Limits and fits

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 offsets 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.

Iso Limits And Fits

Figure 5.6 ISO tolerance ranges for a variety of manufacturing processes

300um 200 100 0 -100 -200 -300um

1________________________________

i Shaft tolerance ranges for | 25mm nominal size and IT7.

A

1

E "«„

■f,B

_______________________4ji*l§

I

Y z

Hole tolerance ranges for fe 25mm nominal size and fT7. |

a

Figure 5.7 ISO shaft and hole tolerance classes for 25mm nominal size and range IT7

Figure 5.7 ISO shaft and hole tolerance classes for 25mm nominal size and range IT7

The alphanumeric tolerance range classes typified in Figure 5.7 can be used to inspect components produced by manufacturing processes. As an example, let us assume we want to inspect a shaft which is to be a 'close-running fit' in a journal as per the left-hand diagram in Figure 5.1. The shaft would be represented by the designation 4>20,00 f7. The upper size limit for class f7 is 19,980mm diameter and the lower size limit for class f7 is 19,959mm diameter. If the shaft were produced on a lathe, there will be a size variability which depends upon the operating conditions and the tool wear. We need to reject any shafts that have a diameter in excess of the upper size limit as well as those which have a diameter that is lower than the lower size limit. This would ensure that the only turned shafts that pass the inspection process are those which meet the requirements if the class is f7. Such an inspection situation is demonstrated by the schematic diagram in Figure 5.8. The basic inspection device is a 'go/no-go' gauge which has one recess corresponding to the upper size limit and another recess which corresponds to the lower size limit for class f7. In this case we are assuming that 10 shafts are manufactured and each is inspected using the go/no-go gauge. To pass inspection, each must be able to enter the left-hand 'go' gauge but not the right-hand 'no-go' gauge. Assuming that the sizes for the 10 shafts are as shown, shafts 1, 2, 3, 4, 7, 8, 9 and 10 pass the f7 inspection test whereas shafts 5 and 6 are rejected because they are undersized and oversized respectively.

Upper size limit for f7 (19,960)

Lower size limit for f7 (19,959)^

Inspection Pass | Pass ] Pass ] Pass | Fail | Fail | Pass | Pass | Pass | Pass ] test

Tolerance band width - 0.021 Tolerance band f7 =

Go-NoGo Gauge

Tolerance band width - 0.021 Tolerance band f7 =

Go-NoGo Gauge

Figure 5.8 Example of a 20,00p go/no-go gauge inspecting 10 shafts from a production line

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Responses

  • dinodas gaukrogers
    What is the upper size limit for a 25mm hole?
    9 years ago
  • nasih
    What are the limits and fits in drawing?
    7 years ago
  • nora
    What are limits in engineering drawings?
    9 months ago
  • erik
    Why are limits and fits critical within engineering?
    6 months ago

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