Figure 5.9 shows the three basic fit 'systems'. The left-hand sketch shows a shaft which will always fit in the hole because the shaft maximum size is always smaller than the hole minimum size. This is called a clearance fit. These have been discussed above with respect to running and sliding fits as per Figure 5.1. In some functional performance situations, an interference fit is required. In this case, the shaft is always larger than the hole. This would be the case for the piston rings prior to their assembly within an engine bore or for a hub on a shaft. In some functional performance situations, a transition fit may be required. Should the shaft and hole final diameters be an interference-clearance fit, the clearances will be very small and the location would be very accurate. If it were an interference-transition fit, on assembly the shaft would 'shave' the hole and thus the location would be very accurate.
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