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Analysis and experiments on the thermal characteristics of the fit of hollow pieces and cylindrical pieces

The tolerance and fit of the hollow pieces and cylindrical pieces is an important primary standard in mechanical engineering. Current standards of tolerance and fit are based on the standard temperature 20°C. Thus the precision of the fit can be achieved only at 20°C. When the temperature changes, the diameter of a cylindrical piece and the inner diameter of a hollow piece changes, too. But the rule of thermal deformation of hollow pieces is different from the rule of cylindrical pieces. Studies on cylindrical pieces and hollow pieces in precision technology are going on and the mathematical models which are based on the approximate formula of Grueneisen have been applied. Based on this models, a theory of an optimal thermal fit is given. Designers should calculate the deviation value of hollow pieces and cylindrical pieces and adjust the tolerance of cylindrical pieces, in order to make sure, that when the temperature changes the clearance or the interference will not change. The rule of adjusting the tolerance of the cylindrical piece is described in detail. Experiments have been done to verify the optimal thermal fit theory. The deformation of the clearance of one plain bearing has been detected. The results of the experiment are presented. The uncertainty of the experimental results is about 1um. The results also show, that the clearance of the plain bearing is changing by the temperature and the value of the deformation is very close to the theoretical value. Thus, based on the experiment results, the theory of an optimal thermal fit is correct and should be used in mechanical engineering design.