Hex key

From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Allen key" redirects here. For the American politician, see Alan Keyes. For the computer scientist, see Alan Kay. For the telephone button, see Number sign.
Hex keys of various sizes.
Socket head screws of various sizes.
same Hex Key, different Screw thread
Part of a series on
Screw drive types
Screw Head - Slotted.svg Slot
Screw Head - Phillips.svg Phillips
Screw Head - Pozidrive.svg Pozidriv (SupaDriv)
Screw Head - Square External.svg Square
Screw Head - Robertson.svg Robertson
Screw Head - Hex External.svg Hex
Screw Head - 12-Point External.png
12-point flange
Screw Head - Hex Socket.svg Hex socket (Allen)
Pin-in-hex socket screw drive 003.png Security hex socket (pin-in-hex-socket)
Screw Head - Torx.svg Torx
T & TX
Screw Head - Torx Tamperproof.svg Security Torx
Screw Head - Tri-wing.svg Tri-Wing
Screw Head - Torq-set.svg Torq-set
Screw Head - Spanner.svg Spanner head
Screw Head - Clutch Type G.svg Clutch
A & G
Screw Head - One-way Clutch.svg One-way
Double-square screw drive 001.png Double-square
Screw Head - Triple Square.svg Triple square
Screw Head - Polydrive.svg Polydrive
Screw Head - Spline.svg Spline drive
Screw Head - Double Hex.svg Double hex
Screw Head - Bristol.svg Bristol
Pentalobular.svg Pentalobe

A hex key or Allen key is a tool of hexagonal cross-section used to drive bolts and screws that have a hexagonal socket in the head (internal-wrenching hexagon drive).

The Allen name is a registered trademark, originated by the Allen Manufacturing Company of Hartford, Connecticut circa 1910, and currently owned by Apex Tool Group, LLC. Its genericized use is discouraged by this company. The standard generic name used in catalogs and published books and journals is "hex key".


The term "hex key" has various synonyms. Some are explained by the geographical and commercial history of the drive type's development: these include Allen, Unbrako, and Inbus key or wrench. The synonym zeta key or wrench refers to the sixth letter of the Greek alphabet. The term hex-head is sometimes used to refer to this type of drive, but this use is not consistent with its more conventional use referring to external-wrenching hexagons.

In the fastener industry, the terms socket head or hex socket head are generally used for the driven part of the driver–driven pair. A less common synonym is female hex.


Some features of hex keys are:

  • The tool is simple, small and light
  • The contact surfaces of the screw or bolt are protected from external damage
  • There are six contact surfaces between bolt and driver
  • The tool can be used with a headless screw
  • The screw can be inserted into its hole using the key
  • Torque is constrained by the length and thickness of the key
  • Very small bolt heads can be accommodated
  • The tool can be manufactured very cheaply, so one is often included with products requiring end-user assembly
  • Either end of the tool can be used to take advantage of reach or torque
  • The tool is L-shaped
  • The tool can be reconditioned using an electric grinder by removing the worn-out part, and then it works like new
  • The hexagon is typically a smaller diameter than would be used with an external cap, and is more likely to round off its contact surfaces.
  • It is much more difficult to turn a damaged (rounded or otherwise) internal fastener than an external one.


Allen Manufacturing Company Inc advertisement for the Allen Safety Set Screw, a brand of set screw, in the Automobile Trade Directory, January 1913.

Extant records suggest (without offering exhaustive documentation) that the idea of a hex socket screw drive was probably conceived as early as the 1860s through 1890s, but that such screws were probably not manufactured until around 1910. Rybczynski (2000) describes a flurry of patents for alternative drive types in the 1860s through 1890s in the US,[1] which are confirmed to include internal-wrenching square and triangle types (that is, square and triangular sockets) (U.S. Patent 161,390), but he explains that these were patented but not manufactured due to the difficulties and expense of doing so at the time. P. L. Robertson, of Milton, Ontario, Canada, first commercialized the square socket in 1908, having perfected and patented a manufacturing method (cold-forming, using the right material and the right die design).

In 1909–1910, William G. Allen patented a method of cold-forming screw heads around a hexagonal die (U.S. Patent 960,244). Published advertisements for the "Allen safety set screw" by the Allen Manufacturing Company of Hartford, Connecticut, exist from 1910.[2] Although it is unlikely that Allen was the first person to think of a hex socket drive, his patent for a manufacturing method and his realized product appear to be the first.

