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A screw, or bolt, is a type of fastener, typically made of metal, and characterized by a helical ridge, known as a ''male thread'' (external thread) or just ''thread'', wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as a ''female thread'' (internal thread), often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects. A screw will almost always have a ''head'' on one end which contains a specially formed shape that allows it to be turned, or ''driven'', with a tool. Common tools for driving screws include screwdrivers and wrenches. The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a ''bearing surface''. There are exceptions; for instance, carriage bolts have a domed head that is not designed to be driven; set screws often have a head smaller than the outer diameter of the screw; J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt. The cylindrical portion of the screw from the underside of the head to the tip is known as the ''shank''; it may be fully threaded or partially threaded.〔.〕 The distance between each thread is called the "pitch". The majority of screws are tightened by clockwise rotation, which is termed a ''right-hand thread''; a common mnemonic device for remembering this when working with screws or bolts is "righty-tighty, lefty-loosey." Screws with left-hand threads are used in exceptional cases. For example, when the screw will be subject to counterclockwise torque (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. The left side pedal of a bicycle has a left-hand thread. More generally, screw may mean any helical device, such as a clamp, a micrometer, a ship's propeller or an Archimedes' screw water pump. ==Differentiation between bolt and screw== There is no universally accepted distinction between a screw and a bolt. A simple distinction that is often true, although not always, is that a bolt passes through a substrate and takes a nut on the other side, whereas a screw takes no nut because it threads directly into the substrate. ''Machinery's Handbook'' describes the distinction as follows: This distinction is consistent with ASME B18.2.1 and some dictionary definitions for ''screw''〔(【引用サイトリンク】 publisher = Cambridge University Press )〕〔(【引用サイトリンク】 url = http://www.allwords.com/query.php?SearchType=3&Keyword=screw&goquery=Find+it%21&Language=ENG )〕 and ''bolt''.〔(【引用サイトリンク】 url = http://www.merriam-webster.com/dictionary/bolt )〕〔(【引用サイトリンク】 publisher = Oxford )〕〔(【引用サイトリンク】 publisher = Cambridge University Press )〕 The issue of what is a screw and what is a bolt is not completely resolved with ''Machinery's Handbook'' distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction, and usage variations.〔(【引用サイトリンク】url=http://www.fastenerexperts.com/bolt-guide/ )〕 Some of these issues are discussed below: 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」の詳細全文を読む スポンサード リンク
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