Engine Assembly
Engine Assembly


Crankshaft Introduction

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.
The crankshaft converts reciprocative motion to rotational motion. It contains counter weights to smoothen the engine revolutions. There are two types of crankshaft, the monolithic type , used for multi-cylinder engines, and the assembled type fabricated from separate elements, which is mainly used for motorcycles.

How a crankshaft is made

Most production engines use a cast iron crankshaft which is made by pouring molten iron into a mould. Forged crankshafts are used in some performance engines. A forged crankshaft is made by heating a block of steel until red hot, and then using extremely high pressure to form it into shape.
Once a crankshaft has been forged or cast, its journals and bearing surfaces are machined perfectly smooth. Oil passageways, or oilways, are drilled. Production engines will generally leave the webs with their original rough casting finish but performance engines will machine every part of the crankshaft to reduce oil drag.
The journals must be harder than their bearings to ensure that any wear occurs on the replaceable bearings and not the crankshaft, which should last for the lifetime of the engine. The manufacturing process will include hardening of these areas through nitriding or heat treatment.
Extremely high performance and custom crankshafts, are machined from a block of solid material, producing a billet crankshaft. Producing a one-off crankshaft by this process would cost a minimum of around $3,000 so it is reserved for competition racing and restoration situations.

Crankshaft Picture

Crankshaft Working Principle

The distance between the center of the main journal and the center of the crankshaft pin is called the crank radius , also called the crank throw . This measurement determines the range of piston travel as the crankshaft rotates - this distance from top to bottom is known as the stroke . The stroke of the piston will be twice the crank radius.
The rear end of the crankshaft extends outside the crankcase and ends with a flywheel flange . This precision machined flange is bolted to the flywheel , whose heavy mass helps smooth the pulsation of the pistons firing at different times. Through the flywheel, the rotation makes its way, through the transmission and final drive, to the wheels. In an automatic, the crankshaft is is bolted to the ring gear , that carries the torque converter, passing drive into the automatic transmission. This is basically the output of the engine - and power is taken to where it’s needed: propellers for boats and aircraft, 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 which extends beyond the crankcase. This shaft will be locked to a toothed gear which drives the valve train through a timing belt or chain [or in highly technical applications, gear sets], and a pulley which provides power through a drive belt to accessories such as the alternator and water pump.

Crankshaft lubrication

Metal-to-metal contact is the enemy of an efficient engine, therefore the main journals and rod journals both ride on a film of oil that sits on the bearing surface.
Supplying oil to the main journal bearing is easy: Oil galleries from the engine block lead to each crankshaft saddle, and a matching hole in the bearing shell allows this oil to reach the journal.
The rod journal bearings require the same lubrication but they are rotating around the crankshaft at an offset. To get the oil to these bearings, oil passageways run inside the crankshaft - through the main journal, diagonally through the web, and out through holes in the rod journals. A groove in the main rod bearing allows oil to be continually forced down the passageway to the rod journals, assisted by being flung outwards by the centrifugal force of the rotating crankshaft.
The clearances between the journals and bearings are the main source of oil pressure in the engine. If the clearances are too high then oil flows out freely, and pressure is not maintained. Clearances that are too low will cause high oil pressure and risk metal-to-metal contact. It is therefore essential that the clearance between the bearings and journals are measured when an engine is rebuilt.
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