Engine bearing

Engine bearing or bearing in automotive application are widely used in support components or power transmission , we will consider here bearing between Cylinder block and The crankshaft which are
known as " engine main bearing " , second the bearing between crankshaft and Connecting rod .

The connecting rod rides on the crankpin via
a plain bearing.The main bearing of the crank-
case supports the main journal of crankshaft.

The plain bearing should also have the ability
to absorb dirt ,metal or other hard particles
that are sometimes carried into the bearings.

The bearing should allow the particles to sink
beneath the surface and into the bearing
material.this will prevent it from scratching
, wearing and damaging the pin surface.

Corrosion resistance is required because
the bearing must resist corrosion from acid,
water and other impurities in the engine oil.

It comprises three layers :

* the backing metal, which is a steel plate facing the connnecting rod .

* an intermediate aluminum alloy layer ( Al-Sn-Si alloy ) that has particulate Sn dispersed in the aluminum-
silicon matrix .

* a soft layer (Sn plating), called overlay.

On the inside. The steel backing plate supports the soft aluminum alloy and the additional soft overlay gives wear resistance during running-in.

The crankshaft is simultaneously subjected to bending and torsion when revolving. Under these conditions ,it tends to twist , and failure can occur as a result of fatigue .The main bearing clearance can be as small as 70 mm, but under these circumstances, the crankshaft deflects fully within the clearance while revolving.

The trend towards reducing crankshaft weight means that the main bearing portion supporting the crankshaft is less rigid. This weakened main bearing cannot support the crankshaft sufficiently, which creates a severe fatigue situation.

The connecting rod connects The piston to the crankshaft to transfer combustion pressure to the crankpin,
There are bearing portions at both ends . The monolithic connecting rod has a needle roller bearing at the big end .

The two-stroke engine requires a needle roller bearing because the big end has less lubricating oil due
to the structure. In Four-stroke engines , lubricating oil is abundant in the crankcase, and the assembly type of connecting rod is used because of the lower cost of this simpler structure.

The needle rollers held in the retainer are inserted into the big end and run on the outer raceway of the
big end and the inner raceway of the crankpin . The roller itself receives high stress and also exerts high Hertzian stress on the rolling surfaces.

The performance and life of bearings are very important in extending the life of the engine.Rolling contact fatigue is likely to take place at high-speed revolutions. Fatigue failure of the pathway surface is called rolling contact fatigue failure. This occurs when the finished smooth surface breaks under repeated rolling.

Bearing life refers to the number of times any bearing will perform a specified operation before failure .

Many factors influence the actual life of the bearing :

Some of the mechanical factors are temperature, lubrication, improper care in mounting, contamination, misalignment and deformation.

Bearing metals contain soft metals such as tin or lead. These soft metals can deform to the shape of the adjacent part ( the crank pin in this instance ) and also create fine oil pools at the rubbing surface.

However,if the bearing consists only of soft metals, it wears out quickly. Appropriate wear properties are provided by small particles of tin dispersed in the harder aluminum alloy.

Lead particles perform this function in the copper-lead alloy.Although bearings containing lead have superior material properties, environmental considerations have led to the development of a Cu-Sn-Ag bearing as an alternative to the Cu-Pb bearing . An Al-Sn-Si alloy is also being used to replace Al-Pb bearings. The use of lead as a bearing metal is decreasing .