The specifications, models and dimensions of the Allen wrench are determined according to the distance s between the opposite sides of the section. For example, for a 2mm Allen wrench, the distance s between the opposite sides of the cross section of the wrench is required to be 1.96-2mm, and for a 4mm Allen wrench, the distance s is required to be 3.95-4mm.
Because the Allen wrench is used to screw the Allen screw, the size of the Allen wrench is actually determined by the size of the Allen hole. If it is too big, it won't fit in. If it is too small, it will be difficult to tighten and easy to slip. The specifications of Allen wrenches are divided into metric and English, and there is little difference in use, but the measurement units are different.
Development History of Allen Wrench
Speaking of the origin of Allen key, we must start with Allen key. In some non-English-speaking countries in Europe, people call "Allenkey" "Unbrakokey", which is actually the earliest brand of hexagon socket head screws. It was founded by American SPS Company around 19 1 1.
SPS Company imported an inner quadrangular screw from England earlier, but the price was very expensive. In order to save money, SPS decided to produce it by itself. H.T.Hallowell, the founder of SPS, said in his memoirs: "We started to try to make some square-hole screws similar to those in Britain, but soon found that such screws would not be accepted in the United States. So we decided to add hexagonal holes to the screws. "
Hallowell's explanation of this history is vague, but later generations speculated that SPS was modified to avoid the patent dispute of internal quadrangular screws.