Functional and non-functional dimensions

Although every aspect of a component has to be dimensioned, some dimensions are naturally more important than others. Some dimensions will be critical to the correct functioning of the component and these are termed functional dimensions. Other dimensions will not be critical to correct functioning and these are termed non-functional dimensions. Functional dimensions are obviously the more important of the two and therefore will be more important when making decisions about the dimension value. Figure 4.1 shows an assembly of a shaft, pulley and body. A shaft is screwed into some form of body and a pulley is free to rotate on the shaft in order to provide drive power via a belt (not shown). The details of the three parts of this assembly are shown below the assembly drawing. The important function dimensions are labelled 'F', and the non-functional dimensions 'NF'. The main function of the assembly is to allow the pulley to rotate on the shaft, driven by the belt. Thus, the bearing diameter and length of the bolt pulley are important and therefore they are functional dimensions because they define the clearances that allow the pulley to rotate on the shaft. The belt will be under tension and the resulting lateral drive force will be transmitted to the shaft. The stresses set up by this force must be resisted by the screw thread in the body. Therefore, the length of engagement of the thread in the body is a functional dimension.
Engineering Drawing for Manufacture

by Brian Griffiths
Publisher: Elsevier Science & Technology Books