Nuts, bolts, and spanners

In the early 1800s, Joseph Whitworth devised a set of standards for screw threads and their associated nuts and bolts. Under the Whitworth system the size of the bolt head and nut is determined by the diameter of the threaded part of the bolt. For engineering drawing purposes, the distance across the flats of the bolt head (S) was set at:

where D 5 diameter of the threaded part of the bolt. The size of the spanner required for these nuts and

bolts is marked on the spanner, near the jaws (see Fig. 30.2). A spanner for a half-inch Whitworth bolt has a distance across the jaws of 0.75 inch.