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A screw drive is the system used to turn a screw.[1] At a minimum, it is a feature on the screw that allows for it to be turned. Usually it also involves a mating tool, such as a screwdriver, that is used to turn it. The following heads are categorized based on commonality, with the less common drives being classified as "tamper-resistant". Most heads come in a range of sizes.
All of these screw drives are characterized by a female tool and a male fastener.
A square screw drive uses square shaped fastener heads. They can be turned with a crescent-type wrench, open-end wrench, or 8- or 12-point[2] sockets. Common in the 19th and early 20th centuries it is less common today due to competition from the similar hexagonal screw. It is also still widely used on theatrical lighting equipment, plumbing on brass plugs, cleanouts, etc. and on gates attached to residential wood fencing.
A hex screw drive uses six-sided fastener heads. The fastener is known as a hex head cap screw. They can be turned with a Crescent-type wrench, combination wrench, or 6- and 12-point sockets. Better than square for locations where surrounding obstacles limit wrenching access, because smaller wrench-swing arcs can be successful.
A pentagon screw drive uses five-sided fastener heads. The fastener is known as a penta screw. They require a special five-point socket in order to be turned. Water meter covers, natural gas valves, and electrical cabinets are commonly secured with penta fasteners. It is also common in the U.S. for fire hydrants to have valves with a pentagon drive.
A thumbscrew is a type of screw drive with either a tall head and ridged or knurled sides, or a flat, vertical head. They are intended to be tightened and loosened by hand, and not found in structural applications. They are often cut for Phillips head or slotted screwdrivers as well.
| Blade width | Fastener size |
|---|---|
| 3⁄32 in (2.4 mm) | 0–1 |
| 1⁄8 in (3.2 mm) | 2 |
| 5⁄32 in (4.0 mm) | 3 |
| 3⁄16 in (4.8 mm) | 4–5 |
| 1⁄4 in (6.4 mm) | 6–7 |
| 5⁄16 in (7.9 mm) | 8–10 |
| 3⁄8 in (9.5 mm) | 12–14 |
| 7⁄16 in (11 mm) | 16–18 |
| 1⁄2 in (13 mm) | 18–24 |
The slot screw drive has a single slot in the fastener head and is driven by a flat-bladed screwdriver. It was the first type of screw drive to be developed, and for centuries it was the simplest and cheapest to make. The slotted screw is common in woodworking applications, but is not often seen in applications where a power tool would be used because a power driver tends to slip out of the head and potentially damage the surrounding material. The tool used to drive a slot is called a slot-head, flat-tip,[3] or flat head.[4]
A cross-recess screw drive has two slots, oriented perpendicular to each other, in the fastener head; a slotted screwdriver is still used to drive just one of the slots. This type is usually found in cheaply made roofing bolts and the like, where a thread of 5 mm (0.20 in) or above has a large flattened pan head. The sole advantage is that they provide some measure of redundancy: should one slot be chewed up in service, the second may still be used.
The following are screw drives based on a cruciform shape, i.e. a cross shape. Other names for these types of drives are cross-head and cross-point.
| Tool size | Fastener size |
|---|---|
| 0 | 0–1 |
| 1 | 2–4 |
| 2 | 5–9 |
| 3 | 10–16 |
| 4 | 18–24 |
Created by Henry F. Phillips, the Phillips screw drive was purposely designed to cam out when the screw stalled, to prevent the fastener damaging the work or the head, instead damaging the driver. This was caused by the relative difficulty in building torque limiting into the early drivers.
The American Screw Company was responsible for devising a means of manufacturing the screw, and successfully patented and licensed their method; other screw makers of the 1930s dismissed the Phillips concept because it calls for a relatively complex recessed socket shape in the head of the screw — as distinct from the simple milled slot of a slotted type screw.
There are five relatively common (and two rather uncommon) Phillips drive sizes that are different from the screw size; they are designated 000, 00, 0, 1, 2, 3, and 4 (increasing in size).[3][5]
The Frearson screw drive, also known as the Reed and Prince screw drive, is similar to a Phillips but the Frearson has a more pointed 75° V shape.[6] One advantage over the Phillips drive is that one driver or bit fits all screw sizes. It is often found in marine hardware and requires a Frearson screwdriver or bit to work properly. The tool recess is a perfect, sharp cross, allowing for higher applied torque, unlike the rounded, tapered Phillips head, which was designed to cam out at high torque. 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 Reed & Prince Manufacturing Company now of Leominster, Massachusetts. [7]
Also called BNAE NFL22-070 after its Bureau de Normalisation de l'Aéronautique et de l'Espace standard number.
