication orifices (oil bore) through which oil flows when the engine is running.

ICE Crankshaft with Bolted-on Counterweights

Image: ICE Crankshaft with Bolted-on Counterweights

The engine torque is not continuous because it’s produced only when each piston is on expansion cycle. Due to this a flywheel is mounted onto the crankshaft in order to smooth the engine torque and reduce vibrations.

On V-type engine on the same conrod journals, two connecting rods are mounted. Because of this arrangement, a V-engine, for the same number of cylinder, is more compact than a straight engine. The length of a V6 engine is shorter than the length of a straight 6 cylinders (L6) engine.

ICE Crank Mechanism Animation

Image: ICE Crank Mechanism Animation (click on it)

Between the crankshaft and the engine block, on the main journals, crankshaft bearings are fitted. Their role is to reduce friction through a layer of anti friction material which comes into contact with the engine block mounts.

Two types of crankshaft are produced, cast and forged. The counterweights can be also forged directly onto the crankshaft or bolted-on (fixed with threaded bolts).

All the pistons of the internal combustion engine are transmitting their forces to the crankshaft. From the mechanical point of view, the crankshaft has to withstand high torsional forces, bending forces, pressures and vibrations.

For any questions or observations regarding this tutorial please use the comment form below.

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One Response

  1. Innocent Mafiele

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