Screw

in mechanics, a wooden or metal cylinder surrounded by a spiral ridge or thread, the turns of which are everywhere equidistant. The cylinder described turns in a hollow cylinder of equal diameter provided with a thread matching the thread of the first. In the case of a bolt, the outside cylinder is called a nut or bur. The jackscrew used in raising great weights is a familiar form of this machine. To compute the force exerted, friction aside, we multiply the power by a quotient obtained by dividing the circumference through which the power moves by the vertical distance between two successive turns of the thread. A force of say twenty-five pounds applied at the end of a wrench a foot in length is capable of balancing a weight of over 864 pounds if exerted on a nut having twelve threads to the inch. (25 lbs. X 12 X 2 X 3.1416 X 12.)

The common pointed screw used in woodwork in place of nails is a modification of the mechanical screw. The force is applied at the circumference of the handle of the screwdriver. The wood which the screw penetrates takes the place of the nut, and the screw cuts the outside thread as it penetrates. By reason of the thread, the wood screw holds better than a nail. The screw propeller or spiral series of flanges by which steamships are driven is also a modification of the screw.

According to the census for 1900 there were thirty-three establishments in the United States engaged in the manufacture of screws of various sorts, with an annual output valued at $4,658,467. Since 1854 screws have been made entirely by machinery at a great reduction of cost. Bits of wire fed into a hopper come out perfect screws. Even the microscopical screw used in watches is cut by an ingenious machine.