List of screw drives

Contents 1 Common types 1.1 External types 1.1.1 Square 1.1.2 Hex 1.1.3 Pentagon 1.2 Slotted types 1.2.1 Slot 1.2.2 Cross 1.3 Cruciform types 1.3.1 Phillips 1.3.2 Frearson 1.3.3 JIS B 1012 1.3.4 French recess 1.3.5 Pozidriv 1.3.6 Supadriv 1.4 Robertson 1.5 Hex socket 1.6 Hexalobular socket 1.6.1 TTAP 1.7 Combination drives 1.7.1 Phillips/square 2 Tamper-resistant types 2.1 Breakaway head 2.2 Bristol 2.3 Clutch 2.4 Double hex 2.5 One-way 2.6 Polydrive 2.7 Proprietary head 2.8 Spanner 2.9 Spline 2.10 Torq-set 2.11 TP3 2.12 Tri-wing 2.13 Triple square 2.14 Protruding obstacle 3 Alternate categorizations 4 Notes 4.1 Bibliography 5 External links

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, which 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.

Common types

External types

All of these screw drives are characterized by a female tool and a male fastener.

Square

A square screw drive uses square shaped fastener heads. They can be turned with a crescent-type wrench, open-end wrench, or 12-point sockets.

Hex

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 sockets.

Pentagon

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.

Slotted types

Slot

Slot drive tool and fastener sizes^[2]

Blade thickness	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. The slotted screw is common in woodworking applications, but is not often seen in applications where a power tool would be used, due to the tendency of a power driver 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,^[2] or flat head.^[3]

Cross

A cross 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.

Cruciform types

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.

Phillips

Phillips drive tool and fastener sizes^[2]

Tool size	Fastener size
0	0–1
1	2–4
2	5–9
3	10–16
4	18–24

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 due to 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 since 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, which are different from the screw size; they are designated 000, 00, 0, 1, 2, 3, and 4 (increasing in size).^[2][4]

Frearson

The Frearson screw drive, also known as the Reed and Prince screw drive, is similar to a Phillips but has a more pointed 75° V shape.^[5] Its advantage over the Phillips drive is that 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.

JIS B 1012

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 to one side of the cross slot.

French recess

Also called BNAE NFL22-070 after its Bureau de Normalisation de l'Aéronautique et de l'Espace standard number.

Pozidriv

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.^[6][7][8] In ANSI standards it is referred to as type IA.^[9] It is very similar to, and essentially compatible with, the Supadriv screw drive.^[10]

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 cross-like features set 45 degrees from the cross. 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.^[6] 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 slot, 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 marker lines on a Pozidriv screwdriver will not fit a Phillips screw correctly, and are likely to slip or tear out the screw head.

Supadriv

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,^[11] 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.^[10][12]

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.^[11]

Robertson

A Robertson, also known as a square,^[13] 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 to make inserting the tool easier. Robertson screws are used mainly in Canada (where they are very common), though they can be found elsewhere.^[14] Robertson screwdrivers are easy to use one-handed, since the tapered socket retains the screw, even if it is shaken. 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. In industry, they speed up production and reduce product damage.^[15]

Canadian P.L. Robertson invented the Robertson screw and screwdriver in 1908 and received a patent in 1909. An earlier square drive wood screw patent, U.S. Patent 161,390, was issued to Allan Cummings on March 30, 1875.

Robertson had licensed the screw in England, but the party with which he was dealing 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.^[16][17][18] 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. However, an alternative explanation for this preference (that has even been aired on a History Television programme) is that Americans simply preferred their own invention as opposed to the "foreign" Robertson screw. 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:

Recess dimensions^[19][20]

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)

Close-up of Robertson drivers

Illustration from Robertson's patent application

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Hex socket

The hex socket screw drive has a hexagonal recess and is driven by a hex wrench, also known as an Allen key, hex key or inbus. Tamper-resistant versions with a pin in the recess are available.

Hexalobular socket

The hexalobular socket screw drive, more commonly known as the torx screw 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 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-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 "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

TTAP is an improved hexalobular screw drive that reduces wobbling between the fastener and the tool. TTAP is backward compatible with standard hexalobular tools.

Combination drives

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 which 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.

Phillips/square

The Phillips/square screw drive, also known as the Quadrex or SPAX 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, which increases the surface area between the tool and the fastener so it can handle more torque.^[21]

Tamper-resistant types

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, which 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 knowledge from possibly performing a repair without having to return the unit to the manufacturer.

Breakaway head

The breakaway head, is a high-security fastener whose head breaks off during installation to leave only a smooth surface.^[22] 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 bolt into the countersunk hole, then either the 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.

Bristol

The Bristol screw drive is a spline shaped with four or six splines.^[23] 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 axis, 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.

Clutch

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

Double hex is a screw drive with a socket shaped as two 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.

This version is even closer to a circle, further decreasing torque and increasing stripping.

One-way

One-way screws are special screws that can only be turned in one direction. They can be installed with a standard slotted screwdriver. One-way screws are commonly used in commercial restroom fixtures, 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; instead a screw extractor is used.

Polydrive

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.

Proprietary head

There are specialty fastener companies that make unusual, proprietary head designs, featuring matching drivers available only from them, and only supplied to registered owners.^[24] These tend to be confined to industrial uses with which the average layperson does not have contact. 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.

Spanner

The spanner^[25] 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 wrench^[26] in the U.S. and a pin spanner in the UK.

Spline

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.

Torq-set

Torq-set is a cruciform screw drive used in torque-sensitive applications.^[27] 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.^[27] 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.^[28]

TP3

TP3 is a type of screw drive that uses a triangular recess in the screw head.^[29] 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".

Tri-wing

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 in which the three "wings" are joined in the center (with no triangular hole). A somewhat similar-looking design in which three short radial slots are not joined in the center is called a tri-groove screw drive. It is commonly used on Nintendo products.

Triple square

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, and Volkswagen.^[30]

6 mm and 8 mm triple square drivers.

End view of 10 mm triple square screw.

Protruding obstacle

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.

Alternate categorizations

There are various other ways to categorize screw drives. One way is by shape of the fastener screw drive:

• External
  • Square
  • Hex
• Socket heads
  • Bristol
  • Clutch
  • Double hex
  • Hex socket
  • Hexalobular socket
  • Polydrive
  • Robertson
  • Spline
  • TP3
  • Triple square

Notes

Bibliography

• Rybczynski, Witold (2000). One Good Turn: A Natural History of the Screwdriver and the Screw. Toronto, Ont., Canada: Harper Flamingo. ISBN 0-00-200031-8. 

External links

• Phillips or Pozidriv?
• Robertson screw on Canadian Broadcasting Corporation's 50 Greatest Canadian Inventions list
• Bristol spline driver manufacturer's catalog
• Spanner Jaw Sizes