TABLE OF CONTENTS The tolerance ranges shown in Figures 5.4, 5.5 and 5.6 are simply ranges. To relate to function they must be put into context and related to some absolute datum. This is the situation demonstrated by the bearings in Figure 5.1. Considering the close-running fit example, the tolerance ranges are IT8 for the hole and IT7 for the shaft. However, it is insufficient to just quote an IT tolerance class on its own. The tolerance class must be related to a datum, in this case the nominal 20mm diameter. The shorthand way of referring to these limits is the designations H8 and f7 . The 8 and the 7 refer to the IT tolerance grades in Figure 5.4. The H and the f give the offset relative to the nominal value. Note that the upper case letter always applies to holes and the lower case letter always applies to shafts.
The relationship between the tolerance grades and their offset is shown in the diagram in Figure 5.7. This is for a nominal size of 25mm diameter and tolerance range IT7. Shaft tolerance ranges are represented by the lower-case letters a to z and holes by the upper-case letters A to Z. Since these are all for the ISO tolerance range IT7, the values should be a7 to z7 and A7 to Z7 respectively. Note that the two sets of bars in Figure 5.7 (for holes and shafts) are the inverse of each other.
