List of screw drives
|Part of a series on|
|Screw drive types|
||Hex socket (Allen)|
||Security hex socket (pin-in-hex-socket)|
T & TX
A & G
A screw drive is the system used to turn a screw. At a minimum, it is a feature on the screw that allows torque to be applied to it. 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, typically distinguished by a number, such as "Phillips #00" or "Torx T5". These sizes do not describe a particular dimension of the drive shape, but are arbitrary designations in the same sense as a "Size 8" shoe or dress.
- 1 Common types
- 1.1 External types
- 1.2 Slotted types
- 1.3 Cruciform types
- 1.4 Robertson
- 1.5 Hex socket
- 1.6 Hexalobular socket
- 1.7 Double-square
- 1.8 Combination drives
- 2 Tamper-resistant types
- 3 Alternative categorizations
- 4 Notes
- 5 References
- 6 External links
Square This screw drive uses square fastener heads. They can be turned with an adjustable wrench, open-end wrench, or 8- or 12-point sockets. Common in the 19th and early 20th centuries, when it was easier and thriftier to manufacture than most other drives, it is less common today (although still easy to find) because the external hex is now cost-competitive and allows better access for wrenching despite nearby obstructions, with fewer degrees of swing on each stroke.
A hex screw drive uses six-sided fastener heads. The fastener is known as a hex head cap screw. It can be turned with an adjustable wrench, combination wrench, and 6- or 12-point sockets. The hex drive is better than square drive for locations where surrounding obstacles limit wrenching access, because smaller wrench-swing arcs can still successfully rotate the fastener.
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 key-like flat sided vertical head. They are intended to be tightened and loosened by hand, and not found in structural applications. They are sometimes also cut for Phillips head and/or slotted screwdrivers as well as having the knurl for finger grip. ASME 18.6.8 covers dimensions for Type A, shoulder under the head, regular and heavy along with Type B, without shoulder, regular and heavy.
|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 simple 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 standard, flat-blade, slot-head, flat-tip, or flat-head screwdriver.
Flat-head in particular can sometimes be confusing because the term flat-head is also used to describe a screw with a flat top, designed to be installed in a countersunk hole; such a flat-headed screw may have a slotted, cross, square recessed, or combination head, causing further confusion about the terminology.
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 deformed 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. (Cross-type slotted screw drives are not considered cruciform because the shape is not recessed.)
|Tool size||Fastener size|
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 of Providence, Rhode Island 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.
The Frearson screw drive, also known as the Reed and Prince screw drive, is similar to a Phillips but the Frearson has a sharp tip and larger angle in the V shape. 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. The Reed & Prince Mfg. Company of Worcester, Massachusetts, was put into bankruptcy in 1987 and liquidated in 1990. Another entity called Reed & Prince Manufacturing Corporation, now of Leominster, Massachusetts, purchased some of the assets including the name at the liquidation sale.
|This section requires expansion. (June 2010)|
Also called BNAE NFL22-070 after its Bureau de Normalisation de l'Aéronautique et de l'Espace standard number.
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 or an "X" to one side of the cross slot.
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.
The Pozidriv, sometimes purposely misspelled Pozidrive to avoid trademark infringement, screw drive is an improved version of the Phillips screw drive. They can be unscrewed with a regular Phillips screwdriver. It is jointly patented by the Phillips Screw Company and American Screw Company. The name is thought to be a portmanteau of positive drive. Its advantage over Phillips drives is its decreased likelihood to cam out, which allows greater torque to be applied. In ANSI standards it is referred to as type IA. It is very similar to, and essentially compatible with, the Supadriv screw drive.
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. 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, 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.
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.
A Robertson, also known as a square, or Scrulox 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, but its other advantages helped popularize the drive.) Robertson screws are commonplace in Canada, though they have been used elsewhere 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. 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.
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 covering both 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. 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, 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. 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:
|Orange (#00)||No. 1 & 2||1/16 in.+||??|
|Yellow (#0)||No. 3 & 4||3/32 in.-||0.0696–0.071 in (1.77–1.80 mm)|
|Green (#1)||No. 5, 6[note 1] & 7||7/64 in.+||0.090–0.091 in (2.3–2.3 mm)|
|Red (#2)||No. 8, 9 & 10||1/8 in.+||0.111–0.1126 in (2.82–2.86 mm)|
|Black (#3)||No. 12 & 1/4||3/16 in.+||0.1315–0.133 in (3.34–3.38 mm)|
|Brown (#4)||5/16 and 3/8||3/16 in.+||0.1895–0.191 in (4.81–4.85 mm)|
Double-square and triple-square drives are 2 or 3 squares superimposed (respectively), forming an 8-pointed or 12-pointed star. In other words, 2 or 3 Robertson squares superimposed at 45° or 30° rotation. The idea is that it's like a square (Robertson) but can be caught at more frequent angles by the driver bit.
The hex socket screw drive has a hexagonal recess and may be driven by a hex wrench, also known as an Allen Wrench, Allen key, hex key or inbus as well as by a hex screwdriver (also known as a hex driver) or bit. Tamper-resistant versions with a pin in the recess are available.
The hexalobular socket screw drive, often referred to by the original proprietary brand name Torx (pronounced "torks") or by the alternate generic name 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.
Double-square drive is 2 squares superimposed, forming an 8-pointed star. In other words, 2 Robertson squares superimposed at 45° rotation. The concept is similar to that of the triple-square, the idea being that it's like a square (Robertson) but can be caught at more frequent angles by the driver bit.
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 also a dedicated tool for it that increases the surface area between the tool and the fastener so it can handle more torque. This drive was formerly also sold by Home Depot under the name Deck Mate, which now signifies another drive.