The JIS B 1012 is commonly found in Japanese equipment. It 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 or an "X" to one side of the cross slot.[8]
Specific "JIS" standardized cruciform-blade screwdrivers are available for this type of screw.
The Mortorq drive, developed by the Phillips Screw Company, is a format used in automotive and aerospace applications. It is designed to be a lightweight, low-profile and high-strength drive, with full contact over the entire recess wing reducing risk of stripping.[9]
The Pozidriv®, sometimes misspelled Pozidrive, screw drive is an improved version of the Phillips screw drive. It is jointly patented by the Phillips Screw Company and American Screw Company. The name is thought to be an abbreviation of positive drive. Its advantage over Phillips drives is its decreased likelihood to cam out, which allows greater torque to be applied.[10][11][12] In ANSI standards it is referred to as type IA.[13] It is very similar to, and essentially compatible with, the Supadriv screw drive.[14]
Phillips drivers have an intentional angle on the flanks and rounded corners so they will cam out of the slot before a power tool will twist off the screw head. The Pozidriv screws and drivers have straight sided flanks.
The Pozidriv screwdriver and screws are also visually distinguishable from Phillips by the second set of radial indentations set 45 degrees from the cross recess. The manufacturing process for Pozidriv screwdrivers is slightly more complex. The Phillips driver has four simple slots cut out of it, whereas in the Pozidriv each slot is the result of two machining processes at right angles. The result of this is that the arms of the cross are parallel-sided with the Pozidriv, and tapered with the Phillips.
This design is intended to decrease the likelihood that the Pozidriv screwdriver will slip out, provide a greater driving surface, and decrease wear.[10] The chief disadvantage of Pozidriv screws is that they are visually quite similar to Phillips, thus many people are unaware of the difference or do not own the correct drivers for them, and use incorrect screwdrivers. This results in difficulty with removing the screw and damage to the recess, rendering any subsequent use of a correct screwdriver unsatisfactory. Phillips screwdrivers will fit in and turn Pozidriv screws, but will cam out if enough torque is applied, potentially damaging the screw head. The drive wings on a Pozidriv screwdriver will not fit a Phillips screw correctly, and are likely to slip or tear out the screw head.
The Supadriv (sometimes found incorrectly as "Supadrive") screw drive is very similar in function and appearance to Pozidriv—indeed, the two are often thought to be identical—and is a later development by the same company. The description of the Pozidriv head applies also to Supadriv. While each has its own driver,[15] the same screwdriver heads may be used for both types without damage; for most purposes it is unnecessary to distinguish between the two drives. Pozidriv and Supadriv screws are slightly different in detail; the later Supadriv allows a small angular offset between the screw and the screwdriver, while Pozidriv has to be directly in line.[14][16]
In detail, the Supadriv screwhead is similar to Pozidriv but has only two identification ticks, and the secondary blades are larger. Drive blades are about equal thickness. The main practical difference is in driving screws into vertical surfaces: that close to a near vertical surface to drive the screws into the drivers, Supadriv has superior bite, making screwdriving more efficient, with less cam out.[15]
A Robertson, also known as a square,[17] or Scrulox[18] screw drive has a square-shaped socket in the screw head and a square protrusion on the tool. Both the tool and the socket have a taper, which makes inserting the tool easier, and also tends to help keep the screw on the tool tip without the user needing to hold it there. (The taper's earliest reason for being was to make the manufacture of the screws practical using cold forming of the heads,[19] but its other advantages helped popularize the drive.) Robertson screws are commonplace in Canada, though they have been used elsewhere[20] and have become much more common in other countries in recent decades. Robertson screwdrivers are easy to use one-handed, because the tapered socket tends to retain the screw, even if it is shaken.[20] They also allow for the use of angled screw drivers and trim head screws. The socket-headed Robertson screws are self-centering, reduce cam out, stop a power tool when set, and can be removed if painted-over or old and rusty.[20] In industry, they speed up production and reduce product damage.[20]
The internal-wrenching square socket drive for screws (as well as the corresponding triangular socket drive) was conceived several decades before the Canadian P. L. Robertson invented the Robertson screw and screwdriver in 1908 and received patents in 1909 (Canada) and 1911 (U.S. Patent 1,003,657). An earlier patent for square-socket- and triangle-socket-drive wood screws, U.S. Patent 161,390, was issued to one Allan Cummings of New York City on March 30, 1875. However, as with other clever drive types conceived and patented in the 1860s through 1890s, it was not manufactured widely (if at all) during its patent lifespan due to the difficulty and expense of doing so at the time.[19] Robertson's breakthrough in 1908 was to design the socket's taper and proportions in such a combination that the heads could be easily and successfully cold formed,[19] which is what made such screws a valid commercial proposition. Today cold forming (via stamping in a die) is still the common method used for most screws sold, although rotary broaching is also common now. Linear broaching to cut corners into a drilled hole (similar to the action of a mortising machine for woodworking) has also been used (less commonly) over the decades.
