The allowance for force-fit is in practice given by the diameter alone, independently on the materials, the outer diameter of the boss and the inner diameter of the shaft. The author starting from the stress equations of thick cylinder of infinite length, constructed a new chart for the determination of the theoretical value of allowance taking these factors into consideration. And the application of the chart to the calculation of the maximum stresses induced by force-fit is also introduced with an example.

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Experimental researches dealing with the forcing fit of steel shafts on steel and cast iron bosses respectively, have been carried out by the author. As regards the test pieces employed in the tests, the diameter and axial length of the finished cylindrical parts of the shafts were determined at 1" and 3/4" each, and the inner diameter and axial height of the bosses were, from the point of view of simplicity in preparing the test pieces, kept constant at 1" and 3/4" respectively, while the radial thickness was varied at 1" and 1/2" to ascertain the effect of the thickness. The diameters of the shafts and the inner diameters of the bosses were measured by a specially prepared micrometer up to the term of 0・0001". Allowances were varied in proper range, from 0・0001" to 0・0030" for a nominal diameter of 1", and the accuracy of measurement was the same as that of the diameters. The forcing fit pressure and slipping resistances against several kinds of external forces, such as axial forces and twisting moment, were measured by suitable testing machines. The permanent set on the bosses was measured immediately after removing the shafts out of the bosses. Thus the author was able b find the limiting value of allowances which practically produce a permanent set on the bosses. At the same time it was also discovered that even if any greater value of allowance than the limiting value should happen to be given, both the forcing fit pressure and the slipping resistance can never be increased at the same rate, and furthermore that this limiting value of allowance depends on the material only and not on the radial thickness of the bosses.

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