Factors Affecting the Grip in Force, Shrink, and Expansion Fits

Abstract

Factors affecting the grip, which depends wholly on the elastic properties of the mating materials, are presented in the first part of the paper by a consideration of force fit practice in the assembly of wheel centres on axles. The degree of accuracy obtained in machining operations is determined by an examination of the out-of-roundness and the out-of-straightness of the cylindrical mating surfaces. The importance of considering the physical properties of the materials used in elastic grip assemblies is indicated by results obtained from specimens taken from wheel centres. Curves showing the residual force fit of elements which failed to satisfy back pressure tests suggest that failure was due to excessive fit allowances. The effect of a reduction in the surface contact area is demonstrated by a series of tests on small steel elements. The nature of the lubricant is shown to have a great effect on the force necessary to produce axial slip in a force fit assembly. The lubricants considered include rape, sperm, Bayonne and “Texaco” motor oils, a mixture of graphite and engine bearing oil, and a cutting lubricant used in metal removal operations. The influence of “skin” or film on the mating surfaces before applying the lubricant is examined in a series of tests in which the mating elements are assembled with surfaces made clean by the use of solvents and by rubbing with a clean cloth. The quality of the grip established by mating the elements, by heating the hollow element, by cooling the solid element, and by pressing the hollow element over the solid element, is considered. Experimental results show that the grip is dependent on a surface film condition which may separate the elements on assembly and may be independent of the method of assembly. The condition of the mating surfaces after the fit is broken is examined and illustrated in each case. Torsional resistance to slip is considered in the latter part of the paper. Force and shrink fit assemblies in which the elements are mated with surfaces lubricated, and with surfaces perfectly dry and free from film, are examined. An analysis of the results of tests provides further evidence of the great influence of a surface film condition on the resistance to slip and affords a means of comparing the axial and torsional coefficients of friction.