Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips; hex, Robertson, and TORX are also common in some applications. Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repair person.
Slot head has a single slot, and is driven by a flat-bladed screwdriver. The slotted screw is common in woodworking applications, but is not often seen in applications where a power driver would be used, due to the tendency of a power driver to slip out of the head and potentially damage the surrounding material.
Cross-head, cross-point, or cruciform has a "+"-shaped slot and is driven by a cross-head screwdriver, designed originally for use with mechanical screwing machines. There are five types:
1. The Phillips screw drive has slightly rounded corners in the tool recess, and was designed so the driver will slip out, or cam out, under high torque to prevent over-tightening. The Phillips Screw Company was founded in Oregon in 1933 by Henry F. Phillips, who bought the design from J. P. Thompson. Phillips was unable to manufacture the design, so he passed the patent to the American Screw Company, who was the first to manufacture it.
2. A Reed & Prince or Frearson screw drive is similar to a Phillips but has a more pointed 75° V shape. Its advantage over the Phillips drive is one driver or bit fits all screw sizes. It is found mainly in marine hardware and requires a special screw driver or bit to work properly. The tool recess is a perfect cross, unlike the Phillips head, which is designed to cam out. It was developed by an English inventor named Frearson in the 19th century and produced from the late 1930s to the mid-1970s by the former Reed & Prince Manufacturing Company of Worcester, Massachusetts a company which traces its origins to Kingston, Massachusetts in 1882, and was liquidated in 1990 with the sale of company assets.
3. A JIS (Japanese Industrial Standard) head, commonly found in Japanese equipment, looks like a Phillips screw, but is designed not to cam out and will, therefore, be damaged by a Phillips screwdriver if it is too tight. Heads are usually identifiable by a single dot to one side of the cross slot. The standard number is JIS B 1012:1985.
4. French Recess, also called BNAE NFL22-070 for Bureau de Normalisation de l'Aéronautique et de l'Espace, a French standards organization.
5. Pozidriv is patented, similar to cross-head but designed not to slip, or cam out. It has four additional points of contact, and does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in Pozidriv screws, but Pozidriv screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Heads are marked with single lines at 45 degrees to the cross recess, for identification. (note that two lines at 45 are a different recess: a very specialised Phillips screw). Pozidriv was jointly patented by the Phillips Screw Company and American Screw Company in the USA. Developed by GKN in the 1960s, the recesas is licenced from Trifast PLC in the rest of the world.
TORX is a star-shaped "hexalobular" drive with six rounded points. It was designed to permit increased torque transfer from the driver to the bit compared to other drive systems. TORX is very popular in the automotive and electronics industries due to resistance to cam out and extended bit life, as well as reduced operator fatigue by minimizing the need to bear down on the drive tool to prevent cam out. TORX screws were found in early Apple Macintosh computers, to discourage home repairs. TORX PLUS is an improved version of TORX which extends tool life even further and permits greater torque transfer compared to TORX. A tamper-resistant TORX head has a small pin inside the recess. The tamper-resistant TORX is also made in a 5 lobed variant. These TORX configurations are commonly used in correctional facilities, public facilities and government schools.
TTAP is an improved "hexalobular" drive for without wobbling and stable stick-fit. TTAP is backward convertible with generic hexalobular drive.
Hexagonal (hex) socket head has a hexagonal hole and is driven by a Hex Wrench, sometimes called an Allen key or Hex key, or by a power tool with a hexagonal bit. Tamper-resistant versions with a pin in the recess are available. Hex sockets are increasingly used for modern bicycle parts because hex wrenches are very light and easily carried tools. They are also frequently used for self-assembled furniture (e.g. from Ikea).
Robertson head, invented in 1908 by P.L. Robertson, has a square hole and is driven by a special power-tool bit or screwdriver. The screw is designed to maximize torque transferred from the driver, and will not slip, or cam out. It is possible to hold a Robertson screw on a driver bit horizontally or even pendant, due to a slight wedge fit. Commonly found in Canada in carpentry and woodworking applications and in Canadian-manufactured electrical wiring items such as receptacles and switch boxes.
Square-drive head is an American clone of the Robertson that has a square hole without taper. Due to the lack of taper, the hole must be oversize relative to the screwdriver, and is much more likely to strip than the Robertson.
Tri-Wing head has a triangular slotted configuration. They were used by Nintendo on several consoles and accessories, including the Game Boy, Wii, and Wii Remote, and on some Nokia phones and chargers to discourage home repair.
Torq-Set or offset cruciform may be confused with Phillips; however, the four legs of the contact area are offset in this drive type. Is commonly used in the aerospace industry.
Spanner drive uses two round holes opposite each other, and is designed to prevent tampering. Commonly seen in elevators in the United States.
Clutch Type A or standard clutch head resembles a bow tie. These were common in GM automobiles of the 1940s and '50s, particularly for body panels.
Clutch Type G head resembles a butterfly. This type of screw head is commonly used in the manufacture of mobile homes and recreational vehicles.
~From www.wikipedia.org~
Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips; hex, Robertson, and TORX are also common in some applications. Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repair person.
Screws are made in a wide range of materials, with steel being perhaps the most common, in many varieties. Where great resistance to weather or corrosion is required, stainless steel, titanium, brass or bronze may be used, or a coating such as brass, zinc or chromium applied. Electrolytic action from dissimilar metals can be prevented with aluminium screws for double-glazing tracks, for example. Some types of plastic, such as nylon or Teflon, can be threaded and used for fastening requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. Even porcelain and glass can have molded screw threads that are used successfully in applications such as electrical line insulators and canning jars.
The United States has its own system, usually called the Unified Thread Standard, which is also extensively used in Canada and in most other countries around the world. At least 85% of the world's fasteners are dimensioned to Unified thread dimensions, and the biggest selection of fastener sizes and materials are found supplied in this standard (Source: World Fastener Review, Industrial Press, 2006). A version of this standard, called SAE for the Society of Automotive Engineers, was used in the American automobile industry. The SAE is still associated with inch-based fasteners by the public, even though the U.S. auto industry (and other heavy industries relying on SAE) have gradually converted to ISO preferred series fasteners for some assemblies from the 1970s onward, because global parts sourcing and product marketing favor international standardization. However, all automobiles sold in around the world contain both metric (engine assemblies) and Imperial fasteners (for example, lug nuts, oxygen sensors, internal electrical assemblies, body fasteners, lamps, steering, brake and suspension parts ).
