The crankshaft is a part of Internal combustion engine or of Stirling engine. It is the principal element of the device connecting rod-crank. It allows the transformation of the alternative linear movement of the piston (S) into a continuous movement of Rotation. The alternative linear movement is due to the successive deflagrations of the fuel-combustive mixture in the combustion chamber.

## General information

The crankshaft is composed of several Tourillon S aligned, on which it turns.

Between these stages are, offset, of the Maneton S, on which the rods are assembled.

The eccentricity $E$, distance between the axis of a crank pin and the axis of a pivot, define the race $C$ piston : there is $C=2E$. This distance partly determines the Cylindrée of the engine.

The crank pins are connected to the stages by masses (let us note that there is not always a pivot between each crank pin - for example, the Renault Dauphine had only 3 pivots for 4 crank pins).

In addition, the masses allow the dynamic balancing of the crankshaft. Their goal is to reduce the vibrations due to the reciprocating motion of the pistons and the dissymmetry of the system of crank.

For the engines of which the cylinders are laid out on line, there is a crank pin for each rod.

## Structure

• one-cylinder Crankshaft of engine

The ignition timing is of 360°.
• Crankshaft of engine twin-cylinder

On the two cylinder motors, the two crank pins can be aligned (ignition timing to 360°) or in opposition (chock with 180°). The presence of a pivot between the two crank pins is not obligatory. On the engines flat (boxer) twin-cylinder, the two crank pins are diametrically opposite (chock with 180°); the same applies to the 4 cylinders (Flat furnace) and the 6 cylinders flat (Flat six).

• Crankshaft of three cylinder motor in line

On the three cylinder motors laid out in lines, the chock is, usually, of 120°. This type of crankshaft is used on the engines of Smart.

A chock with 180° was also seen (certain models of Laverda 1000).

• Crankshaft of straight-4 engine

On the four cylinder motors, the two external crank pins are aligned, and the two interior crank pins are aligned between them, with 180° with outsides.
• Crankshaft of 6 cylinder motor in lines

On the 6 cylinder motors, the chock is of 120°; the crank pins are aligned 2 to 2. It is the case of the crankshaft in photograph.
• On the V-engine , there is, generally, two rods per crank pin, either side by side (slightly lengthened crank pin), or intersected (Harley-Davidson).

## Types and modes of assembly

### Cast solid crankshaft with stage (S) hydrostatic

This type of crankshaft offers better a rigidity and makes it possible to put at it a greater number of crank pins and thus of pistons. The hydrostatic bearings are expensive to implement since they require an oil circulation under pressure. This is why these crankshafts are used for the engines of strong cubic capacity. It is the case of the cars, boats and machines of building site.

A cast solid crankshaft imposes the use of dismountable rods whose two half landings are screwed around the crank pins. Two half-rings are inserted in the parts of the rod; their specific form makes it possible to maintain the continuity of oil film between the rod and its crank pin.

The same applies to the connection between the frame, commonly called driving block, and the crankshaft on the level of the pivots. Oil under pressure is brought to all the pivots; it is then conveyed with the crank pins via a drilling crossing the masses.

### Assembled crankshaft, housing bearing

This type of crankshaft is inexpensive because the machining of the parts is simple. It is composed of an axis acting of crank pin and two shaft masses. The assembled crankshafts are not very rigid, they thus are generally used for the one-cylinder engines of weak cubic capacity. It is the case for example Mobylettes, scooters and slicers.

The assembly of the crankshaft is carried out once the cast solid rod was introduced on the axis being used as crank pin. The Connection pivot between the rod and the axis is ensured by a Cage needles.

## Manufacture

The crankshafts are generally out of cast iron GS moulded for the engines of specific low power (until 40kW/l). For the more powerful engines, overfed or turbocompressé (fuel rating from 40 to 60 kW/l), the crankshafts out of strongly allied steels are forged.

The crankshafts thus obtained are monoblocs and impose the use of dismountable rods.

The rubbing parts, pivots and crank pins, are machined very precisely by attaching a great importance in their surface quality. They undergo then a surface heat treatment in order to increase hardness by it and thus to reduce the speed of wear.

In the case of the hydrostatic bearing crankshafts, of drillings in skew connect the crank pins to the pivots nearest while passing through the masses to balancings. These drillings make it possible to bring oil under pressure to the interface rod-crank pin. The openings of these drillings are visible above on the photograph.

The dynamic balancing, obtained coarsely by the geometry of the masses, is adjusted by not very major drillings on the periphery as of these last. These drillings are observable on the photograph.

## Establishment in the vehicle

The majority of the engines use only one crankshaft, but it sometimes happens to use some several (two crankshafts one behind the other on 1000 Ariel, four in squares on the Suzuki 500 of Grand Prix, etc).

Usually, it is said that an engine is transverse or longitudinal according to the position of the axis of the crankshaft compared to the axis of the vehicle.

## Distribution of times of the engine

• the crankshaft allows the distribution, between the cylinders, of various times (admission, compression, combustion/relaxation, exhaust). For example, in a 2 cylinder motor, the crankshaft will have to be built so as to have the following distribution:
• cylinder 1:
• time 1: admission or combustion/relaxation
• time 2: compression or exhaust
• cylinder 2:
• time 1: compression or exhaust
• time 2: admission or combustion/relaxation

The choice between admission or relaxation and compression or exhaust is carried out by a Camshaft which manages the opening and the closing of the Soupape S.

## See too

### External bonds

• Calculation of the internal and external dynamic stresses of an internal combustion engine
• Calculation of kinematics and dynamic stresses of the distribution

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