Hex key


A hex key, Allen wrench or Allen key, is a simple tool used to drive bolts and screws with hexagonal sockets in their heads.
The tool is usually formed of a single piece of hexagonal rod of hard steel, with blunt ends that are meant to fit snugly into the screw's socket, bent in an "L" shape with unequal arms. The tool is usually held and twisted by the long arm, creating a large torque at the tip of the short arm. Reversing the tool lets the long arm reach screws in hard-to-reach places.
Each key is meant to be used with screws of a specific socket size, with rather tight tolerances; so the tool is commonly sold in kits that include half a dozen or more keys of different sizes. Usually the length of the key increases with the size of the socket, but not necessarily in direct proportion.
Variants of the tool have the short end inserted in a transverse handle, which may contain multiple keys that can be folded into the handle when not in use.
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. The standard generic name used in catalogues and published books and journals is "hex key".

Nomenclature

Explained by the geographical and commercial history of the drive type's development, the term "hex key" is best known as "Allen" in the UK, Australia and the USA, and also in Spain,
as "Inbus" in Germany,
and as "Unbrako" key or wrench in Scandinavia. In Italy, it is known as brugola, for the company Officine Egidio Brugola.
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.

Features

Some features of hex keys are:
The scant documentation available indicates that the idea of a hex socket screw drive was probably conceived as early as the 1860s to the 1890s, but that such screws were probably not manufactured until around 1910. Rybczynski describes a flurry of patents for alternative drive types in the 1860s to the 1890s in the US, which are confirmed to include internal-wrenching square and triangle types , but he explains that these were patented but not manufactured because of 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 suitable cold forming method, using the right material and the right die design.
In 1909–1910, William G. Allen too patented a method of cold-forming screw heads around a hexagonal die. Published advertisements for the "Allen safety set screw" by the Allen Manufacturing Company of Hartford, Connecticut, exist from 1910. 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, Howard T. Hallowell Sr, presents a version of events in which SPS developed a hex socket drive in-house, independently of Allen, circa 1911. From this came the 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 Britain, but they were very expensive. 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. Hallowell said that " a while we experimented with a screw containing a square hole like the British screw but soon found these would not be acceptable in this country . Then we decided to incorporate a hexagon socket into the screw ." 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. 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 socket head set screw, Hallowell had the idea to make a socket head cap screw. 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 ." SPS gave their line of screws the Unbrako brand name, chosen for its echoing of the word .
Hallowell said that acceptance of the internal-wrenching hexagon drive was slow at first, but that it eventually caught on quite strongly. This adoption occurred first in tool and die work and later in other manufacturing fields such as defense, 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 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.".
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.
It appears that the internal-wrenching hexagon drive may have been independently reinvented in various countries. At the least, the design was patented in various countries by various patentees, and its name varies. There is another name in Italian, stemming from the name Officine Egidio Brugola, a company who first commercialized Allen's products in Italy.

Hex key standard sizes

Hex keys are measured across-flats, which is the distance between two opposite flat sides of the key.
Damage to the fastener or the tool can result from using a hex wrench that is too small for the socket, for example a 5 mm tool used in a 5.5 mm socket. Because hex-style hardware and tools are available in both metric and Imperial and customary sizes, 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 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 ¾" that they are completely interchangeable in practical use.

Metric sizes

Standard metric sizes are defined in ISO 2936:2014 "Assembly tools for screws and nuts—Hexagon socket screw keys", also known as DIN 911, and, measured in millimeters are:
Metric hex wrench sizes are sometimes incorrectly 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 thread size of a metric screw or bolt.
Metric hex keys are sorted in 1st choice sizes and 2nd choice sizes. A rationale for using only metric 1st choice sizes instead of a mix with 2nd choice sizes is that the 1st choice sizing intervals satisfy the constraint of being safe sizes. This means that it should not be possible to use a hex key with the wrong size, and thereby minimizing the risk of damaging either the screw head or the tool itself if the wrong tool should be used. A tool with a size too small will then simply spin freely inside the larger sized screw head. This is not true when mixing 1st and 2nd choice metric hex keys, or when mixing metric and inch-based hex keys, and as such these combinations do not constitute safe sizes.
;Table of some common metric hex key sizes from 0.7 to 10 mm. Italic values are rounded.:
1st choice2nd choiceInches
0.7 mm0.028"
0.9 mm0.035"
1 mm0.039"
1.25 mm0.049"
1.3 mm0.051"
1.5 mm0.059"
2 mm0.079"
2.5 mm0.098"
3 mm0.118"
3.5 mm0.138"
4 mm0.157"
4.5 mm0.177"
5 mm0.197"
5.5 mm0.217"
6 mm0.236"
7 mm0.276"
8 mm0.315"
9 mm0.354"
10 mm0.394"

1st choice column shows some of the sizes which are most commonly found on hex screws with ISO metric threads

2nd choice sizes are rarely used.

Inch sizes

American sizes are defined in ANSI/ASME standard B18.3-1998 "Socket Cap, Shoulder, and Set Screws ".
;Table of some common SAE hex key sizes from 0.028" to 3/8". Italic values are rounded.:
Inch sizeMillimetersInches
0.028"0.71 mm0.028"
0.035"0.89 mm0.035"
0.050"1.27 mm0.050"
1/16"1.59 mm0.0625"
5/64"1.98 mm0.078125"
3/32"2.38 mm0.09375"
7/64"2.78 mm0.109375"
1/8"3.18 mm0.125"
9/64"3.57 mm0.140625"
5/32"3.97 mm0.15625"
3/16"4.76 mm0.1875"
7/32"5.56 mm0.21875"
1/4"6.35 mm0.25"
5/16"7.94 mm0.3125"
3/8"9.53 mm0.375"

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

Variants