Shrink-Fit Stress Systems in Built Crankshafts

Davidson, William R. S (1951) Shrink-Fit Stress Systems in Built Crankshafts. PhD thesis, University of Glasgow.

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Among applications of the well-known practice of shrink-fitting, the assembly of large marine crankshafts from relatively small forgings is perhaps the most outstanding example, having regard to the stringent operating conditions in which the component must function satisfactorily. Variations in shrink-fitting practice and failures attributable to slipping of the grip have directed attention to the lack of information on crankshaft assemblies and instigated an investigation, the results of which are presented in this thesis. A major part of the investigation comprises a survey of published literature relevant to the subject. It has been found that little is yet known about stress or grip conditions at the shrink-fit, and that the rules for sizes and proportions have evidently no theoretical foundation. The scope of the survey has therefore been extended to include work on all types of interference fits, from v/hich it is evident that, while a useful amount of empirical data on factors affecting grip strength exists, treatment of interface pressure and stress has been limited to the application of elastic thick cylinder theory. With a view to analysing stress conditions in assemblies overstrained by fit allowances which cause permanent enlargement of the hollow element, a part of the survey is devoted to examining work on thick cylinders subjected to internal fluid pressure. A number of features of shrink-fitting, such as prevention of free axial shrinkage, overstrain of the hollow element, and the influence of temperature on the elastic constants of the material, have been analysed as part of the theoretical work of the present investigation. On the basis of certain reasonable assumptions, a solution for the stress in a representative crank web shape has been obtained, using Relaxation Methods of analysis. Stress concentration and radial stiffness values, which correlate the complex shape to the rotationally-symmetrical, follow readily from the solution. The experimental investigations comprise tests on rings-and-plugs, on model crank webs, and on large crank webs removed from a vessel which had been scrapped. The results indicate that prevention of free axial shrinkage does, in fact, occur to a marked degree. Furthermore, the presence of axial grip has been detected in large crank webs which had suffered extensive cold-working due to pulsating bending actions during service. Model experiments have demonstrated the influence of surface finish on the grip strength to be quite unimportant. Friction values in the large crankshaft, on the mating surfaces of which tool-marks were clearly visible, were comparable with those in model webs with near-perfect honed surfaces. Interface pressure and stress values predicted from theoretical considerations were in sound agreement with values measured by electrical strain gauges and inferred from deformations. A list of design formulae and data, and recommendations for improvements to shrink-fitting practice, based on the results of the investigation, are presented.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Mechanical engineering, Naval engineering
Date of Award: 1951
Depositing User: Enlighten Team
Unique ID: glathesis:1951-78849
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:47
Last Modified: 30 Jan 2020 14:47

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