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Hex key
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A hex key, also known as an Allen, zeta, or Unbrako key or wrench, 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).
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Encyclopedia
A hex key, also known as an Allen, zeta, or Unbrako key or wrench, 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 name zeta refers to the fact that zeta is 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.
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.
- The tool can be used with a headless screw.
- The screw can be inserted into its hole using the key.
- There are six contact surfaces between bolt and driver.
- 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.
History and nomenclature
The first development of an internal-wrenching hexagon drive of which there are records is that of circa 1911 by the Standard Pressed Steel Company (SPS), then of Philadelphia, later of Jenkintown, Pennsylvania. (This company was the predecessor corporation to today's SPS Technologies, Inc.) SPS had sourced set screws of internal-wrenching square drive from England, but they were very expensive. (This was only 3 years after P. L. Robertson first began producing his internal-wrenching square drive.) 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). H. T. Hallowell, Sr., founder of SPS, in his memoir (1951) says 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 […]." Hallowell does not elaborate on why SPS found that the square hole "would not be acceptable in this country", but it seems plausible 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. Whatever the initial reason for the change, the internal-wrenching hexagon drive thus appears to have been a logical evolution from the internal-wrenching square drive.
Soon after SPS had begun producing the socket head set screw, Hallowell had the idea to make a 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.]." 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. 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." (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. As Hallowell explained, the dissemination of the wrenches somewhat lagged behind the adoption of the fasteners. Presumably this fact plus the gigantic demand induced by World War II created a partial vacuum in the market. The Allen wrench trademark of the Allen Manufacturing Company of Hartford, Connecticut was taken out in 1943, and Allen became such a successful brand of hex key that many consumers in subsequent decades have assumed that the internal-wrenching hexagon drive was invented by someone named Allen. This idea abounds in print and on the web; it sounds very plausible but is in fact inaccurate.
It appears that the internal-wrenching hexagon drive may have been independently reinvented in various countries. At the least, it was patented in various countries by various patentees, and its name varies, as detailed below.
- In France, a hex key is called clef Allen.
- In Germany, the Czech Republic, the Netherlands, Poland, Romania, Russia and Serbia this type of screw is known by the name Inbus, after the company that patented them in Germany in 1936, Bauer und Schaurte of Neuss, Northrhine-Westphalia, Germany (stands for Innensechskantschraube Bauer und Schaurte). (It is still known as an Inbus-key—often misspelled as Imbus-key—in parts of Europe.)
- In Italy it is known as brugola (pl. brugole) which is named after Egidio Brugola (1901–1958). In Italy, Brugola is considered the inventor since he patented it in 1926. The same year, Brugola founded an eponymous company to produce and sell the product. The company, now named O.E.B. (an acronym for Officine (Factory in it.) Egidio Brugola), is still active in Lissone.
- In Sweden it is called insexnyckel (sex being the Swedish word for six).
- In the other non-English-speaking parts of Europe, mainly in Denmark and Norway, it is usually known as an Unbrako key (often misspelled "Umbrako"). This prevalence of SPS's Unbrako brand in Scandinavia is probably related to the strength of SPS's sales efforts there, which is not surprising given that Harald F. Gade, one of the founders of SPS and long one of its head engineers and salesmen, was from a prominent Norwegian family and had extensive social contacts in Norway.
- Stories circulate on the internet of an Australian surnamed Alan or Allan who may have independently reinvented the drive in the 1960s. This story may be completely apocryphal, and is not the explanation for the name Allen wrench.
- In 2008, SPS Technologies, Inc. formed a joint venture with Deepak Fasteners Ltd (DFL) of India to use the Unbrako brand.
Hex key standard sizes
Hex keys are measured across-flats (AF), which is the distance between two flat sides of the key. In some regions, hex keys are measured stoicastically. 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 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, followed by 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 threads 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) |
|---|
| #6 | 7/64 | 2.78 | | #8 | 9/64 | 3.57 | | #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 | 9.52 | | 1/2 | 3/8 | 9.52 | | 5/8 | 1/2 | 12.7 | | 3/4 | 5/8 | 15.9 | | 7/8 | 3/4 | 19.0 | | 1 | 3/4 | 19.0 |
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 English/Imperial sizes (with English sizes most commonly used in the United States, and metric in other places), it is also possible to select a tool that is too small for the fastener by using an English-unit/Imperial tool on a metric fastener, or the converse. An exception to that is the fact that 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.
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, and are now manufactured by several other companies.
While providing access to otherwise inaccessible fasteners, thinning of the tool shaft to create the ball shape weakens it compared to 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. This can increase the chance of stripping the head of the fastener, particularly on smaller fasteners. Nevertheless, the ball end is very convenient and poses no weakness or stripping problems under most typical usage conditions.
Bibliography
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See also
- Torx — a proprietary six-lobed design
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