1、机械工程专业英语 English In Mechanical Engineering,Prepared by Xiaoping Pang From College of Mechanical Engineering Chongqing University,14 Basic Concepts of Mechanisms,A combination of interrelated parts(相关零件的组合体) having definite（确定的） motions and capable of performing useful work may be called machine. A m
2、echanism is a component of a machine consisting of two or more bodies arranged so that the motion of one compels the motion of the others. Kinematics is the study of motion in mechanisms without reference to the forces that act on the mechanism. Dynamics is the study of motion individual bodies and
3、mechanisms under the influence of forces and torques. The study of forces and torques in stationary systems (and systems with negligible inertial effects) is called statics. Synthesis is the procedure by which a product (a mechanism, for example) is developed to satisfy a set of performance requirem
4、ents. If a product configuration is tentatively specified and then examined to determine whether the performance requirements are met, the process is called analysis. The design of mechanisms involves both synthesis and analysis.,14 Basic Concepts of Mechanisms,The design process begins with the rec
5、ognition of a need. A set of requirements is then listed. Detailed analyses of displacements, velocities, and accelerations are usually required. This part of the design process is followed by an analysis of forces and torques. The design process may continue long after（很久以后） the first models have b
6、een produced and may include redesigns of components that affect velocities, accelerations, forces, and torques. In order to successfully compete from year to year, most manufactures must continuously modify their product and their methods of production. Increases in the production rate, upgrading o
7、f product performance, redesign for cost and weight reduction are frequently required. Success may hinge on（决定于） the accuracy of the kinematic and dynamic analysis.,14 Basic Concepts of Mechanisms,Many of the basic linkage configurations have been incorporated into（成为。的一部分 ） machines designed centur
8、ies ago, and the terms we use to describe them have changed over the years. Thus, definitions and terminology are not consistent throughout the technical literature. In most cases, however, meanings will be clear from the context of the descriptive matter. A few terms of particular interest to the s
9、tudy of kinematics and dynamics of machines are defined below.,14 Basic Concepts of Mechanisms,Link A link is one of the rigid bodies or members joined together to form a kinematic chain. The term rigid link, or sometimes simply link, is an idealization used in the study of mechanisms that does not
10、consider small deflections due to strains in machine members. A perfectly rigid or inextensible link can exist only as a textbook type of model of a real machine member. For typical machine parts, the maximum changes in dimension are on the order of only a one-thousandth of the part length. We are j
11、ustified（合理的） in neglecting this small motion when considering the much greater motion characteristic of most mechanisms. The word link is used in a general sense to include cams, gears, and other machine members in addition to cranks, connecting rods and other pin-connected（铰接） components.,14 Basic
12、 Concepts of Mechanisms,Frame The fixed or stationary link in a mechanism is called the frame. When there is no link that is actually fixed, we may consider one as being fixed and determine the motion of the other links relative to it. In an automotive engine, for example, the engine block (see Fig.
13、14.1) is considered the frame, even though the automobile may be moving. Degree of Freedom The number of degrees of freedom of a linkage is the number of independent parameters required to specify the position of every link relative to the frame or fixed link. If the instantaneous configuration of a
14、 system may be completely defined be specifying one independent variable, that system has one degree of freedom. Most practical mechanisms have one degree of freedom.,14 Basic Concepts of Mechanisms,An unconstrained rigid body has six degrees of freedom: translation in three coordinate directions an
15、d rotation about three coordinate axes. If the body is restricted to motion in a place, there are three degrees of freedom: translation in two coordinate directions and rotation within the plane. Lower and Higher Pairs Connections between rigid bodies consist of lower and higher pairs of elements. T
16、he two elements of a lower pair have theoretical surface contact with one another, while the two elements of a higher pair have theoretical point or line contact (if we disregard deflections). Kinematic Chain A kinematic chain is an assembly（集合） of links and pairs (joints). Each link in a closed-loo
17、p kinematic chain is connected to two or more other links. A linkage failing to meet the closed-loop criterion is an open-loop kinematic chain.,14 Basic Concepts of Mechanisms,Linkage Since we may wish to examine kinematic chains without regard to their ultimate（最后） use, it is convenient to identify
18、 any assemblage of rigid bodies connected by kinematic joints as a linkage. Thus, both mechanisms and machines may be considered linkages. In some references, however, the term linkage is restricted to kinematic chains made up of lower pairs. Planar Motion and Planar Linkages If all points in a link
19、age may be described as a planar linkage. If we examine the linkage made up of the crank, connecting rod, and piston in a piston engine, we see that it is a planar linkage. Most of the mechanisms in common use may be treated as planar linkages.,14 Basic Concepts of Mechanisms,Spatial Motion and Spat
20、ial Linkages The more general case in which motion cannot be described as taking place in parallel planes is called spatial motion, and the linkage may be described as a spatial or three-dimensional (3D) linkage. Inversion The absolute motion of a linkage depends on which of the links is fixed, that is, which link is selected as the frame. If two otherwise equivalent linkages(等效连杆机构) have different fixed link, then each is an inversion of the other. Relative motion will be the same in both linkages.,
