Crankshaft Design Considerations and Proportional Dimensions

The present design consideration is to increase the stiffness of the crankshaft and reduce its overall length by incorporating narrow journals of large diameter. For the required wall thickness and coolant passages, the minimum cylinder centers can be around 1.2 times the cylinder bore diameter for an engine having its stroke equal to the bore. The maximum diameter of the big-end for the connecting-rod assembly that can pass through the cylinder is 0.65 times
of the bore. The proportions of the crankshaft are as follows :

Cylinder bore diameter = D

Cylinder centre distance = 1.20 D

Big-end journals diameter = 0.65 D

Main-end journal diameter = 0.75 D

Big-end journal width = 0.35 D

Main-end journal width = 0.40 D

Web thickness = 0.25 D

Fillet radius of journal and webs = 0.04 D

To increase the fatigue life of the shaft, the fillet radius between journals and webs should be as large as possible but not less than 5% of the journal diameter. The overlap between the diameters of the big-end crankpin and the main-end journal depends on the length of the stroke i.e. the crank-throw. A long-stroke engine has very little overlap, requiring thicker web sections, and a short-stroke engine has considerable overlap which strengthens the shaft.

Collars are machined on the webs adjacent to the journals to accurately align the crankshaft and the bearings with the correct amount of side-float and, if necessary, to absorb the crankshaft end-float. Most crankshafts dimensions are such that the nominal stresses in the material under operating conditions do not exceed 20% of the tensile strength in bending and 15% in torsion. Crankshaft journals are ground to provide a surface finish better than 0.5 urn, to minimize bearing wear.

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