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=                                Torx                                =
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                            Introduction
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Torx (pronounced ) is a trademark for a type of screw drive
characterized by a 6-point star-shaped pattern, developed in 1967 by
Camcar Textron. 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 the numeral '6', not the capital
letter 'G'). Torx Plus, Torx Paralobe and Torx ttap are improved head
profiles.

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. However, as torx 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
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By design, Torx head screws resist cam out better than Phillips head
or slot head screws. Whereas the tendency of Phillips drivers to cam
out under excessive torque has been listed as a feature preventing
damage to the screw-head or driver, Torx heads were designed to
'prevent' cam-out. The development of better torque-limiting automatic
screwdrivers for use in factories allowed this change. Rather than
rely on the tool to slip out of the screw head when a desired torque
level is reached (which risks damage to the driver tip, screw head, or
workpiece), torque-limiting driver designs achieve a desired torque
consistently.

The Torx design allows for a higher torque to be exerted than a
similarly sized conventional hex socket head without damaging the head
or the tool. The diagram 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 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 will 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.

For the Torx type of design, the angle is much closer to 90° 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, which can be
an advantage in applications where space to accommodate the head is
limited.


                               Sizing
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Torx head sizes are described using the capital letter "T" followed by
a number ranging from T1 to T100. Some manufacturers and resellers
head sizes are also described using "TX" or "Tx" in front of the
number. A smaller number corresponds to a smaller point-to-point
dimension of the screw head (diameter of circle circumscribed on the
cross-section of the tip of the screw driver). Common sizes include
T10, T15, and T25, while T35 and T47 tend to see specialized use. 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 ranging from E4 to
E44. The "E" numbers are different from the "T" numbers of the same
size: for example, an E4 Torx socket fits a T20 head.

Properties of various Torx drives        Size    Point-to-point distance
Maximum torque range     ~ E Torx
(in)     (mm)    (lb·ft)        (N·m)
T1      0.90 mm 0.02 -
T2      1.00 mm 0.07 -
T3      1.20 mm 0.14 -
T4      1.35 mm 0.22 -
T5      1.50 mm 0.43 -  E2
T6      1.75 mm 0.75 -
T7      2.10 mm 1.4 -
T8      2.40 mm 2.2 -
T9      2.60 mm 2.8 -
T10     2.80 mm 3.7 -
T15     3.35 mm 6.4 -
T20     3.95 mm 10.5 -  E4
T25     4.50 mm 15.9 -  E5
T27     5.10 mm 22.5 -
T30     5.60 mm 31.1 -  E6
T35     5.90 mm E7
T40     6.75 mm 54.1 -  E8
T45     7.93 mm 86 -
T47     GM-Style
T50     8.95 mm 132 -   E10
T55     11.35 mm        218 -   E12
T60     13.45 mm        379 -   E16
T70     15.70 mm        630 -   E18
T80     17.75 mm        943 -   E20
T90     20.20 mm        1334 -
T100    22.40 mm        1843 -  E24


                              Variants
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Although the brand "Torx" generally refers to the standard
6-star-driver or -socket, there are many variations to the original
design, including a number made or licensed by Torx. Other tool
manufacturers have been producing 8-, 10- and 12-point star drivers
and sockets for many decades. The Torx brand is often used universally
to describe these and other star driver variations.


Security Torx
===============
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 from being inserted.
However, the pin allows a smaller flat head screw driver to be used
successfully between two adjacent lobes.

Security Torx has its own set of variations, and many other variations
of Torx drives are available in Security or TR versions. These include
five- and seven-lobed TR heads.


Torx Paralobe
===============
Torx Paralobe is a further developed Torx Drive System with 6% longer
flanks which results in a 20% higher applicable torque.


Torx Plus
===========
A Torx successor, Torx Plus, was introduced around 1990 when the
original Torx patent was expiring. The Torx Plus patent subsequently
expired in 2011. The lobes are more square to allow for higher torque
and to minimize wear. The name is shortened to IP (Internal Plus) with
sizes ranging from 1IP to 100IP  (sometimes listed as IP1 to IP100 )
and EP (External Plus) with sizes ranging from 1EP to 42EP as well as
smaller sizes ranging from H7EP to H2EP and includes five-lobed
tamper-resistant variants. 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.
* A tamper-resistant version of Torx Plus exists having five lobes
rather than six, plus a solid post in the center, and is used for
security as the drivers are uncommon. Though Acument (formerly
Textron) lists no designation, TS or IPR may be seen. The screw may
alternatively be a Pentalobe.
* Torx Plus Maxx Stems is a highly specialized variant used on the
ends of fasteners opposite the bolt-head, and provides higher torque
than other drive systems allow. Torxstem is a stud with the Torx Plus
Maxx drive on both ends.


Torx ttap
===========
A proprietary version of Torx called Torx ttap was developed in 2006
and is licensed by Acument Intellectual Properties. It features a
second recess to create a "stick-fit" engagement (branded Frixion
Fit), designed to minimize wobbling (branded Stable Drive) without
pressing and the need for magnetic bits, a feature that can be
important to certain industrial users. Standard Torx drivers can be
used to drive Torx ttap screws, but Torx ttap drivers will not fit
standard Torx screws.


AudiTorx
==========
AudiTorx is a tamper-proof fastener where a convex and smooth fastener
head is topped with a break-away Torx drive that snaps off when the
engineered torque is reached, leaving a rivet-like bolt head that
cannot be easily removed. The main application for these fasteners is
in the railroad industry.


External (inverted) Torx
==========================
An External Torx version exists, also known as Inverted Torx, where
the screw head has the shape of a Torx screwdriver bit, and a Torx
socket is used to drive it. The external "E" Torx nominal sizing does
not correlate to the "T" size, (e.g. an E40 socket is too large to fit
a T40 Torx bit, while an E8 Torx socket will fit a T40 Torx bit).

Properties of various External Torx drives
Size     Point-to-point distance         Standard fastener selection
(in)     (mm)    SAE     Metric
E4      3.8 mm  #6      M3
E5      4.7 mm  #8      M4
E6      5.6 mm  #10     M5
E7      6.1 mm
E8      7.4 mm  1/4"    M6 & M7
E10     9.3 mm  5/16"   M8
E12     11.1 mm 3/8"    M10 & M11
E14     12.8 mm 7/16"   M12
E16     14.7 mm 1/2"
E18     16.6 mm 9/16"   M14
E20     18.4 mm 5/8"    M16
E24     22.1 mm 3/4"    M18 & M20
E28     7/8"    M22
E32     1"      M24 & M27
E36     1-1/8"  M30
E40     1-1/4"  M33
E44     1-3/8"  M36


                        Competitive variants
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AW drive is a hexalobular-type screw head similar to Torx, with a
tapered profile to aid in centering, developed by the Würth Group in
Germany. It is available in five sizes: AW 10, AW 20, AW 25, AW 30 and
AW 40.

T-Star plus has a similar screw drive design to Torx ttap and was
introduced in 2005 by German fastener manufacturer Altenloh, Brinck
& Co under its brand name 'Spax'.


                              Gallery
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File:Torx-Bits_T15-T20-T25-T30.jpg|Torx bits T15, T20, T25, and T30
File:Torx 01 KMJ.jpg|A Torx wrench
File:Torx.jpg|Closeup of Torx screwdriver tip
File:18-03-22-Schrauben-M6x20 RRK3361.jpg|A Torx T30 bolt


                              See also
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* Wrench


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