Screws are a broad category of mechanical fasteners with a threaded shaft, designed to screw into a part. This includes wood screws and self-tapping screws, which have a tapered shaft with sharp threads designed to cut a mating thread in the part to which they are fastened. It also includes machine screws, which much more closely resemble bolts, but their entire shaft is normally threaded.
Screws are sometimes delineated by their use. They mate with a threaded hole in one of the parts being fastened. Fasteners that are either self-tapping or which have their entire shaft threaded are normally called screws.
To remain securely fastened, they generally reply on friction at the threads. In the case of self-tapping screws, radial expansion of the hole causes much of this friction. For machine screws, friction is mostly caused by axial force resulting from the torque applied to the head and the subsequent tension in the screw combined with compression of the parts. Machine screws may alternatively, or additionally, use a locking nut or thread-locking adhesive to prevent loosening.
Machine screws vary from: Hex screw, hex socket countersunk head screw, countersunk head cross screw, hex socket cap screw, and pan head cross screw.
Machine screws have a standard thread on a parallel shaft, designed to be screwed into a tapped hole in a part, they may also be used with a nut. The most common type of machine screw is a hex screw — which is very similar to a hex bolt, but without the shank. There are many types of machine screws for specific applications, generally differing in the shape of the head:
- Countersunk head screw: A tapered head designed to be countersunk so that it lies flat with the surface of a part, with a.
- Cap head screw: A barrel-shaped head with a socket, designed to fit into a counterbored hole.
- Pan head screw: A domed head fitted with a socket
- Grub screw: A type of set screw that has its entire outer surface threaded, with no head, allowing it to be screwed all the way through a hole to lock against a part below the hole.
Socket headed screws may have many different socket types. The order in which the shape of the head and the socket are given in the name may vary. Some of the most common socket types are:
- Hex socket or ‘Allen key’
Self-tapping screws including wood screws
Self-tapping screws is a term generally used for screws designed to cut their own thread in sheet metal, although in a general sense wood and plastic screws are also self-tapping. These types of screws often have a sharp pointed end and sharp threads on a tapered shaft. They may have a section of the unthreaded shank, like a bolt. Common types include:
- Wood screws
- Self-tapping screws
- Coach screw: This is a large self-tapping screw with a hex head to enable high torque when fitting. Confusingly, this is quite different to a coach bolt, which is a machine screw with a domed head and square shank.
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