Invention of the screw

By Joseph A. McGeough

Although Archimedes is credited with inventing the screw in the 3rd century BCE, his screw was not today’s fastener but actually two other screw-type devices. One was a kind of water pump; still used today for large-volume low-lift industrial applications, the device is now called the inclined screw conveyor. The second was the “endless screw,” actually the worm of a worm and gear set, one of the ancients’ five devices for raising heavy weights. With the state of the mechanical arts as it was then, Archimedes’ concept of the screw was actually as a motion-transforming device and was more hypothetical than practical.

By the 1st century BCE, heavy wooden screws had become elements of presses for making wine and olive oil and for pressing clothes. The character of the screw took on a new dimension, for these screws were used to exert pressure; their modern counterparts are called power screws. These press screws were turned by means of hand spikes thrust into radial holes in the cylindrical end. The problem of making the internal thread of the nut prevented the use of small threaded fasteners in metal construction. The external thread, however, was readily, if tediously, made by filing.

Metal screws and nuts appeared in the 15th century. The square or hexagonal head or nut was turned with an appropriate box wrench; a T-handled socket wrench was developed in the 16th century. Some screws used in 16th-century armour have slots (nicks) in which a screwdriver may have been used, although this tool is not shown. Deep notches on the circumferences of the heads of other armour screws suggest that some type of pronged device was used to turn them. Slotted, round headed screws were used in the 16th century, but few screw-and-nut-fastened clocks are in evidence earlier than the 17th century. Metal screws were called machine, or machinery, screws since they were made of metal and mated with threaded holes.

The wood screw differs from the machine screw in that the wood into which it is turned is deformed into a nut. It must, however, be started in a hole made by awl or drill. Aside from a few and sometimes doubtful artefacts from Roman times, the wood screw is not mentioned until the mid-16th century, when it appears in a mining treatise. Here a screw tapered to a point, carrying a slotted head and looking very familiar except for its left-handed thread, is described so casually as to suggest that it was a common article. It is remarked that the screw is superior to the nail, which is also shown being driven by a claw hammer. There is no mention of a screwdriver.

Screwdrivers and wrenches

The simple screwdriver was preceded by a flat-bladed bit for the carpenter’s brace (1744). The handled screwdriver is shown on the woodworker’s bench after 1800 and appears in inventories of tool kits from that date. Screwdrivers did not become common tools until 1850 when automatic screw machines began the mass production of tapered, gimlet-pointed wood screws. In its early form, the screwdriver was made from flat stock; its sometimes scalloped edges contributed nothing to function. Being flat, the blade was easy to haft but weak when improperly used for prying. The present form of the screwdriver, round and flattened only at the end, was devised to strengthen the shaft and make use of readily available round-wire stock.

Early box and socket wrenches fit only a particular nut or screw with flat surfaces on the head. The open-end wrench may have rectangular slots on one or both ends. In their earliest forms, such wrenches, with straight, angled, or S-shaped handles, were made of wrought iron. Cast iron came into use around 1800. Modern wrenches are drop forgings and come in many formats.

The limitations of fixed-opening wrenches were addressed as early as the 18th century, when sliding-jaw types were developed to accommodate a range of flats. In these, the end of an L-shaped handle provided the fixed jaw, and the parallel jaw was arranged to slide along the handle until it engaged the flats. In the first models, the sliding jaw was fixed into position by a wedge that was hammered into place. By the early 19th century, patents for screw wrenches began to proliferate; in these the sliding jaw was positioned and held by means of a screw whose axis was parallel to the handle. The most common example is the monkey wrench, whose name appeared in tool catalogues in the 1840s but may have been in use before that time. A convenient variation of this type of wrench is the thin and angled Crescent wrench, a modern innovation. The plumber’s pipe wrench is a serrated-jaw variation of the monkey wrench, whose additional feature of a pivotable movable jaw enables it to engage round objects, such as rods and pipes.

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