Fatigue Strength of Cast Crankshafts
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BENDING fatigue-strength of a castiron crankshaft is unaffected by surface-finish changes, but is greatly influenced by fillet radius. These are two of the conclusions reached by Mr. H. R. Mills, Ph.D., B.Sc., M.I.Mech,E., principal research engineer of the Motor Industry Research Association, and Mr. R. J. Love, Wh.Sc., a research engineer of the same organization.
In their paper, "Fatigue Strength of Cast Crankshafts," which they read to members of the Automobile Division of the Institution of Mechanical Engineers, in London, on Tuesday, the authors dealt with investigations which they had carried out at the Association's laboratories, The section of the work described was concerned with the influence of crankshaft proportions and surface finish on bending fatiguestrength, and the design features examined were: (I) Radius and surface finish fillets; (2) thickness, breadth and profile of webs; (3) size of lightening holes; (4) length of crankpin; (5) crankthrow or overlap. Tests were also carried out on test bars to check the basic properties of the material and its consistency between melts.
Testing for Fatigue
The tests for determining fatigue strength were carried out on machines of the constant-strain type, in which one end of the crankthrow under test was held rigidly, while a loading arm. pressed into the bore of the other end.
was vibrated. The appearance of a fatigue crack in a test specimen was taken as the criterion of failure, and marked the completion of a particular test.
When testing forged crankshafts, it had been found that the appearance of a fatigue crack was usually followed fairly quickly by complete failure of the crankthrow, but with cast-iron crankshafts a crack would grow only a small amount after its initiation.
Cracking always occurred in one of the fillets of the web nearer the clamp, sometimes on the crankpin side and sometimes on the main-journal side.
In addition to the fatigue test, a value for the flexibility of the crankthrow was determined. This was done because the bending properties of a crankshaft appeared to be determined by two. factors—bending fatigue-strength and bending flexibility.
Bending loads in an engine were shared by the crankcase and the crankshaft, and the proportion of these loads carried by the crankcase would be higher, the greater the bending flexibility of the crankshaft in relation to that of the crankcase. The best service from a crankshaft subjected to bending loads in a particular crankcase would, therefore, be dependent on the combination of high bending fatiguestrength and good bending flexibility.
Regarding the average composition of the material in the crankshafts tested, the authors gave the following percentage figures:—Total carbon, 2.95; silicon, 1.3; sulphur, 0.1; phosphorus. 0.07; manganese, 1.1; copper, 1.25: molybdenum, 0.45.
Tensile specimens of I.128-in. diameter, machined from separately cast bars from most of the melts, gave an average value for ultimate tensile stress of 26 tons per sq. in., and specimens machined from one crankshaft gave the same average value. Specimens of 0.564-in, diameter, machined from bars from about half of the different melts, gave an average value of 24 tons per sq. in., and specimens from crankshafts from about half of the melts, gave an average value of 22 tons per sq. in.
In addition, the results of tests carried out at the foundry on 0.798-in.-diameter specimens, machined from special " waisted " castings made from each melt, gave an average value of 29 tons per sq. in.
It could not be said, therefore, that .the tensile tests showed any significant difference between melts, but the material in the castings was, perhaps, a little weaker than in the test bars. Dealing with surface finish, it was well known, said the authors, that this had a pronounced effect on fatigue strength where steel -components were concerned. Ground fillets, turned fillets, and " very rough," fillets were used in the tests, the results showing no significant differences in the matter of strength due to surface finish.
With regard to holes drilled for lightness, it appeared, said the authors, that there was an optimum size for these in respect of bending fatigue-strength, and this would he about 0.4 of the journal diameter. With some earlier tests on forged crankshafts, not conducted by the authors, it was found that the lightening holes, having diameter as large as 0.59 of the crankpin and main journal, raised the bending fatiguestrength of a particular crankshaft by at least 16 per cent.
The general proportions of that crankshaft were, however, quite different from the cast crankshafts with which the authors were concerned, and the hole in the crankpin was slightly eccentric; this, they said, might appreciably affect the optimum bore size.
Further Conclusions Further conclusions arrived at after these tests, in addition to those already mentioned, were:—(1) Increase of crankweb thickness or breadth gave an increase. of bending fatigue-strength, but the improvement was not so great as the resulting increase of crankweb section modulus, Above certain values, a greater web thickness •or breadth, gave only a small increase in strength.
(2) Crankpin length did not influence bending fatigue-strength.
(3) The introduction of lightening holes below a certain size, resulted in an increase in bending fatigue-strength.
(4) The nominal limiting stress for the crankshaft was influenced considerably by crankshaft size. Reducing all the linear dimensions of the shaft by half resulted in a 28-per-cent. increase of nominal limiting fatigue-strength.