Engineering Drawing for Manufacture phần 7 pptx

96 Engineering drawing for manufacture

Tolerance band width - 0,021

Tolerance band f7 = .o,o~,

-o.o2o

f7 =-o.o41

Lower size limit for f7 (19.959) ] I Go-NoGo Gauge

r- ~ r" ~ r" r" t- r" ~ r- J I tt

test

Figure 5.8 Example of a 20,00f7 go~no-go gauge inspecting 10 shafts from a

production line

5.4.1 Fit systems

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 interferencefit 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 tran￾sition 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.

5.4.2 The "shaft basis" and the "hole basis' system of fits

In all the examples given above, the discussion has been concerning

'shafts' and 'holes'. It should be remembered that this does not

necessarily apply to shafts and holes. These are just generic terms

that mean anything that fits inside anything else. However,

whatever the case, it is often the case that either the shaft or the hole

is the easier to produce. For example, if they are cylindrical, the

shaft will be the more easily produced in that one turning tool can

produce an infinite number of shaft diameters. This is not the case

with the cylindrical hole in that each hole size will be dependent on

a single drill or reamer.