Lesson 7

Mechanism

A mechanism has been defined as "a combination of rigid or resistant bodies so formed and connected that they move upon each other with definite relative motion."

Mechanism n.机械装置 , 机构 , 机制 , 机理

Resistant adj.抵抗的 ; 反抗的 ; 耐久的 ; 坚固的 , 稳定的

definite adj.明确的 , 一定的

Mechanisms form the basic geometrical elements of many mechanical devices including automatic packaging machinery, typewriters, mechanical toys, textile machinery, and others. A mechanism typically is designed to create a desired motion of a rigid body relative to a reference member.

Geometrical adj. 几何学的 , 几何的Geometry n.几何学packagingn. 包装textile n.纺织品adj. 纺织的

Kinematic design of mechanisms often is the first step in the design of a complete machine. When forces are considered, the additional problems of dynamics, bearing loads, stresses, lubrication, and the like are introduced, and the larger problem becomes one of machine design.

kinematic adj. 运动学(上 )的staticsdynamicskinematicset al 等人以及其他人，etc. (et cetra) (=and so on) 等等 , 及其他and the likeadv. 等等

The function of a mechanism is to transmit or transform motion from one rigid body to another as part of the action of a machine, there are three types of common mechanical devices that can be used as basic elements of a mechanism

transmit vt.传输 , 转送 , 传达 , 传导 , 发射 , 遗传 , 传播 , 传递vi. 发射信号 , 发报transform vt. 转换 , 改变 , 改造 , 使 ... 变形n.[数 ] 变换(式 ), [语 ] 转换Laplace ~拉普拉斯变换FFTFast Fourier ~ 快速傅氏变换

Gear Systems Gear systems, in which toothed members

in contact transmit motion between rotating shafts. Gears normally are used for the transmission of motion with a constant angular velocity ratio, although noncircular gears can be used for nonuniform transmission of motion.

gear n.齿轮 , 传动装置机构Landing ~Gear up throttle down v. 调整 , 适合 , 换档noncircular非圆形的nonuniform adj. 不一致的 , 不均匀的

follower

Cam

Cam Systems Cam systems, where a uniform motion of

an input member is converted into a nonuniform motion of the output member. The output motion may be either shaft rotation, slider translation, or other follower motions created by direct contact between the input cam shape and the follower. The kinematic design of cams involves the analytical or graphical specification of the cam surface shape required to drive the follower with a motion that is a prescribed function of the input motion.

Cam 凸轮Camshaft 凸轮轴~ gear 凸轮装置slider n. 滑块 , 滑雪 (冰 ) 者translationn. 翻译 , 译文 , 转化 , 转换[物 ][数 ] 平移Follower n. 从动件，随动件追随者 , 信徒Analytical adj.分析的 , 解析的~ geometry n.[数 ] 解析几何学graphical adj.图形的 , 绘画的prescribe v. 指示 , 规定 , 处(方 ), 开 (药 ) Prescription

Plane and Spatial Linkages

They are also useful in creating mechanical motions for a point or rigid body. Linkages can be used for three basic tasks.

( 1 ) Rigid body guidance. A rigid body guidance mechanism is used to guide a rigid body through a series of prescribed positions in space.

spatial adj.空间的

linkage n.联接，连杆机构

Guidance n.指导 , 领导导向

(2) Path generation. A path generation mechanism will guide a point on a rigid body through a series of points on a specified path in space.

(3) Function generation. A mechanism that creates an output motion that is a specified function of the input motion.

path n.小路 , 路线 , 轨道 , 通道generation n. 产生 , 发生 , 一代 , 一代人[数 ](线 , 面 , 体的 ) 形成、实现trajectory n. 轨道 , 弹道 , 轨线

Mechanisms may be categorized in several different ways to emphasize their similarities and differences. One such grouping divides mechanisms into planar, spherical, and spatial categories. All three groups have many things in common; the criterion which distinguishes the groups, however, is to be found in the characteristics of the motions of the links.

categorize v.加以类别 , 分类Category n.种类 , 别 , [逻 ] 范畴Catalog n.目录 , 目录册emphasize vt. 强调 , 着重Criterion n.( 批评判断的 )标准 , 准据 , 规范distinguish v.区别 , 辨别

A planar mechanism is one in which all particles describe plane curves in space and all these curves lie in parallel planes; i.e. , the loci of all points are plane curves parallel to a single common plane. This characteristic makes it possible to represent the locus of any chosen point of a planar mechanism in its true size and shape on a single drawing or figure. The plane four-bar linkage, the plate cam and follower, and the slider-crank mechanism are familiar examples of planar mechanisms. The vast majority of mechanisms in use today are planar.

Locus / loci (pl.)n.地点，所在地 , [数 ] 轨迹Trajectory n. [ 物 ]( 射线的 ) 轨道 , 弹道 , 轨线

plane four-bar linkage

slider-crank mechanism

A spherical mechanism is one in which each link has some point which remains stationary as the linkage moves and in which the stationary points of all links lie at a common location; i.e. , the locus of each point is a curve contained in a spherical surface, and the spherical surfaces defined by several arbitrarily chosen points are

all concentric.

arbitrarily adv.武断地 , 随机地，任意地 , 专横地locus n.地点，所在地 , [ 数 ] 轨迹~ of an equation方程的轨迹~ of singular points奇点的轨迹

The motions of all particles can therefore be completely described by their radial projections, or "shadows," on the surface of a sphere with properly chosen center. Hooke's universal joint is perhaps the most familiar example

of a spherical mechanism.

ProjectioProjectionn投影 投影

Spatial mechanisms, on the other hand, include no restrictions on the relative motions of the particles. The motion transformation is not necessarily coplanar, nor must it be concentric. A spatial mechanism may have particles with loci of double curvature.

coplanar adj.

[ 数 ] 共面的

Curvature n. 弯曲 , 曲率

Any linkage which Any linkage which contains a contains a screw pairscrew pair, for , for example, is a spatial example, is a spatial mechanism, since the mechanism, since the relative motion within a relative motion within a screw pair is helical.screw pair is helical.

Spacial Mechanism