Engine crankshaft

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 which used for multi-cylinder engines,and the
assembled type fabricated from separate Elements
,which is mainly used for motorcycles.

The type of crankshaft determines what kind of
Connecting rods are used .

Most of crankshaft has been a steel forging ,nodular
cast iron crankshafts are also accepted normal
practice in automotive engines . the crankshaft is
supported in Main bearing .

The strength of the shaft depends primarily on that
of the material from which it is made.

A crankshaft may have as few as two main journal bearings, even in four cylinder engines provided their rating is low. However, in highly-rated engines there is usually, in addition, one between each pair of crank throws, though some four-cylinder units have only three bearings. In the latter event, the shaft has to be very stiff , otherwise at certain speeds and loads it would whip , heavily loading the central bearing as it
does so, possibly causing it to fail.

This tendency can be completely eliminated, or at least reduced, by balance weight on the crank webs each side of that bearing.

The larger the number of bearings to support the shaft uniformly along its length , the more slender,and therefore lighter, can it be without risk of whip due to Bending . More bearings, however, will entail increased
cost and, since the accuracy of their alignment is critical.

The crankcase structure supporting them must be very stable under variations of temperature and load. Frictional drag of the bearings on the shaft increases as the square of the diameter but only linearly with length.

The maximum number of main bearings is one more then the number of cylinders there may be less.

A major factor governing the dimensions of the shaft is the torsional stiffness needed to raise its natural frequencies of vibration , together with their harmonics , above the rotational speed of the engine . For any given length of shaft both the bending and torsional stiffness depend on the diameters and overlap of the main and big end journals and the thicknesses and widths of the webs . The lengths of the journals are a function of their loadings and the Strengths of the bearing materials.

The strength of the shaft depends primarily on that of the material from which it is made. Measures such as the incorporation of generous fillet radii between the webs and journals , and perhaps rolling these fillets to induce in them Residual compressive stresses , can improve Fatigue strength , which is also affected by heat and hardening treatments.