11. The Fatigue Strength of Large Cast Steel Crank Throws for Marine Diesel Engines

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A study has been made of the infiuence of minor casting defects on the fatigue properties of cast steel throws for large crankshafts. The rotating-beam fatigue strength of unnotched and notched specimens of 60 to 100 mm diameter were investigated using a 5,500 kg. M rotary bending fatigue testing machine. The test specimens are cut out from the pin, in the direction parallel with the pin centreline, from two crank throws of more than 650 mm pin diameter. The cast steel crank throws are made from vacuum degassed steel and heat treated to the strength level of 50 kg/mm^2 by double normalizing and tempering. It was found that the rotary bending fatigue strength varied between±16kg/mm^2 and ±13kg/mm^2 according to the number and distribution of pinholes, and shrinkage porosities on the surface of the machined test specimens. The former figure applies to a casting having defects less than I mm deep and less than 2 mm diameter, as revealed by the liquid penetration test, the latter figure, ±13 kglmm^2 applies to the largest defect, 3 mm deep, revealed by visual test. The corresponding figure for a sound casting was found to be ±17 kg/mm^2 to ±18kg/mm^2, using test specimens taken from keel blocks cast from the same melt and having surface defects of less than 1 mm, as indicated by the liquid penetration test. The foregoing figures have been used for comparison with those of estimated values of working stress in the fillet area of crank pin and web.
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11. The Fatigue Strength of Large Cast Steel Crank Throws for Marine Diesel Engines

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