In his autobiography, the founder of the Standard Pressed Steel Company (SPS; now SPS Technologies, Inc.), Howard T. Hallowell Sr, presents a version of events[3] in which SPS developed a hex socket drive in-house, independently of Allen, circa 1911. From this came the Unbrako line of products. This account from Hallowell does not mention the Allen patent of 1910, nor the Allen safety set screw product line. Hallowell does describe, however, the same inspiration also mentioned in connection with Allen for a wave of adoption of the hex socket head, beginning with set screws and followed by cap screws. This was an industrial safety campaign, part of the larger Progressive Movement, to get headless set screws onto the pulleys and shafts of the line shafting that was ubiquitous in factories of the day. The headless set screws would be less likely to catch the clothing of workers and pull them into injurious contact with the running shaft.

SPS at the time was a prominent maker of shaft hangers and collars, the latter of which were set in place with set screws. In pursuit of headless set screws with a better drive than a straight slot, Hallowell said, SPS had sourced set screws of square-socket drive from England, but they were very expensive.[4] (This was only 2 years after Robertson's Canadian patent.) This cost problem drove SPS to purchase its first screw machine and make its screws in-house, which soon led to SPS's foray into fastener sales (for which it later became well known within the metalworking industries). Hallowell said that "[for] a while we experimented with a screw containing a square hole like the English screw but soon found these would not be acceptable in this country [the U.S.]. Then we decided to incorporate a hexagon socket into the screw […]."[5] Hallowell does not elaborate on why SPS found that the square hole "would not be acceptable in this country", but it seems likely that it would have to have involved licensing Robertson's patent, which would have defeated SPS's purpose of driving down its cost for internal-wrenching screws (and may have been unavailable at any price, as explained at "List of screw drives > Robertson"). The story, if any, of whether SPS's methods required licensing of Allen's 1910 patent is not addressed by Hallowell's memoir. The book does not mention which method—cold forming or linear broaching—was used by SPS in these earliest years. If the latter was used, then Allen's patent would not have been relevant.

Soon after SPS had begun producing the [hex] socket head set screw, Hallowell had the idea to make a [hex] socket head cap screw (SHCS). Hallowell said, "Up to this moment none of us had ever seen a socket head cap screw, and what I am about to relate concerns what I believe was the first socket head cap screw ever made in this country [the U.S.]."[6] SPS gave their line of screws the Unbrako brand name, chosen for its echoing of the word unbreakable.

Hallowell said that acceptance of the internal-wrenching hexagon drive was slow at first (painfully slow for SPS's sales), but that it eventually caught on quite strongly.[7] This adoption occurred first in tool and die work and later in other manufacturing fields such as defense (aircraft, tanks, submarines), civilian aircraft, automobiles, bicycles, furniture, and others.

Concerning the dissemination of the screws and wrenches, Hallowell said that "the transition from a square head set screw [Hallowell refers here to the then-ubiquitous external-wrenching square drive] to a hexagon socket head hollow set screw[,] for which had to be developed special keys or wrenches for tightening or loosening the screw, was the cause of more profanity among the mechanics and machine manufacturers than any other single event that happened. […] I am sure that the old-timers who read this book will remember this period vividly."[8] (These transitional growing pains echo those experienced many decades later with the adoption of the Torx drive).

World War II, with its unprecedented push for industrial production of every kind, is probably the event that first put most laypersons in contact with the internal-wrenching hexagon drive. (Popular Science magazine would note in 1946 that "Cap screws and setscrews with heads recessed to take hexagonal-bar wrenches are coming into increasing use.")[9]

It appears that the internal-wrenching hexagon drive may have been independently reinvented in various countries. At the least, the design (or methods of manufacturing it) was patented in various countries by various patentees, and its name varies. For example, in various European countries, it is known by the name Inbus (often misspelled *Imbus), after the company that patented them in Germany in 1936, Bauer und Schaurte of Neuss (Inbus stands for Innensechskantschraube Bauer und Schaurte). Similarly, there is another name in Italian (brugola), stemming from the name Officine Egidio Brugola, a company who first commercialized Allen's products in Italy.