Robertson had licensed the screw design to a maker in England, but the party that he was dealing with intentionally drove the company into bankruptcy and purchased the rights from the trustee, thus circumventing Robertson. He spent a small fortune buying back the rights. Subsequently, he refused to allow anyone to make the screws under license. When Henry Ford tried out the Robertson screws he found they saved considerable time in Model T production, but when Robertson refused to license the screws to Ford, Ford realized that the supply of screws would not be guaranteed and chose to limit their use in production to Ford's Canadian division.[21][22][23] Robertson's refusal to license his screws prevented their widespread adoption in the United States, where the more widely licensed Phillips head has gained acceptance. The restriction of licensing of Robertson's internal-wrenching square may have sped the development of the internal-wrenching hexagon, although documentation of this is limited.
Robertson-head screwdrivers are available in range of tip-sizes. A given driver's suitability for use with a particular size of screw and recess size is indicated upon its handle according to the following color code:
| Colour | Screw types | Fraction | Range |
|---|---|---|---|
| Orange (#00) | No. 1 & 2 | 1/16 in.+ | 1.77–1.80 mm (0.070–0.071 in) |
| Yellow (#0) | No. 3 & 4 | 3/32 in.- | 2.29–2.31 mm (0.090–0.091 in) |
| Green (#1) | No. 5, 6 & 7 | 7/64 in.+ | 2.82–2.86 mm (0.111–0.113 in) |
| Red (#2) | No. 8, 9 & 10 | 1/8 in.+ | 3.34–3.38 mm (0.131–0.133 in) |
| Black (#3) | No. 12 and larger | 3/16 in.+ | 4.81–4.85 mm (0.189–0.191 in) |
The hex socket screw drive has a hexagonal recess and is driven by a hex wrench, also known as an Allen Wrench, Allen key, hex key or inbus. Tamper-resistant versions with a pin in the recess are available.
The hexalobular socket screw drive, more commonly known as the Torx screw drive (and sometimes, incorrectly, referred to as star drive), uses a star-shaped recess in the fastener 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 because of 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-Plus is an improved version of Torx that 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 five-lobed variant. These "5-star" Torx configurations are commonly used in correctional facilities, public facilities and government schools, but can also be found in some electronic devices.
TTAP is an improved hexalobular screw drive that reduces wobbling between the fastener and the tool. TTAP is backward compatible with standard hexalobular tools.
Some screws have heads designed to accommodate more than one kind of driver, sometimes referred to as combo-head or combi-head. The most common of these is a combination of a slotted and Phillips head, often used in attaching knobs to furniture drawer fronts. Because of its prevalence, there are now drivers made specifically for this kind of screw head. Other combinations are a Phillips and Robertson, a Robertson and a slotted, a torx and a slotted, and a triple-drive screw that can take a slotted, Phillips or a Robertson. The Recex drive system claims it offers the combined non-slip convenience of a Robertson drive during production assembly and Phillips for after market serviceability. Quadrex is another Phillips/Robertson drive. Phillips Screw Company offers both Phillips and Pozidriv combo heads with Robertson.
