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LO 8-8 Identify causes of bending and torsional stress of crankshafts and describe the methods used to minimize crankshaft mechanical stress

1. Fundamentals of Crankshafts

1.3. Crankshaft Journal Hardening

LO 8-08: Identify causes of bending and torsional stress of crankshafts and describe the methods used to minimize crankshaft mechanical stress.

 

Crankshaft Journal Hardening

Crankshafts in diesel engines are commonly forged from high carbon steel alloys, which produce a denser and stronger component. Following the machining of crankpins, counterweights, and other surfaces, journal surfaces are hardened to improve the durability of a diesel crankshaft. The two most common processes used to harden bearing surfaces of the crankshaft are induction hardening and nitriding.

Induction Hardening
To induction harden a crankshaft, the journal is wrapped in a coil of wire that has high amperage alternating current passed through it. The journal surface will have current induced into it by mutual induction, much like the secondary winding of a power transformer. This current flows over the journal surface, heating the metal surface and changing its molecular structure. Quenching the heated surfaces with water produces a very hard surface finish. Induction hardening causes discoloration of the metal surfaces heated during this process Figure 61. Typically, induction hardening is effective to no more than 0.03" (0.76 mm) deep. It may go deeper, but the surface will be less hard with increasing depth. This is one reason why machining diesel cranks is typically not an accepted practice; the hardening material is removed by machining, and most rebuilders are not able to restore it during rebuilding and repair. This also explains why replacement bearings are generally unavailable for crankshafts with journals machined smaller or undersized to repair damaged journal surfaces. However, at an OEM rebuild level, plasma-spray welding of the crankshaft with alloy steel fillers (which is then machined to the correct journal dimension) is an accepted technique for diesel engine crankshaft restoration.

   Fig08_61

FIGURE 08-61: Note the fillet radius at the edge of each journal surface. This crankshaft also has discoloration around hardened journals, which is not evidence of damage from overheating but is caused by heating during induction hardening.

 

  Technicians Tip  
  Grinding of diesel crankshafts to undersized dimensions to repair damaged journal surfaces is common in spark ignition, gasoline-fueled engines. Diesel cranks, however, are almost always hardened, and grinding to repair journal damage will remove the surface hardening. Furthermore, a fillet radius ground into each crankshaft journal is difficult for most machine shops to restore. For these reasons, most manufacturers do not recommend the practice, and undersized crankshaft bearings are not commonly available for diesel crankshafts.  

 

Nitriding
Another crankshaft hardening technique is nitriding. Chevy diesels, including the Duramax, use this technique. The process involves heating the shaft and introducing cyanide salt or liquid into the heating chamber with the crankshaft. Slowly cooling or quenching the crank with these substances in its heated condition causes the nitrogen and carbon to molecularly bond with metal surfaces to create an extremely hard journal. This hardened finish does not extend more than 0.012" (0.305 mm) deep but is one of the hardest journal surface finishing techniques.

  Technicians Tip  
  Crankshafts are made strong and are capable of resisting tremendous bending and twisting forces; however, they can bend if improperly stored. Storing a crankshaft on its side will allow a crankshaft to distort over time. Always store a crankshaft on its end, standing it vertically with the flywheel flange placed against the floor. To prevent injury from a falling crankshaft, always tie a crankshaft to a stationary object, such as a workbench or post.  

 

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