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. The Allen wrench trademark of the Allen Manufacturing Company was taken out in 1943, Allen became such a successful brand of hex key that many consumers in following decades have assumed (reasonably but incorrectly) that the internal-wrenching hexagon drive was invented by someone named Allen. 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.
Industry-Standard Numbers for Open-End Wrenches
Open-end wrenches were one of the first tools to be offered in standardized sizes, and from an early date these tools were assigned "standard" model numbers. These numbers actually evolved from the more-or-less sequential model numbers assigned by J.H. Williams & Company, generally credited as being the first tool maker to offer wrenches in standard sizes. As the system proved to be useful, other manufacturers started adopting the same numbering, and soon the wrench models were considered as "Trade Numbers" or "Industry-Standard Numbers".By the time of the 1912 15th Edition of the Williams catalog, open-end wrenches could still be covered by two-digit model numbers. However, additional size combinations were needed within a few years, and the system was expanded by adding a "7" prefix to some numbers, plus an alphabetic prefix (A, B, or C) to others.Not all manufacturers adopted the Williams system, at least not at first. Billings & Spencer was probably the most significant manufacturer with a separate numbering system, and they offered even a few more size combinations in their 11xx model series.
| Standards for Opening SizesThe wrench model number specified a combination of opening sizes, but the means of describing the sizes added further confusion. In the late 19th and early 20th centuries, wrench sizes were commonly named by the nominal size of the bolts whose nuts they fit, rather than the size (across the flats) of the nuts themselves. For example, a "1/2" opening might fit a 7/8 (across the flats) nut threaded on a 1/2 inch bolt.To further compound the confusion, there were at least three different standard systems for nuts and bolts: * The U.S.S. or U.S. Standard, * The S.A.E. or Society of Automotive Engineers standard, and * The Hex Cap screw standard used by hardware manufacturers.The latter convention was often indicated by a hexagon symbol followed by "C" or "CAP".These multiple standards meant that some wrench openings might be marked with three size indications. For example, a number 23 wrench would be marked "3/16 U.S.S." for the small opening, and "1/4 U.S.S.", "5/16 Hex Cap", and "5/16 S.A.E." on the larger end.This confusing system of multiple size standards persisted until the late 1920s, when it was phased out in favor of the across-flats size.
| Specification Table for Industry-Standard Double-Open WrenchesThe table below shows the specifications for the industry-standard double-open wrench models, with the Billings & Spencer 11xx model numbers included as well. The opening sizes are shown in each of applicable sizing systems, along with the nominal milled opening and decimal equivalent.
| Table 1. Specifications for Industry-Standard Double-Open Wrenches with B. & S. Models
| Hex Cap
| 1/8 - 3/16
| 5/16 - 3/8
| 0.313 - 0.375
| 1/8 - 3/16
| 1/8 -
| 5/16 - 13/32
| 0.313 - 0.406
| 1/8 -
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