An Allen wrench or Allen key is a familiar brand name for a hex key or hex head wrench, a tool that drives a screw or bolt called a “socket head cap screw.” This type of screw, with a recessed hexagonal head is common in bicycles, motorcycle engines and, increasingly, build-it-yourself furniture, according to Popular Mechanics.com. The hex head wrench is also known by the brand name “Unbrako,” sometimes misspelled “Umbrako.”
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According to John Bates of the Traditional Tool Group in an article for SuperLiving.com.au, the SPS Company of the United States is thought by some to have invented the socket head cap screw. The company, established around 1911, used the brand name Unbrako, as in “unbreakable,” for the screw and became a world leader in its mass production.
The “Allen screw” name for the socket head cap screw and “Allen key” for the wrench originate from the products of the Allen Manufacturing Company of Hartford, Connecticut. According to Bates, it is widely reported that the company trademarked the name “Allen wrench or key” for its range of hex wrenches in 1943.
Like the Unbrako name, the name of the popular Allen brand influenced terminology regarding the tool. The Allen Manufacturing Company no longer exists, but the Danaher Corporation of Washington, DC. took over the name and continued production of the Allen wrench.