The implications of tolerances for design

The above explains the need for tolerances since nothing can be made perfectly. The following examples show how tolerances and clearances can be used together to make sure parts assemble. Figure 4.16 shows an example of the influence of hole clearances on position, dimensions and tolerances. The example consists of two plates bolted together. The top plate has two counter-bored clearance holes in it. The lower plate has two M5 threaded holes in it into which bolts are screwed. This example is concerned with the tolerance for the hole centre distance and the necessary clearances on the bolt in the upper plate. Let us assume that the hole spacing for the counter-bored holes in the top plate is invariant at 22,5mm. The tolerance associated with the threaded holes centre spacing in the lower plate is 22,5 ± 0,5mm. This tolerance of ± 0,5mm is accommodated by the clearances on the bolt head and body of the counter-sunk holes in the top plate. These counter-sunk holes are over-sized to accommodate the hole centre spacing variability. The bolt shank diameter is 5mm and the head diameter is 8mm and the corresponding bolt hole diameters in the upper plate are 5,5mm and 8,5mm. This means that each bolt is 'free' to move ± 0,25mm about the nominal value of 22,5mm to accommodate spacing variabilities.

2xM5 < 22,5 * 0,5 fthread crs)
 \////\ \$ f/ ± v///,. =fc 22,5 (thread crs) 22,5 (C/B hole crs) Figure 4.16 The influence of hole clearances on hole centre position dimensions and tolerances The three small diagrams in Figure 4.16 show the three cases of nominal dimension, maximum dimension and minimum dimension. The top-right diagram shows the nominal situation where the threaded hole centre distance in the lower plate is the nominal value of 22,5mm. In this condition the bolts have an equi-spaced clearance on either side of the holes in the top plate. In the lower left-hand figure, the threaded holes centre distance is at the lowest value (i.e. 22,5 - 0,5 = 22,0mm). In this case the bolts and plates will still assemble because the clearances of the bolts in the upper plate allowed the bolts to be closer together. The lower right-hand figure case shows the situation when the threaded hole centre distance in the lower plate are in their maximum dimension condition (i.e. 22,5 + 0,5 = 23,00). In this case assembly is still possible because the clearances in the upper hole are such that the bolts can be positioned at their maximum spacing. It should be noted that the tolerance of 22,5 ± 0,5mm is a generous tolerance and has been given this value for convenience of drawing and understanding.