Combined slotted/pozidriv heads are so ubiquitous in electrical switchgear to have earned the nickname 'electricians screws' (the first screwdriver out of the toolbox is used - the user does not have to waste valuable time searching for the correct driver). Their rise to popular use has been in spite of the fact that neither a flat screwdriver or pozidriv screwdriver are fully successful in driving these screws to the required torque. Some screwdriver manufacturers offer matching screwdrivers and call them 'contractor screwdrivers', although the original concept of not needing to search for a particular driver is defeated as a contractor screwdriver is useless for non-combination heads. Slotted/Phillips (as opposed to slotted/pozidriv) heads occur in some North American made switchgear.
The Phillips/square screw drive, also known as the Quadrex screw drive, is a combination of the Phillips and Robertson screw drives. While a standard Phillips or Robertson tool can be used, there is a dedicated tool for it that increases the surface area between the tool and the fastener so it can handle more torque.[26]
Most of the following screw drives are considered tamper-resistant because of their obscurity. The exceptions to this are the breakaway and one-way screw drives that require special tools that destroy the fastener during removal. The first class of tamper-resistant drives is commonly used on equipment such as home electronics, to prevent easy access, reducing the incidence of damage or improper repair. Equally this can prevent people with the relevant technical knowledge from possibly performing a repair without having to return the unit to the manufacturer. However, widespread recent availability of assorted drive bits (including security types) minimizes this advantage, at least for some security types.
The breakaway head is a high-security fastener whose head breaks off during installation to leave only a smooth surface.[note 1] It consists of a countersunk flat-head screw, with a thin shank and hex head protruding from the flat head. The hex head is used to drive the screw into the countersunk hole, then either a wrench or hammer is used to break the shank and hex head from the flat head—or it is driven until the driving head shears off, leaving only a smooth screw head exposed. This type of screw is commonly used in prison door locks, automobile ignition switches, and street signs. This drive type has the disadvantage of not being torque controlled; many applications fail due to either too little torque being applied to correctly fasten the joint, or too much torque being required to shear the head, resulting in damage to the material being fastened.
The Bristol screw drive is a spline with four or six splines.[27] The grooves in the wrench are cut by a square-cornered broach, giving a slight undercut to the outer corners of the driver. The main advantage to this drive system is that almost all of the turning force is applied at right angles to the fastener spline face, which reduces the possibility of stripping the fastener. For this reason Bristol screw drives are often used in softer, non-ferrous metals. Compared to an Allen drive, Bristol drives are less likely to strip for the same amount of torque, however the Bristol drive is not much more strip resistant than a Torx drive. It was created by the Bristol Wrench Company.
This type of drive is commonly used in avionics, higher-end communications equipment, cameras, air brakes, construction and farm equipment, astronomy, and military equipment. A Bristol screw with a pin in the center is also used as a security screw in the PlayStation 3.
There are two types of clutch screw drives: type A and type G. Type A, also known as a standard clutch resembles a bow tie. These were common in GM automobiles, trucks and buses of the 1940s and 1950s. Type G resembles a butterfly. This type of screw head is commonly used in the manufacture of mobile homes and recreational vehicles.
Double hex is a screw drive with a socket shaped as two coaxial offset hexes. It is shaped similar to triple square and spline screw drives, but they are incompatible. Standard hex keys can be used with these sockets.
The radial 'height' of each arris is reduced, compared to a six-point, although their number is doubled. They are potentially capable of allowing more torque than a six-point, but greater demands are placed on the metallurgy of the heads and the tools used, to avoid rounding off.
The line screw drive is a Japanese system with male, female, and female tamper-resistant screw configurations. The fasteners are commonly called line head screws. They are found on IBM computers and Nintendo systems. The female sizes are designated: ALR2, ALR3, ALR4, ALR5, ALR6; the male: ALH2, ALH3, ALH4, ALH5, ALH6; and the tamper-resistant female have a "T" at the end of the designation (e.g. ALR3T).[1]
One-way screws are special screws that can only be turned in one direction. They can be installed with a standard flat-blade screwdriver. One-way screws are commonly used in commercial restroom fixtures and on vehicle registration plates, to prevent vandals from tampering with them. One-way screws are only practical when the need for removal is unlikely. They cannot be removed with conventional tools because the slot is designed to cause cam out when even minimal torque is applied in the direction to unscrew it. Instead a screw extractor is used.
