CRANKSHAFT

Is either fully built(all individual and later shrink fit), semi built(with one crank throw forged or cast as a single unit), fully forged

      C/shaft is usually cast and for large engines is forged from 0.4% carbon steel with Nitralloy( 1.5% CR, 1% Al and 0.2%MO). Molybdenum refines the grain structure. The C/shaft is heated in Ammonia for 4days and the nitrogen in ammonia dissociates and combines with CR and AL to form hard nitrates on surface.

 Fold forged method using hydraulics is used to forge a large single billet with its ends flattened. care is taken that it defect free at the bends and cooling is done at a controlled rate.  Usually forged to get continuous grain structure and we achieve no sub-surface defects due to repeated heating and pressing. 

Identical crank thros are then shrink fitted with the main journals at desired angles. Usually, cold shrinking is preferred over hot shrinking as hot shrinking gives residual stresses and oxidation products are formed in btw the interface. Shrink fit allowance is usu 1/570 - 1/660 of journal dia so that it provides necessary radial pressure. The thickness can be reduced as it is forged but the area around the interference fit is increased as there are large tensile hoop stress present in material after shrink fit.

By virtue of its geometry, c/shafts are subjected to fatigue stresses of combined gas load, bending and torsional stresses. The junction bte the pin and web is where the highest likely possiblity of repetition and reversal of stresses.

To minimise the stress, a fillet (usu 5% of journal dia) passing into the web so as not to disturb the bearing surface area of the pin.

   The fillets are cold rolled or strain hardened to 220BHN, to induce compressive stress, again to increase fatigue resistance.

The running surfaces are induction hardened. flame hardened to abt 480BHN.

C/shaft is subjected to -

- tensile and compressice stresses at TDC and BDC due to gas load

- bending stresses due to gas combustion load at different crank angles 

- Gas load also imparts flexing in and out of the webs leading to axial vibration.

- torsional stresses due to varying periodic firing impulses from the different units which twist the shaft at varying torque

        Moreover, these all are cyclic and varying leading to fatigue stresses.