A combined slotted and Torx drive screw was used in electronics manufacturing, notably Compaq used this type to combine the benefits of Torx in manufacturing and the commonality of flat drive in field repair situations. The slot was closed on the ends to avoid the flat blade tool slipping and damaging the electronics.
Most of the following screw drives are considered tamper-resistant because of their obscurity. The exceptions to this are the double hex and triple square screw drives that can be removed with standard hex or square screw bits. 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. True tamper resistant screw drives include the breakaway head, one way, TA, and TP3 screw drives.
The breakaway head is a high-security fastener whose head breaks off during installation, immediately after driving, to leave only a smooth surface. 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, which are not necessarily tamper resistant. 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. Variants with a pin in the center are often found in video game systems: such as PlayStations and others.
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. Type G is not necessarily tamper resistant because type G can be removed with a flat-blade screwdriver.
|This section requires expansion with: plan of claw head comparable to screw head plans in other sections. (April 2012)|
Two projections opposite each other.
Double hex is a screw drive with a socket shaped as two coaxial offset hexes. Its shape is 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 also known as game bit screws, due to their use on videogame consoles. 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).
One-way screws are special screws that can only be turned in one direction. They are sometimes called one-way clutch screws, but should not be confused with true "clutch" screws. They can be installed with a standard flat-blade screwdriver, but cannot be easily removed using standard tools. 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 are difficult to remove 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 special removal tool (a one-way screw extractor) must be used. Alternatively, a one-way screw can be removed by using a drill bit, by using a rotary tool and cutting disk to extend the slot, by using locking pliers, or by drilling two holes in the slot and removing the screw with a pin spanner (snake-eyes driver).
The pentalobe screw drive (often mistaken for 5-point torx screw drives) is a five-pointed tamper-resistant system being implemented by Apple in its products. Apple's first use of the pentalobe drive was in mid-2009. They used a pentalobe screw to secure the battery in the MacBook Pro; smaller versions are now used on the iPhone 4, the iPhone 4S, the iPhone 5, the iPhone 5s, the MacBook Air, and the MacBook Pro with Retina Display. Inexpensive pentalobe screwdrivers, manufactured by third parties, are relatively easy to obtain. Pentalobe screw sizes include TS1 (0.8mm, used on the iPhone 4, the iPhone 4S, the iPhone 5, and the iPhone 5s), TS4 (1.2mm, used on the MacBook Air and the MacBook Pro with Retina Display), and TS5 (1.5mm, used on the 2009 MacBook Pro battery). The TS designation is ambiguous as it is also used for Torq-set screw drive.
The polydrive screw drive, also known as ribe, 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. 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 deter theft. Security fasteners are also available for bicycle wheels and seats.
Protruding obstacle or pin type
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, requiring a tool with a corresponding hole to drive the fastener. This can also prevent attempts at turning the screw with a small flat-bladed screwdriver. Sometimes the hole is slightly off-center.
The spanner or snake eyes (trademarked) 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 London Underground in the United Kingdom, and the Montreal Metro in Montreal, Quebec. The driving tool is called a spanner driver or spanner screwdriver in the U.S. and a pin spanner in the UK. They are also often used for soft spikes on golf shoes.
The knife and gun manufacturer 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, cylinder head bolts, and other engine bolts.
Torq-set is a cruciform screw drive used in torque-sensitive applications. 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 lines 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-blade screwdriver will not fit the head. It is used in aerospace applications, e.g., the B-2 stealth bomber. 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.
The 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 include TA14, 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, elevators 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. It is used on fast food promotional toys and video games, die-cast toys and Roomba battery packs. There are four sizes: A = 2 mm, 2.3 mm, 2.7 mm, and 3.2 mm.
The tri-point, security screw head is similar to the Phillips screw head, but it has three points rather than four. These specalized screws are usually used on electronics equipment, including the Nintendo Gamecube, Nintendo Game Boy Advance, Game Boy Advance SP, Sanyo and Kyocera cellular telephones and Fuji digital cameras. 
The tri-wing, also known as triangular slotted, is a screw with three slotted "wings" and a small triangular hole in the center. The design was adapted by some parts of the aerospace industry, led by Lockheed in the early 1970s on the L-1011, but met with mixed results due to complaints of insert damage during installation. McDonnell Douglas also used this as a primary fastener on its commercial aircraft. In the present day it is usually used on electronics equipment.
A variation has the three "wings" joined in the center, but with no triangular hole. It is commonly used on Nintendo products. Some Nokia Phone chargers use tri-wing screws. Apple uses this on the 2010 and 2011 Macbook Pro battery access - Y1 size. Microsoft's Zune also uses this type on some models. Nikon also uses this type of screws on some equipment like the SB-700 flash.
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[further explanation needed] threads is called an Opsit screw.
The triple square, also known as XZN, is a type of screw drive with 12 equally spaced tips, each with a 90 degree angle. Common sizes are 6, 8, 10, and 12 mm. Its name derives from overlaying three equal squares to form such a pattern with 12 right-angled tips (a 12-pointed star). In other words, 3 Robertson squares superimposed at 30° rotation. The concept is similar to that of the double-square—in both cases, the idea being that it's like a square (Robertson) but can be caught at more frequent angles by the driver bit. These screws can be removed with standard Robertson bits.
Triple square drives are often used in high-torque applications, such as cylinder head bolts and drive train components. The heads are hardened and tempered to withstand the driving torque without destroying the star points.
There are various other ways to categorize screw drives. One way is by shape of the fastener screw drive:
- Line (ALH)
- Socket heads
- Double hex
- Hex socket
- Hexalobular socket
- Line (ALR)
- Triple square
- Some specialty #6 Robertson screws require a Red #2 driver.
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