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A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a ''straight'' thread and the latter called a ''tapered'' thread. A screw thread is the essential feature of the screw as a simple machine and also as a fastener. The mechanical advantage of a screw thread depends on its ''lead'', which is the linear distance the screw travels in one revolution.〔http://books.google.com/books?id=IRdIAAAAIAAJ&pg=PA137〕 In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation. ==Applications== Screw threads have several applications: * Fastening * *Fasteners such as wood screws, machine screws, nuts and bolts. * *Connecting threaded pipes and hoses to each other and to caps and fixtures. * Gear reduction via worm drives * Moving objects linearly by converting rotary motion to linear motion, as in the leadscrew of a jack. * Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a micrometer. * Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew of a lathe. In all of these applications, the screw thread has two main functions: * It converts rotary motion into linear motion. * It prevents linear motion without the corresponding rotation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「screw thread」の詳細全文を読む スポンサード リンク
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