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

Crankshaft Purpose and Construction

The main purpose of the crankshaft in a reciprocating engine is to convert the linear motion of the connecting rod assemblies into smooth, rotational movement Figure 58. The mechanical stress caused by rotational speeds, combustion pressure, thrust loads, and vibration require the crankshaft to be exceptionally strong and precisely manufactured. The two connecting surfaces to the crankshaft are the main and the rod bearing journal ( Figure 59A and Figure 59B).

FIGURE 08-58: Balance holes in crankshaft counterweights are made during crankshaft production. To balance a crankshaft for smooth operation, some holes are drilled to lighten the crankshaft and to remove material in heavy spots. Other holes are filled with heavier metal plugs that add weight to localized areas of the crankshaft.		A	B
		FIGURE 08-59: A. The crankshaft is supported along its centerline by the main bearing journals. B. Connecting rods attach to the crankshaft crankpins. Main bearing caps clamp the crankshaft into the block. Note the numbers and directional markings that orient the caps on the crank.

Main bearing journals are located along the crankshaft centerline, and main bearing caps support the crankshaft as it rotates. Rod journals are offset from the crankshaft centerline main journals. The distance from the centerline of the rod journal to the centerline of the main bearing journal is known as the length of the crankshaft throw . The length of throw determines the stroke length of the engine. The arrangement of the rod journals, or throws, is established to provide even spacing between power impulses Figure 60. For example, a six-cylinder engine will have 120 degrees of rotation spacing between the rod journals. The throws are further arranged to evenly distribute the firing order over the length of the shaft to balance out vibration and stresses. For example, in in-line six-cylinder engines, the throws of cylinders are paired 1 and 6, 2 and 5, and 3 and 4. Using a standard firing order of 1-5-3-6-2-4 distributes power impulses evenly throughout the crankshaft during the two rotations it makes to complete a combustion cycle in all cylinders.

FIGURE 08-60: Arrangement of crankpin spaces for even firing impulses. An in-line six-cylinder engine’s space crank throws at 120 degrees.

In-line engines are preferred in vocational and HD diesel engines because the crankshaft is supported by more main bearings than in V-block configurations. This means the in-line block is better able to handle higher cylinder pressure with greater durability than V-block configurations due to more and better distributed clamping force supplied by main bearing caps. The advantage of a V-block lies in its more compact design. In V-bank engines, cylinder banks are leaned 60 degrees, 72 degrees, or 90 degrees from the vertical centerline, resulting in an engine with less overall height. A short block length and height permits lower hood profiles. It also allows an engine to be installed either transversely or longitudinally in a chassis. The longer length of an in-line six-cylinder engine means it can only be installed longitudinally in large vans and pickup trucks. However, an in-line six-cylinder engine, in addition to having a stronger crankcase, is also among the best balanced configurations for an internal combustion reciprocating engine, possessing excellent primary and secondary balance characteristics.

When the engine is operating, tremendous inertia is generated by the reciprocating mass of the connecting rod and piston assembly. To balance these forces, large counterweights are formed on the crankshaft 180 degrees opposite the rod journal.

Crankshaft Fillet Radius
A fillet radius is a critical feature unique to a diesel crankshaft. It is a circular-shaped radius ground into each bearing journal where it meets the crankshaft cheek. Because this region is one of the most stressed areas of the crankshaft (due to shaft bending and twisting), leaving a sharp 90-degree angle between them could induce a crack through a stress riser. Imparting a smooth, circular radius to this area strengthens the crankshaft and minimizes the possibility of a fracture.