The pentalobe screw drive is a five-pointed tamper-resistant system being implemented by Apple in its products.[28] Pentalobe screws were first used by Apple in mid-2009, holding the battery in the MacBook Pro; smaller versions are now used on the iPhone 4 and the MacBook Air. However, cheap pentalobe screwdrivers, manufactured by third-parties, have become relatively easy to obtain.[29]
The polydrive screw drive is spline shaped with rounded ends in the fastener head. The tool has six flat-tip teeth at equal spacing; the sizes are determined by the diameter of the star points. Its primary advantage over older screw drives is that it resists camming out. It is used primarily in the automotive industry in high-torque applications, such as brakes and driveshafts.
There are specialty fastener companies that make unusual, proprietary head designs, featuring matching drivers available only from them, and only supplied to registered owners.[30] These tend to be confined to industrial uses that are unavailable to the average layperson. One example familiar to laypersons is the attachment for the wheels and/or spare tires of some types of car; one of the nuts on each wheel may require a specialized socket, provided with the car, to prevent theft. Security fasteners are also available for bicycle wheels and seats.
The spanner[31] screw drive uses two round holes opposite each other and is designed to prevent tampering. This type is seen in elevators and restrooms in the United States. The driving tool is called a spanner driver or spanner screwdriver[32] in the U.S. and a pin spanner in the UK. Many people have found that a bent paper clip, of a low wire gauge can easily be used to unfasten these screws.
The knife and gun company Microtech uses a variation of this with 3 round holes arranged in a triangle shape.
The spline screw drive has twelve splines in the fastener and tool. Spline drives are sized via numbers. Its primary advantage is its ability to resist camming out, therefore it is used in high-torque applications, such as tamper-proof lug nuts. It is named after its resemblance to a spline used on shafting to transmit power.
Torq-set is a cruciform screw drive used in torque-sensitive applications.[33] The Torq-set head is similar in appearance to a Phillips drive in that it has a cross with 4 arms. In Torq-set however, the arms of the cross are offset from each other, so they do not align to form intersecting slots across the top of the head. Because of this, a regular Phillips or flat-head screwdriver will not fit the head. It is used in aerospace applications.[33] Phillips Screw Company owns the name and produces the fasteners.
The applicable standards that govern the Torq-set geometry are National Aerospace Standard NASM 33781 and NASM 14191 for the ribbed version. The ribbed version is also known as ACR Torq-set.[34]
TA is a type of screw drive that uses a triangle-shaped recess in the screw head. Note that the sides of the triangle are straight, which differs from TP3 fasteners. Sizes are TA18, TA20, TA23, and TA27. These screws are often found in children's toys, particularly from fast food restaurants. They can also be found in devices such as vacuum cleaners, fan heaters and camping stoves, among others, to help restrict access to the device internals.
TP3 is a type of screw drive that uses a Reuleaux Triangle-shaped recess in the screw head.[35] It is used on Nintendo Gameboy, fast food promotional toys and video games, die-cast toys and Roomba battery packs. There are four sizes: A=0.079", 0.091", 0.106", and 0.126".
The tri-wing, also known as triangular slotted, screw drive has three radial slots. It is usually used on electronics equipment. Tri-wing, as the name suggests, is a screw with three "wings" and a small triangular hole in the center. A variation is a kind where the three "wings" are joined in the center (with no triangular hole). It is commonly used on Nintendo products. A somewhat similar-looking design where three short radial slots are not joined in the center is called a tri-groove screw drive. The version with same design but reverse threads is called an Opsit screw.[36]
Triple square, also known as XZN, is a type of screw drive with 12 equally spaced tips, each with a 90 degree angle. Its name derives from overlaying 3 equal squares to form such a pattern with 12 right-angled tips. Common sizes are 6, 8, 10, and 12 mm. Triple square drives are used in high torque applications, such as cylinder head bolts and drive train components. Triple square fasteners are commonly found on German vehicles such as Audi, BMW, Mercedes, Porsche, and Volkswagen.[37]
A protruding obstacle screw drive is a common modification to socket and cruciform style drives to make the fastener more tamper resistant by inserting a pin in the fastener screw drive. The tool then requires a corresponding hole to drive the fastener. Usually the hole is in the center, but some are slightly off-center.
There are various other ways to categorize screw drives. One way is by shape of the fastener screw drive: