Inbus

A hex key, Allen key or Allen wrench is a tool used to drive bolts and screws with hexagonal sockets in their heads.

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 genericised use is discouraged by this company. The standard generic name used in catalogues and published books and journals is "hex key".

Explained by the geographical and commercial history of the drive type's development, the term "hex key“ is best known as Allen in the USA (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), known as Allen in Spain (pronounced with Spanish double L), Inbus in Germany (The term “INBUS” is a registered trademark, originated by the German company Bauer & Schaurte in 1934 and currently owned by INBUS IP GmbH, Breckerfeld, Germany) and Unbrako key or wrench in Scandinavia. 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:

Extant records suggest (without offering exhaustive documentation) 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 (2000) 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 (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).

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