CRANKSHAFT

CONSTRUCTION

Crankshaft:The stresses which arise in a crankshaft are

mainly due to bending and torsion. The actual stressing is complex which may

consist of a number of stresses acting in more than one plane.

The stresses are summarised as follows:a) That arising out of bending from combustion

load, static weight of piston, connecting rod etc. and inertia forces.

All components of a crank i.e. the pin, webs and main bearing journals are stressed due to bending.

The effect of bending is examined approximately on the simplified assumption of a straight shaft analogy.

It considers one crank as a straight circular beam which is supported at a span taken from the ends of adjacent main bearings.

The load on crankpin is considered as concentrated at the centre.

The journals in the bearing are considered as fixed or encastre. The webs are subjected to end fixing moments due to the crank pins being encastre in webs.

b) that due to axial bending from propeller axial thrust variation or from unbalanced couples.

C) that due to transmission of variable torque producing twisting of the main bearings journals and crankpins subjecting all those components to shear..

d) that due to high tensile hoop stress set up on the webs and radial compressive stress on the pins by virtue of the shrinkage of webs on journals.

e) A direct shear stress on the crank pin and main bearing journals owing to the firing load. The major stresses as calculated on simplified assumptions are cyclic stresses which subject the material to fatigue.

Failure mostly occurs owing to progressive fracture caused by repeated bending

and from reversed torsional stresses. Severe bending stresses arise out of one or more main bearings being lower or

higher from the rest.

The normal calculations do not take

into account the acceptable misalignment due to wear down of main bearings.

Material of Construction:High tensile strength together with ductility,

resistance to wear, corrosion, high endurance to fatigue are some of the desirable properties for material of crankshaft.

Plain carbon’ steels with percentage of carbon varying from 0.2 to 0.4 are used for crankshaft of slow speed large diesel engines of cast throw, semi-built construction.

For forged throw the p.c. of carbon may be raised to a maximum limit of 0.5%. Higher speed engines requiring greater hardness at crank pin and journal surfaces use alloy steel.

Working Stress:

The calculation of working stress is a complex procedure. With the materials of above description the working stress could be as follows:

50.3 MN/M2 or 5.13 kgf/mm2 in case of carbon steel and not more than 69 MN/m2 or 7 kgf/mm2 in case of alloy steel.

The stress calculation involves —(a) Calculation of cyclic nominal bending stress

corrected for inertia of reciprocating masses.(b) Calculation of cyclic nominal shear stress based

on conventional torque variation in each cycle.

(c) The working stress calculation should take into account:

i) a dynamic magnification factor for torsional vibration by a stress value above the nominal stress

ii) a fatigue strength reduction factor due to the presence of any oil hole or a shoulder.

iii) a stress concentration factor for the oil holes and fillets at the underside of the crankpin with the crankwebs.

Construction:

Large diesel engine crankshafts are mostly semi-built but may also be fully built in some cases. Engines of smaller powers have their crankshafts drop forged.

In the fully-built construction all components, i.e. webs and pins, are separately made and then shrunk together.

In the semibuilt construction a crank throw consisting of the webs, its backward extension and the pin are forged out or cast in one piece.

In applying forging, the crankshaft is best formed by the fold-forge method.

The ingot is first forged down to the shape of a long cylindrical form.

A portion from both ends are flattened to the approximate shape of webs.

It is then folded taking care that the bends remain free from defects.

Forging produces a sound material by closing in small cavities.

The process of working produces change in the macrostructure of the metal that leads to a finer grain size.

The metallic fibres flow in longitudinal direction which results in a product of higher mechanicalstrength.

The forgings are gradually and uniformly hot worked.

They are made from ingots of plain carbon steel or alloy steel.

The main bearing journal ns are shrunk in. The shrinkage grip alone is relied on to transmit

the power. The surfaces- are smoothly prepared with proper

shrinkage allowance so that maximum surface grip is available.

The grip pressure is carefully estimated. The limiting factor being the yield point at th

surface under grip. An interference fit of 1.4/1000 is not to be

exceeded in large diesel engine crankshafts.

Inspection tests carried out on pins, pin fillets and journals of the finished crankshaft are the magnetic particle and ultrasonic tests which are meant to detect, any surface and sub-surface flaws.

Large crankshafts are built in two parts joined together by flanged couplings. A fully assembled crankshaft will also carry the gear or sprocket wheel for the camshaft drive, a flywheel at the after end and the thrust shaft. The machining of oil holes should be smooth. The outer portion is reamered and rounded off. Fillets are curved out smoothly and cold rolled at the fillet radius to strengthen the fillet from bending fatigue effect. A high degree of smoothness is given at pin and journal surfaces so that notching arising out of machining tool is removed from surfaces. The high carbon content of the material produces a hard wear resisting surface.

Sometimes casting of the crank throw is preferred to forgings for economic reasons.

Casting gives a coarse grained structure of the material.

The mechanical properties and the homogenuity would be inferior.

It always contains micro-defects which are sensitive to notch effect and do not contribute to fatigue life.

Both casting and forgings however, are to be fully annealed. A casting process of B & W engine crankshaft is described below: