Hex key

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Hex keys of various sizes.

Hex screws of various sizes.

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A Hex key, Allen wrench, Allen key or hex head wrench is a tool used to drive hex screws and bolts, which have a hexagonal socket in the head.

Some of the features of this type of tool 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 bolt can be inserted into its socket using the key.
Ther 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.

Using a single tool instead of many others reduces tooling costs for the manufacturer and is more convenient for the end user. Many manufacturers have adopted hex key bolts for assembling bicycles or for self-assembly furniture.

Generically known as a hex key, this tool is also commonly referred to by one of many trademark or brand names. "Allen wrench" was originally a trademark of Allen Manufacturing Company in Hartford, Connecticut in 1943. In non-English speaking parts of Europe, it is usually known as an "Unbrako key" (also often misspelled as "Umbrako"), which is a brand name established in 1911. The brand is owned by SPS Technologies. In Germany and the Netherlands, this type of screw is known under the name Inbus™, after the company that patented them in 1936, Bauer & Schaurte Karcher in Neuss (Innensechskantschraube Bauer und Schaurte). (It is still known as Inbus-key—often misspelled as Imbus-key—in parts of Europe.) In Italy it is known as brugola, after Egidio Brugola, who invented it in 1926.^[1]

[edit] Hex key standard sizes

(American National Standard)
Machinery's Handbook, 26^th Edition

Nominal	Socket Size
6	7/64
8	9/64
10	5/32
1/4	3/16
5/16	1/4
3/8	5/16
7/16	3/8
1/2	3/8
5/8	1/2
3/4	5/8
7/8	3/4
1	3/4

Standard metric sizes are 0.7, 0.9, 1.3, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 7, 8, 9, 10 mm and larger.

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".

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 sizes (with English sizes most commonly used in the United States), it is also possible to select a tool that is too small for the fastener by using an English-unit tool on a metric fastener, or the converse.

Some hex keys have a ball on one end, which allows the tool to be used at an angle off-axis to the screw. They were invented in 1964 by the Bondhus Corporation^[2], 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.