These are designed to engage with the tool used to tighten them. Bolt headsįor more details on this topic, see List of screw drives.īolts use a wide variety of head designs, as do screws. Many bolts are held fixed in place during assembly, either by a tool or by a design of non-rotating bolt, such as a carriage bolt, and only the corresponding nut is turned.
traditional wood screws), precluding the use of a nut, or when a sheet metal screw or other thread-forming screw is used.Ī screw must always be turned to assemble the joint. This is most obviously so when the thread is tapered (i.e. Where a fastener forms its own thread in the component being fastened, it is called a screw. No more than two turns of the thread should be within the hole. Too long prevents the nut from being tightened down correctly. Too short places the dowel shear load onto the threads, which may cause fretting wear on the hole. The grip length should be chosen carefully, to be around the same length as the thickness of the materials, and any washers, bolted together. screws, but this is incidental to its use, rather than defining. The presence of the unthreaded shank has often been given as characteristic of bolts vs. For this reason, many bolts have a plain unthreaded shank (called the grip length) as this makes for a better, stronger dowel.
This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. Many threaded fasteners can be described as either screws or bolts, depending on how they are used.īolts are often used to make a bolted joint. Screws in contrast are used with components, at least one of which contains its own internal thread, which even may be formed by the installation of the screw itself. The defining distinction, per Machinery's Handbook, is in their intended purpose: Bolts are for the assembly of two unthreaded components, with the aid of a nut. There are several practical differences, but most have some degree of overlap between bolts and screws. The distinction between a bolt and a screw is commonly misunderstood. Main article: Screw § Differentiation between bolt and screw
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