At some time, and in varying degress, the crankshaft is exposed to all forms of mechanical stressing. On the larger engines the crankshafts, for many years, been manufactured by forging, from a single billet, the combined ‘webs’ and ‘bottom end’ comprising one ‘throw’. These were then assembled into the composite structure of the crankshaft by ‘shrink fitting’ the relevant main bearings journals between each throw. These shrink fits, in the region of 1/600 the shaft diameter, used to be achieved by heating up the web and then entering the pin when the required expansion had taken place. However, with this method there was the possibility that slight products of oxidation, created by the heating, might become trapped in the interface such that the integrity of the grip was marginally reduced. To avoid this, liquid nitrogen or similar cooling agents have recently been adopted to cool the pin sufficiently to be entered into the web.
The more obvious forms of mechanical stressing that the crankshaft is subjected to are torsional stressing and bending stressing. Torsional stressing, being a result of the forces applied by the connecting rod to the bottom end, varies in magnitude with both the changes in cylinder pressure and the angle of thrust applied by the connecting rod during the power stroke. The compression stroke, acting as a resistance to turning, further compounds this variation in torque, so that, were the shaft not adequately dimensioned, early fatigue failure through cyclic torsional stressing would be likely to occur.
‘Torsional’ vibration indicates a situation where an applied turning moment causes the shaft to ‘wind up’ (twist along its length) and then unwind again as the stiffness of the shaft re-asserts itself over the applied torque.
The bending stresses generated within the shaft system are easy to visualise, especially if one main bearing is lower than it should be (due to wear, or lack of support from chocks). The firing forces will deflect the shaft into the housing causing the shaft to bend, which in turn causes the fibres at the outside surface of the curvature to be put into tension whilst those on the inside are compressed. Throughout the revolution these stresses are reversed, leading to a cyclic stressing that may lead to fatigue failure, particularly if there are flaws on the journal that may act as crack initiation centres. The most obvious of these are any drillings in the shaft, made to provide lubrication passages. Naturally, the area around such holes is subjected not only to the torsional effects but also to the bending stresses mentioned above.
Apart from torsion and bending stresses, the shaft will also be subjected to shear forces, particularly over the TDC position, where the piston rod, connecting rod and webs are in alignment and the turning moment is negligible. At this point in the cycle, the crank throw is thrust downwards creating a shearing effect on the two main journals.
Crankshafts should be checked for the following:
- surface damage of journals by:
- scoring by impurities in lubricating oil or particles embedded in white metal.
- corrosion (usually apparent as discoloration), possibly from weak acids caused by oxidation of lubricating oil (weak); bacteria in oil; products of combustion (trunk engines).
- cracks at fillet radii, oil holes or other areas where stress concentration occur.
Caused by cyclic torsional stresses; crankshaft misalignment (worn main bearings or loose chocks); overloading of the engine.
- slipped shrink fit (check witness marks).
Caused by liquid in cylinder during starting; propeller collision with submerged object; extreme and pudden overloading od a unit or units (possibly as a result of a major fault in the fuel injection and timing system).
This occurs infrequently but is a problem that primarily concerns the bottom end journal, though in extreme cases the mains and even the crosshead pins may be affected. It is caused by a combination of reduced effectiveness of lubricating oil and the directional thrust of the connecting rod, which is at a maximum somewhere around 45 deg after TDC, with a result that the journal wears oval.
One of the most important and radical changes of recent times has been the production of crankshafts by ‘welding’ together pre cast or forged sections of shaft. This process, accepted by all the major classification societies and regulating bodies, has the advantage of eliminating the need for a shrink fit. The procedure is simply to forge a throw, similar to that described earlier through this time with sections of shaft formed at the lower end on the webs. These ‘stubs’, when welded to the adjacent throw form the main bearing journals. The welding techniques employed are such that these shafts are more than able to withstand the variations in stress mentioned above. As there is no longer any need to provide a depth of material around the pin, in which to absorb the hoop stresses, these shafts are much lighter in structure than the traditional shrink fit shafts.