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Fasteners
A fastener is a hardware device that mechanically joins or affixes two or more objects together.
Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container, etc. There are also special purpose closing devices, e.g. a bread clip. Fasteners used in these manners are often temporary, in that they may be fastened and unfastened repeatively.
Screw
A screw is a shaft with a helical groove orthread formed on its surface. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.
Threaded Screw
Screw used as a threaded fastener consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a helical ridge or thread formed on it. The thread is essentially an inclined plane wrapped around a shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix (taps and dies), or the screw may create it when first driven in (a self-tapping screw). The head is specially shaped to allow a screwdriver or wrench (UK: spanner) to grip the screw when driving it in. It also stops the screw from passing right through the material being fastened and provides compression.
Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse.
A screw that is tightened by turning it clockwise is said to have a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Examples include rotating items such as the left hand grinding wheel on a bench grinder or the left hand pedal on a bicycle (both looking towards the equipment).
Threaded fasteners are traditionally made by a cutting action such as taps and dies provide, however recent advances in tooling allows them to be made by rolling the blank (a section of rod) between two specially machined dies. The thread form and shape of the fastener are squeezed onto the blank. This method work hardens> the threads and saves material. A rolled thread is obvious after manufacture because the outside diameter of the thread is greater than the diameter of the blank material. Bicycle spokes, which are just very long thin bolts, always use rolled threads for strength.
Types Of Screws & Bolts
- A hex cap screw has a protruding hexagonal head, designed to be driven by a spanner or wrench.
- A socket cap screw has a hexagonal recessed drive, usually with a cylindrical head, but can also be found with a rounded button head or a countersunk flat head. Socket cap screws can be torqued more tightly than other drives without stripping, and they are usually made from a high strength steel alloy.
- A Wood screw has a tapered shaft, allowing it to penetrate undrilled wood, or a wallplug in brickwork etc.
- Lag screw, lag bolt or coach screw refers to a large wood screw with a hexagonal head, driven by a wrench rather than a screwdriver.
- A Machine screw has a cylindrical shaft, threaded its entire length, and fits into a nut or a tapped hole; a small bolt.
- Self-tapping screws or thread cutting screws have sharp threads that cut into a material such as sheet metal or plastic. They are sometimes notched at the tip to aid in chip removal during thread cutting.
- A Self-drilling screw is similar to a self-tapping screw, but has a drill-shaped point to cut through the material without prior drilling.
- Thread rolling screws have a lobed (usually triangular) cross section. They form threads by pushing outward during installation. They may have tapping threads or machine threads.
- A Drywall screw is a specialized self-tapping screw designed to bind drywall to wood or metal studs, but it has proven to be a versatile construction fastener.
- A Set screw, used to prevent loosening due to vibration, is available with thumb screw, square head, Allen head (inset socket) and, most commonly, headless (a grub screw in UK parlance, designed to be inserted flush with or below the surface of the work piece). Alternatively defined to be a screw whose thread reaches the head (if any), as opposed to a bolt.
- Dowel screw is a wood screw with two pointed ends and no head, used for making hidden joints between two pieces of wood.
- A stud is similar to a bolt but without the head. Studs are threaded on both ends. In some cases the entire length of the stud is threaded, while in other cases there will be an unthreaded section in the middle. It may be anchored in concrete, for example, with only the threads on one end exposed.
- A carriage bolt or coach bolt has a domed or countersunk head, and the shaft is topped by a short square section under the head. It is used in wood where the square section is pulled into the wood as the nut is tightened, preventing the bolt from turning. The rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.
- A stove bolt is similar to a carriage bolt, but usually used in metal. It requires a square hole in the metal being bolted to prevent the bolt from turning.
- The Superbolt is a form of very large fastener where tension in the bolt is developed by a special nut containing individual jackscrews. This is applied in large structural joints where hand tools or portable tools are insufficient to develop the required tension of a simple bolt and nut.
- Acme screw form has threads that are stronger and broader than standard V-profile threads, making them much better for load carrying, linear actuating (also see ball screws), and quick threading. It is often used for vise screw
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