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Torx (pronounced "torks"), developed in 1967 by Camcar Textron, is the trademark for a type of screw head characterized by a 6-point star-shaped pattern. A popular generic name for the drive is star, as in star screwdriver or star bits. The official generic name, standardized by the International Organization for Standardization as ISO 10664, is hexalobular internal. This is sometimes abbreviated in databases and catalogs as 6lobe (starting with numeral "6", not a capital "G"). Torx Plus is an improved head profile.
Torx screws are commonly found on automobiles, motorcycles, bicycle brake systems (disc brakes), hard disk drives, computer systems and consumer electronics. Initially, they were sometimes used in applications requiring tamper resistance, since the drive systems and screwdrivers were not widely available; as drivers became more common, tamper-resistant variants, as described below, were developed. Torx screws are also becoming increasingly popular in construction industries.
Principles of operation
By design, Torx head screws resist cam-out better than Phillips head or slot head screws. Where Phillips heads were designed to cause the driver to cam out, to prevent overtightening, Torx heads were designed to prevent cam-out. The reason for this was the development of better torque-limiting automatic screwdrivers for use in factories. Rather than rely on the tool slipping out of the screw head when a torque level is reached, thereby risking damage to the driver tip, screw head and/or workpiece, the newer driver design achieves a desired torque consistently. The manufacturer claims this can increase tool bit life by ten times or more.
The Torx design allows for a higher torque to be exerted than a similarly-sized conventional hex socket head without damaging the head and/or the tool. The diagram on the right depicts the interaction between the male and female components of a conventional hex drive and a Torx drive. The clearance between the components is exaggerated for clarity. The diagram does not show a true Torx profile, but illustrates the general shape and geometry.
The green circle, passing through the six points of contact between the two components, represents the direction of the rotational force being exerted at each of those points. Because the plane of contact is not perpendicular to this circle, a radial force is also generated which tends to "burst" the female component and "crush" the male one. If this radial force component is too great for the material to withstand, it will cause the corners to be rounded off one or both components or split the sides of the female part. The magnitude of this force is proportional to the cotangent of the angle (depicted in orange) between the green circle and the contact plane. It can be seen that for the Torx type of design, the angle is much closer to 90 degrees than in the case of the hex head, and so for a given torque the potentially-damaging radial force is much lower. This property allows the head of the fastener to be smaller for the same required torque and this can be of advantage in applications where space to accommodate the head is limited.
|Part of a series on|
|Screw drive types|
||Hex socket (Allen)|
||Security hex socket (pin-in-hex-socket)|
T & TX
Torx head sizes are described using the capital letter "T" followed by a number. A smaller number corresponds to a smaller point-to-point dimension of the screw head. Common sizes include T10, T15 and T25, although they reach as high as T100. Only the proper driver can drive a specific head size without risk of damaging the driver or screw. The same series of Torx drivers is used to drive SAE, metric and other thread system fasteners, reducing the number of bit sizes required.
The "external" variants of Torx head sizes (see below) are described using the capital letter "E" followed by a number. The "E" numbers are different from the "T" numbers of the same size: for example, an E4 Torx socket fits a T20 head.
|Size||Point to point distance||Maximum torque range||Equiv.
|T1||.031||.81||.01 – .02||.02 – .03|
|T2||.036||.93||.05 – .07||.07 – .09|
|T3||.046||1.10||.10 – .13||.14 – .18|
|T4||.050||1.28||.16 – .21||.22 – .28|
|T5||.055||1.42||.32 – .38||.43 – .51|
|T6||.066||1.70||.55 – .66||.75 – .90|
|T7||.078||1.99||1.0 – 1.3||1.4 – 1.7|
|T8||.090||2.31||1.6 – 1.9||2.2 – 2.6|
|T9||.098||2.50||2.1 – 2.5||2.8 – 3.4|
|T10||.107||2.74||2.7 – 3.3||3.7 – 4.5|
|T15||.128||3.27||4.7 – 5.7||6.4 – 7.7|
|T20||.151||3.86||7.74 – 9.37||10.5 – 12.7||E4|
|T25||.173||4.43||11.7 – 14.0||15.9 – 19||E5|
|T27||.195||4.99||16.6 – 19.8||22.5 – 26.9|
|T30||.216||5.52||22.9 – 27.6||31.1 – 37.4||E6|
|T40||.260||6.65||39.9 – 48.0||54.1 – 65.1||E8|
|T45||.306||7.82||63.4 – 76.1||86 – 103.2|
|T50||.346||8.83||97.4 – 117||132 – 158||E10|
|T55||.440||11.22||161 – 189||218 – 256||E14|
|T60||.519||13.25||280 – 328||379 – 445||E16|
|T70||.610||15.51||465 – 516||630 – 700||E18|
|T80||.690||17.54||696 – 773||943 – 1048||E20|
|T90||.784||19.92||984 – 1094||1334 – 1483|
|T100||.871||22.13||1359 – 1511||1843 – 2048||E24|
- A version known as Security Torx, Tamper-Resistant Torx (often shortened to Torx TR) or pin-in-Torx contains a post in the center of the head that prevents a standard Torx driver (or a straight screwdriver) from being inserted.
- An external Torx version exists, where the screw head has the shape of a Torx screwdriver bit, and a special Torx socket is used to drive it. A standard hex socket can often be used as an expedient, but this practice risks causing fastener damage. External Torx heads are found primarily on items like automobile engines and transmissions. However, the external E Torx nominal sizing bears no obvious relation to the T size, e.g. an E40 socket is too large to fit a T40. In fact an E8 female Torx socket will fit a T40 Torx head.
- A Torx variant, Torx Plus, introduced about 1990 when the original Torx patent was expiring, is designed to allow greater torque, and to minimize wear. The specifications for these licenses are held by Textron. Standard Torx drivers can be used to drive Torx Plus screws, but not to full torque because of the loose fit. Torx Plus drivers will not fit into standard Torx screws. The name is shortened to Torx TX.
- A tamper resistant version of Torx Plus exists, having five lobes rather than six, and a solid post in the center. These fasteners are used for security, as the drivers are still hard to obtain, and are given the designation Torx TS.
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TTAP, which is also hexalobular, is designed to minimize wobbling without the need for magnetic bits, a feature that can be important to certain industrial users. Standard Torx drivers can be used to drive TTAP screws, but TTAP drivers will not fit standard Torx screws.
AW, similar type of screw head developed by the Würth group in Germany.
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- U.S. Patent 3,584,667 filed 1967-03-21
- Camcar eventually became part of Textron Fastening Systems in the 1990s. In 2006 Textron Fastening Systems was sold to Platinum Equities, LLC, of Beverly Hills, California. They renamed the company Acument Global Technologies, which as of 2010 includes Avdel, Camcar, Ring Screw, and others.
- ISO 10664:2005, ISO.org, retrieved 2012-01-14
- Fast and Secure, Memagazine.org, retrieved 2012-01-14
- Chart of Torx fasteners and tools, Wihatools.com, retrieved 2012-01-14