Screws（螺纹连接件） have been used as fasteners（紧固件） for a long time. As indicated（显示）in the section on simple machines, a screw acts with a wedging action（楔作用） and consists of a circular cylinder (or truncated cone) with a helical groove in it. When used for fastening, all screws are provided with means for applying a torque (twisting moment) to them; this may consist of a square of hexagonal head for a wrench, a head with a slot or a cross-shaped recess for a screwdriver, a cylindrical head with a recess fitting a special wrench, or a slot or a recess on one end fitting a screwdriver or special wrench and no head.
Screws with screwdriver slots have heads several shapes, such as flat, oval, round, and fillister (cylindrical with round top). Wood screws are tapered, and self-tapping screws have tapered ends and a variety of special points. The later are usually hardened and cut or form a mating internal thread when screw into the proper size hole.
Screws with hexagonal heads are usually known as cap screws. Screws with other types of heads are also known as cap screws in the larger size and as machine screws in the smaller sizes. All of these screws are used to clamp machine parts together when one of the parts has an internal thread. If neither part is threaded a bolt must be used: this consists of a screw with a hexagonal head, a nut (hexagonal ring with an internal thread), and usually a washer (flat ring). A stud has threads on both ends and no means for turning. It is permanently screwed into one member and clamps by means of a nut on the other end. All of these fasteners stretch when tightened, and the tensile load created in this way clamps the members together.
Set screws differ in form and function from the other types of screws. They are hardened and have cup, cone, oval, flat, and dog (cylindrical) ends or points. They are usually headless and are frequently used as substitutes for keys or in conjunction with keys in shaft-mounted members. The set screw fits in a threaded radial hole in the hub, and the point is tightened down into a hole, a flat, or a dimple in the shaft or key. Set screws are stressed in compression and in action they create a helical wedge between the connected parts.
Vocabulary
1.screw n. 螺钉；螺旋 vt. 拧（紧）
screw driver 螺钉旋具；改锥
2.fastener n. 紧固件；钮扣
3.indicate vt. 指示；表明
4.wedging action 楔紧作用
5.truncate vt. & a. 截短（的）；截平（的）
truncate cone 截锥体
6.groove n. 槽沟
7.hexagonal head 六角头
8.wrench n. 扳手 vi.扳动
9.twisting moment 扭矩
10.oval n. 椭圆形（的）
11.fillister n. 凹槽 v. 刨槽
12.mating internal thread 匹配的内螺纹
13.stud n. 螺栓
14.clamp n. 夹具；夹子 vt. 夹紧
15.nut n. 螺母
16.conjunction n. 结合； 连接
17.substitute n. 代替者 vt. （用）…代替
18.dimple n. 凹（座、痕）
19.stretch vt. 伸展；拉伸
20.set screw 紧定螺钉
21.in conjunction with 和…一起；连同…一起
