1、 黄石理工学院 毕业设计 (论文 ) 外文文献翻译 Introduction to mechanism The function of 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. (1) Gear system, i
2、n which toothed members in contact transmit motion between rotating shafts. (2) Cam system, where a uniform motion of an input member is converted into a nonuniform motion of the output member. (3) Plane and spatial linkages are also useful in creating mechanical motions for a point or rigid body. A
3、 kinematic chain is a system of links, which are either jointed together or are in contact with one another in a manner that permits them to move relative to one another. If one of the links is fixed and the movement of any other link to a new position will cause each of the other links to move to d
4、efinite predictable position, the system is a constrained kinematic chain. Otherwise, the system is an unconstrained kinematic chain. A mechanism or linkage is a constrained kinematic chain, and is a mechanical divice that has the purpose of transferring motion and force from a source to an output.
5、A linkage consists of links (or bar),generally considered rigid,which are connected by joints, such as pin (or revolute) or prismatic joints, to form open or closed chains (or loops). Such kinematic chains,with at least one link fixed, become (i) mechanisms if at least two other links remain mobilit
6、y, or (ii) stuctures if no mobility remains. In other words, a mechanism permits relative motion between its “rigid links”； a structure does not. Since linkages make simple mechanisms and can be designed to perform complex tasks, such as nonlinear motion and force ransmission, they will receive much
7、 attention in mechanism study. Mechanisms are used in a great variety of machines and devices. The simplest closed-loop linkage is the four-bar linkage, which has three moving links (plus one fixed link) and four pin joints. The link that is connected to the power source or prime mover and has one m
8、oving pivot and one ground pivot is called the input link. The output link connects another moving povit to another ground povit. The coupler or floating link connected the two moving pivots, thereby “coupling” the input to the output link. The four-bar linkage has some special configurations create
9、d by making one or more links infinite in length。 The slider-crank （ or crank and slider） mechanism is a four-bar chain with a slinder replacing an infinitely long output link。 The internal combustion engine is built based on this mechanism。 Other forms of four- bar mechanisms exist in which a slide
10、r is guided on a moving link rather than on a fixed link. These are called inversions of the slider-crank， produced when another link（ the crank， coupler， or slider） is fixed link。 Although the four-bar linkage and slider-crank mechanism are very useful and found in thousands of applications， we can
11、 see that these linkages have limited performance level。 Linkages with more members are often used in more demanding circumstances。 However it is often difficult to visualize the movement of a multiloop linkage， especially when other components appear in the same diagram。 The fist step in the motion
12、 analysis of more complicated mechanisms is to sketch the equivalent kinematic or skeleton diagram。 The skeleton diagram serves a purpose similar to that of the electrical schematic or circuit diagram in that it displays only the essential skeleton of 黄石理工学院 毕业设计 (论文 ) 外文文献翻译 the mechanism， which， h
13、owever， embodies the the key dimensions that affect its motion。 The kinematic diagram takes one of two forms： a sketch（ proportional but not exactly to scale）， and the scaled kinematic diagram（ usually used to further analysis： position， displacement， velocity，acceleration， force and torque transmis
14、sion， etc。）。 for convenient reference， the links are numbered（ starting with ground link as number 1）， while the joints are lettered。 The next step in the kinematic analysis of mechanisms is to determine the number of degree of freedom of the mechanism。 By degree of freedom we mean the number of ind
15、ependent inputs required to determine the positions of all links of the mechanism with respect toground。 There are hundreds of thousands of different linkage types that one could invent。 Envision a bag containing a large variety of linkage components： binary， ternary， quaternary， and so on， links ；
16、pin joints，slide joints； cams and cam followers； gears， chains， sprockets， belts， pulleys， and so on。（ spherical and helical joints as well as other connections that allow three-dimensional relative motion are not included， as only planar motion in parallel planes are discussed here）。 Furthermore， i
17、magine the possibility of forming all sorts of linkage types by putting these components together。 Are there any rules that help govern how these mechanisms are formed？ Actually most mechanism tasks require a single input to be transferred to a single output。 Therefore， single-degree-of-freedom mech
18、anisms are the forms used most frequently。 For example， it is easy to see intuitively that a four-bar linkage is a single-degree-of-freedom linkage。 The process of drawing kinematic diagrams and determining degrees of freedom of mechanisms are the first steps in both the kinematic analysis and synth
19、esis process。 In kinematic analysis， a particular given mechanism is investigated based on the mechanism geometry plus possibily other known characteristics。 Kinematic synthesis， on the other hand， is the proess of designing a mechanism to accomplish a desired task。 Here， both choosing the type as w
