Screw
A screw is a type of threaded fastener, a device used to hold objects together. It consists of a shaft, which may be cylindrical or conical, and a head. The shaft has a helical ridge formed on it that assists in fixing the screw and stops it from falling out. The head is used to grip the screw when driving it in, and stops the screw from passing right through the material being fastened.
Screws are more versatile than nails: they can be taken apart and replaced. Nails hold strictly by friction, while screws are a specialized form of wedge or inclined plane that either fits into a corresponding plane in a nut, or form a corresponding plane in the wood or metal as it is inserted.
A related concept to a screw is a bolt, called a machine screw in the smaller forms. The combination of bolt and nut is a very common way of holding together temporary as well as permanent constructions or assemblies. When screws and bolts cannot be used, riveting, welding, soldering or glueing are all alternatives.
A screw is almost always tightened by turning it clockwise. (Is this true throughout the world?) Reverse-threaded screws are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a normal thread.
Mechanics
A screw is often considered to be a specialized application of the wedge or inclined plane. In this view, the screw is a wedge wound around an interior cylinder or shaft. The technical analysis (see also statics, dynamics) to determine the pitch, thread shape or cross section, coefficient of friction (static and dynamic), and holding power of the screw is very similar to that performed to predict wedge behavior. See also simple machine.
Types of Screw
- Wood screw has a tapered shaft allowing it to penetrate undrilled wood.
- Machine screw has a cylindrical shaft and fits into a nut or a tapped hole, a small bolt.
- Self-tapping screw has a cylindrical shaft and a sharp thread that cuts its own hole, often used in sheet metal or plastic.
- Drywall screw, a specialized self-tapping screw with a cylindrical shaft that has proved to have uses far beyond its original application.
Shapes of Screw Head
- Pan head disc with chamfered outer edge
- Cheese head disc with cylindrical outer edge
- Countersunk conical, with flat outer face and tapering inner face allowing it to sink into the material, very common for wood screws
- Button or dome head flat inner face and hemispherical outer face
- Mirror screw head countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for attaching mirrors
Types of Screw Drive
A variety of screwdrivers and related tools exist to drive screws into the material to be fixed.
- Slot to be driven by a flat-bladed screwdriver
- Cross-head X-shaped slot driven by a cross-head screwdriver
- Phillips another type of cross-head pattern, patented, designed originally for use with mechanical screwing machines, the driver will ride out, or cam out, under strain.
- Pozidriv, patented, similar to cross-head but with better resistance to slipping, or cam out)
- Hexagonal or hex the screw head has a hexagonal hole and is driven by a hexagonal wrench, sometimes called an Allen key, or a power tool with a hexagonal bit
- Square the screw head has a square hole and is driven by a special power-tool attachment