bolts and screws
Some of the many types of bolts and screws in common industrial and domestic use. Materials used range from brass and various steels to polymer plastics.
Bolts and screws are devices in which the principle of the screw thread, which may be traced back as far as Archimedes, is applied to the fastening together of objects. A screw is essentially conical, with a sharp point and widening toward the head – which is usually shaped to take a screwdriver – with a helical ridge. If the point is pressed into the material (usually wood) and the screw longitudinally rotated by means of a screwdriver, the screw will be driven into the wood and will be held in place by friction.
A bolt is essentially cylindrical, again with a helical ridge, and has a broad head usually shaped to take a spanner or wrench. It is used in conjunction with a nut, a member containing a pre-threaded hole into which the bolt fits. The objects to be fastened are held together by the pressure of the bolthead on one side, the nut on the other. The distance between consecutive turns of a screw thread is termed the pitch of the thread.