20、ell as the dimensions of the new mechanism can be part of kinematic synthesis。 The ability to visualize relative motion， to reason Why a mechanism is designed the way it is， and the ability to improve on a particular design are marks of a successful kinematician。 Although some of this ability comes
21、in the form of innate creativity， much of it is a learned skill that improves with practice。 Movement analysis One of the simplest and most useful mechanisms is the four-bar linkage。 Most of the following description will concentrate on this linkage， but the procedures are also applicable to more co
22、mplex linkages。 We already known that a four-bar linkage has one degree of freedom。 Are there any more that are useful to know about four-bar linkage？ indeed there are！ these include the Grashof criteria， the concept of inversion， dead-center position（ branch points）， branching， transmission angle a
23、nd their motion feature， including positions， velocities and accelerations。 The four-bar linkage may take form of a so-called crank-rocker or a double-rocker or a double-crank（ drag-link） linkage， depending on the range of motion of the two links connected to the ground link。 The input crank of a cr
24、ank-rocher type can rotate continuously through 3600 ，while the output link just “ rocks”（ or oscillates）。 As a particular case， in a parallelogram linkage，where the length of the input link equals that of the output link and the lengths of the coupler and the ground link are also the same， both the
25、 input and output link may rotate entirely around or switch into a crossed configuration called an antiparallelogram linkage。 Grashof, s criteria states 黄石理工学院 毕业设计 (论文 ) 外文文献翻译 that the sum of the shortest and longest links of a planar four-bar linkage cannot be greater than the sum of the remainin
26、g two links if there is to be continuous relative rotation between any two links。 Notice that the same four-bar linkage can be a different type， depending on which link is specified as the frame（ or ground）。 Kinematic inversion is the process of fixing different links of a chain to create different
27、mechanisms。 Note that the relative motion between links of a mechanism does not change indifferent inversions。 Besides having knowledge of the extent of the links， it would be useful to have a measure of how well a mechanism might“ run” before actually building it。 Hartenberg mentions that“ run”is a
28、 term that means effectiveness with which motion is imparted to the output link ； it implies smooth operation， in which a maximum force component is available to produce a force or torque in an output member。 The resulting output force or torque is not only a function of the geometry of the linkage，
29、 it is generally the result of dynamic or inertia force which is often several times as large as the static force。 For the analysis of low-speed operations or for an easily obtainable index of how any mechanism might run， the concept of the transmission angle is extremely useful。During the motion of
30、 a mechanism， the transmission angle changes in value。 A transmission angle of 0 degree may occur at a specific position， on which the output link will not move regardless of how large a force is applied to the input link。 In fact， due to friction in the joints， the general rule of thumb is to desig
31、n mechanisms with transmission angle of large than a specified value。Matrix-based definitions have been developed which measure the ability of a linkage to transmit motion。 The value of a determinant（ which contains derivatives of output motion variables with respect to an input motion variable for
32、a given linkage geometry） is a measure of the movability of the linkage in a particular position。 If a mechanism has one degree of freedom (e.g. a four-bar linkage), then prescribing one position parameter, such as the angle of the input link, will completely specify the position of the rest of the
33、mechanism (discounting the branching possibility). We can develop an analytical expression relating the absolute angular positions of the links of a four-bar linkage. This will be much more useful than a graphical analysis procedure when analyzing a number of positions and /or a number of different
34、mechanisms, because the expressions will be easily programmed for automatic computation. The relative velocity or velocity polygon method of performing a velocity analysis of a mechanism is one of several methods available. The pole represents all points on the mechanism having zero velocity. Lines
35、drawn from the pole to points on the velocity polygon represent the absolute velocities of the corresponding points on the mechanism. A line connecting any two points on the velocity polygon represents the relative velocity for the two corresponding points on the mechanism. Another method is the ins
36、tantaneous center or instant center method, which is a very useful and often quicker in complex linkage analysis. An instantaneous center or instant center is a point at which there is no relative velocity between two links of a mechanism at that instant. In order to locate the locations of some ins
37、tant centers of a given mechanism, the Kennedys theorem of three centers is very useful. It states that the three instantaneous centers of three bodies moving relative to one another must lie along a straight line. The acceleration of links of a mechanism is of interest because of its effect on inertia force, which in turn influences the stress in the parts of a machine, bearing loads, vibration, and noise.