Hex key standard sizes[edit]

Hex keys are measured across-flats (AF), which is the distance between two opposite (parallel) flat sides of the key. Standard metric sizes are defined in ISO 2936:2001 "Assembly tools for screws and nuts—Hexagon socket screw keys", also known as DIN 911, and, measured in millimeters (mm) are:

  • 0.7, 0.9, 1.0, 1.25, 1.3, 1.5
  • 2 to 6 in 0.5 mm increments
  • 7 to 22 in 1 mm increments
  • 24, 25, 27, 30, 32, 36, 42 and 46 mm.

Metric hex wrench sizes are sometimes referred to using the designation "M" followed by the size in millimeters of the tool or socket, e.g. "M6", although this may be confused with the standard use of "M6" which refers to the size of a metric screw or bolt.

American sizes are defined in ANSI/ASME standard B18.3-1998 "Socket Cap, Shoulder, and Set Screws (Inch Series)". Values given here are taken from Machinery's Handbook, 26th Edition, section "Fasteners", chapter "Cap and Set Screws", table 4 (p. 1601).

Screw size (nominal) Socket size (inches) Approximate socket size (mm)
No. 6 7/64 2.78
No. 8 9/64 3.57
No. 10 5/32 3.97
1/4 3/16 4.76
5/16 1/4 6.35
3/8 5/16 7.94
7/16 3/8 9.53
1/2 3/8 9.53
5/8 1/2 12.70
3/4 5/8 15.88
7/8 3/4 19.05
1 3/4 19.05

Note that numerous other sizes are defined; these are the most common.

Using a hex wrench on a socket that is too large may result in damage to the fastener or the tool. An example would be using a 5 mm tool in a 5.5 mm socket. Because hex-style hardware and tools are available in both metric and Imperial and customary sizes (the latter sometimes labeled "SAE"), it is also possible to select a tool that is too small for the fastener by using an Imperial/customary tool on a metric fastener, or the converse. There are some exceptions to that. For example, 4 mm keys are almost exactly the same size as 5/32", and 8 mm keys are almost exactly the same size as 5/16", which makes 4 mm and 8 mm preferred numbers for consumer products such as self-assembly particle-board furniture, because end users can successfully use an imperial key on a metric fastener, or vice versa, without stripping. 19 mm keys are so close to the same size as 3/4" that they are completely interchangeable in practical use.


Hex keys of various sizes with ball ends.

A security version of the hex head includes a pin in the center. These fasteners are said to have a "center pin reject" feature to prevent standard hex wrenches from working. A special driver must be used to fasten or remove these fasteners. The TORX head's security variant also has such a pin for the same reason.

Some hex keys have a ball on one end, which allows the tool to be used at an angle off-axis to the screw. This type of hex key was invented in 1964 by the Bondhus Corporation,[10] and is now manufactured by several other companies.

While providing access to otherwise inaccessible fasteners, thinning of the tool shaft to create the ball shape renders it weaker than the straight-shaft version, limiting the torque that can be applied. The tool also makes point contact with the fastener as opposed to the line contact seen in the straight style tools.

Manufacturing methods[edit]

  • Hex socket screw heads are usually made by stamping the head with a die, plasticly deforming the metal. Other ways to generate the hex socket include linear broaching and rotary broaching. Broaching the heads with a linear broach is essentially the metalworking analog of mortising wood with a mortising machine; a hole is drilled and then the corners are broached out. This operation often leaves little telltale curled chips still attached at the bottom of the socket. These are negligible for most applications.
  • Hex keys are made by imparting the hexagon cross-section to steel wire (for example, with a die), then bending and shearing.

See also[edit]


  1. ^ Rybczynski 2000, pp. 79–81.
  2. ^ Alloy Artifacts, Various Tool Makers (section on Allen Manufacturing Company), retrieved 2012-01-25. 
  3. ^ Hallowell 1951, pp. 51–52.
  4. ^ Hallowell 1951, p. 51.
  5. ^ Hallowell 1951, p. 52.
  6. ^ Hallowell 1951, p. 57.
  7. ^ Hallowell 1951, pp. 54,57–59.
  8. ^ Hallowell 1951, p. 54.
  9. ^ Premiere ball end tools, Bondhus Corporation 


External links[edit]