Shrink-fit Investigations on Simple Rings and on Full-scale Crankshaft Webs

Abstract

This paper presents the results of experimental investigations on shrink-fit effects in full-scale crankshaft webs and of ancillary studies of the behaviour of simple ring-and-plug arrangements and model two-pin web assemblies. These small-scale tests allow such variables as surface conditions and fit allowances to be more fully and systematically examined; the range of surface finish used is from superfine to dry-scrape standards; mating surface conditions from lubricated to chemically dry; and fit allowances from 0.5 to 4.0 mils, per inch diameter. Three methods of dismantling are used: trepanning, axial force, and torsion. The first allows the most accurate determination of the permanent distortion and the others provide evidence as to the effectiveness of the grip. The coefficients of friction deduced therefrom depend on the bore pressures. Experimental bore pressures are obtained by assuming that the measured relief of bore strain on removal of the pins is elastic. In addition, all results are compared with available theory. For this purpose the thick-cylinder equations of Lame, and Allen and Sopwith, are used for the elastic and over-strained conditions respectively. Some degree of experimental verification of theory was sought through the medium of a special preliminary experiment on bore strains under fluid pressure conditions. This experiment is included. In the full-scale investigations, webs from marine Diesel-engine crankshafts before and after service are examined. The fit allowances are necessarily limited in number but vary between 1.0 and 2.0 mils, per inch diameter. The procedure involves the measurement, by means of electrical-resistance strain gauges patterned over the surface of the web, of the relief of stress when the pins are cut out and again when the web is cut through. The variations of radial and circumferential stresses are shown and compared for different fits and with certain theoretical considerations. Dismantling of selected ‘after service’ webs, by axial forces up to 2,000 tons, provides information on coefficients of friction and loss of fit due to plastic flow. The main findings from the experimental work are summarized and reviewed. Experimental results are found to agree with theory in all cases where the ring or web was stressed within the elastic range. Where overstraining has occurred, however, experiment and available theory are, in general, not in agreement; with rings unexpectedly high permanent strains are revealed. The stress patterns obtained for overstrained webs are complex; yielding in the bridge-piece between the pins results in uneven radial pressures at the bore which, in turn, are low relative to the circumferential stresses at the bore; loss of fit is again excessive. There are definite indications that one explanation of the divergence is the inadequacy of overstrained thick-cylinder theory as at present established. In the large webs, a further complication is provided by the evidence of an additional stress system owing, presumably, to uneven flame heating of the webs in preparation for shrinking. Data on derived coefficients of friction at the mating surfaces are summarized. The investigation has been confined to the static effects of shrink fitting, but evidence of bell-mouthing in the bores of the used crankwebs indicates that the dynamic effects of service stresses also require examination. It is considered that one cause of this bellmouthing is the excessive fit allowances used in current marine engineering practice.