外文翻译--运动的综合,凸轮和齿轮-精品

外文翻译：Kinematic Synthesis ，Cams and Gears 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. Kinematic design, or kinematic syntheses, 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.A kinematician defined kinematics as “the study of the motion of mechanisms and methods of creating them.” The first part of this definition deals with kinematic analysis. Given a certain mechanism, the motion characteristics of its components will be determined by kinematic analysis. The statement of the tasks of analysis contains all principal dimensions of the mechanism, the interconnections of its links, and the specification of the input motion or method of actuation. The objective is to find the displacements, velocities, accelerations, shock or jerk (second acceleration) , and perhaps higher accelerations of the various members, as well as the paths described and motions performed by certain elements. In short, in kinematic analysis we determine the performance of a given mechanism. The second part of definition may be paraphrased in two ways:1. The study of methods of creating a given motion by means of mechanisms.2. The study of methods of creating mechanisms having a given motion.In either version, the motion is given and the mechanism is to be found. This is the essence of kinematic synthesis. Thus kinematic synthesis deals with the systematic design of mechanisms for a given performance. The area of synthesis may be grouped into two categories.1. Type synthesis. Given the required performance, what type of mechanism will be suitable? (Gear trains? Linkages? Cam mechanisms? ) Also, how many links should the mechanism have? How many degrees of freedom are required? What configuration id desirable? And so on. Deliberations involving the number of links and degrees of freedom are often referred to as the province of a subcategory of type synthesis called number synthesis.2. Dimensional synthesis. The second major category of kinematic synthesis is best defined by way of its objective: Dimensional synthesis seeks to determine the significant dimensions and the starting position of a mechanism of preconceived type for a specified task and prescribed performance.Significant dimensions mean link lengths or distances on binary, ternary, and so on, links, angles between axis, cam-contour dimensions and cam-follower diameters, eccentricities, gear rations, and so forth. A mechanism of preconceived type may be a slider-crank linkage, a four-bar linkage, a cam with flat follower, or a more complex linkage of a certain configuration defined topologically but not dimensionally. Thereare three customary tasks for kinematic synthesis: function generation, path generation and motion generation.In function generation mechanisms rotation or sliding motions of input and output links must be correlated. For an arbitrary function )(x f y =, a kinematic synthesis task may be to design a linkage to correlate input and output such that the input moves by x , the output moves by )(x f y = for the range 10+<<n x x x . In the case of rotary input and output, the angles of rotation ϕ and ψ are the linear analogs of x and y respectively. When the input link is rotated to a value of the independent x , the mechanism in a “black box” causes the output link to turn to the corresponding value of the dependent variable )(x f y =. This may be regarded as a simple case of a mechanical analog computer. A variety of different mechanisms cou ld be contained within the “black box”. However, the four -bar linkage is not capable of error-free generation of an arbitrary function and can match the function at only a limited number of precision points. It is widely used in industry because the four-bar linkage id simple to construct and maintain.In path generation mechanism a point on a “floating link” is to trace a path defined with respect to a fixed frame of reference. If the path points are to be correlated with either time or input-link positions, the task is called path generation with prescribed timing. An example of path generation mechanisms id a four-bar linkage designed to pitch a baseball or tennis ball. In this case the trajectory of point p would be such as to pick up a ball at a prescribed location and to deliver the ball along a prescribed path with prescribed timing for reaching a suitable throw-velocity and direction.There are many situations in the design of mechanical devises in which it is necessary either to guide a rigid body through a series of specified, finitely separated positions or to impose constraints on the velocity and/or acceleration of the moving body at a reduced number of finitely separated positions. Motion-generation or rigid-body guidance mechanism requires that an entire body be guided through a prescribed motion sequence. The body to be guided usually is a part of a floating link, of which not only is the path of a point p prescribed, but also the rotation of a line passing through the point and embedded in the body,. For instance, the line might represent a carrier link in a automatic machinery where a point located on the carrier link has a prescribed path while the carrier has a prescribed angular orientation. Prescribing the movement of the bucket for a bucket loader id another example of motion generation mechanisms, the path of tip of the bucket is critical since the tip must perform a scooping trajectory followed by a lifting and a dumping trajectory. The angular orientation of the bucket are equally important to ensure that load is dumped from the correct position.A cam is a convenient device for transforming one motion into another. Thismachine element has a curved or grooved surface which mates with a follower and imparts motion to it. The motion of the cam (usually rotation) is transformed into follower oscillation, translation, or both. Because of the various cam geometries and the large number of cam and follower combinations, the cam is an extremely versatile mechanical element. Although a cam and follower may be designed for motion, path, or function generation, the majority of applications utilize the cam and follower for function generation.The most common cam types according to cam shapes are: disk or plate translating (two-dimensional or planar), and cylindrical (three-dimensional or spatial) cams. Followers can be classified in several ways: according to follower motion, such as translation or oscillation; according to whether the translational (straight-line) follower motion is radial of offset from the center of the cam shaft; and according to the shape of the follower contact surface (e. g. , flat-face, roller, point (knife-edge), spherical, planar curved, or spatial-curved surface).In the case of a disk cam with a radial (in-line) translating roller follower the smallest circle that can be drawn tangent to the cam surface and concentric with the camshaft is the base circle. The tracer point is a point at the center of the roller center and the normal to the pitch curve. The pressure angle is the angle between the direction of the path of the roller center and the normal to the pitch curve through the center of the roller and is the complement of the transmission angle. Neglecting friction, this normal is collinear with the contact force between the cam and follower. As in a linkage, the pressure angle varies during the cycle and is a measure of the ability of the cam to transfer motive effort to the follower. A large pressure angle will produce an appreciable lateral force exerted on the stem of the follower, which, in the presence of friction, would tend to bind the follower in the guide.Numerous applications in automatic machinery require intermittent motion. A typical example will call for a rise-dwell-return and perhaps another dwell period of a specified number of degrees each, together with a required follower displacement measured in centimeters or degrees. The designer’s job is to lay out the cam accordingly. The first decision to be made is to choose the cam follower type. The specified application may dictate the combination of the cam and follower. Some factors that should enter into the decision are: geometric considerations, dynamic considerations, environmental considerations and economic matters. Once a type of cam and follower pair has been selected, the follower motion must be chosen. Therefore, the velocity, acceleration, and in some cases further derivatives of the displacement of the follower are of great importance.Gears are machine elements that transmit motion by means of successively engaging teeth. Gears transmit motion from one rotating shaft to another, or to a rack that translates. Numerous applications exist in which a constant angular velocity ratio (or constant torque ratio) must be transmitted between shafts. Based on the variety of gear types available, there is no restriction that the input and the output shafts need be either in-line or parallel. Nonlinear angular velocity ratios are also available by using noncircular gears. In order to maintain a constant angular velocity, the individual tooth profile must obey the fundamental law of gearing: for a pair of gears to transmita constant angular velocity ratio, the shape of their contacting profiles must be such that the common normal passes through a fixed point on the line of the centers.Any two mating tooth profiles that satisfy the fundamental law of gearing are called conjugate profiles. Although there are many tooth shapes possible in which a mating tooth could be designed to satisfy the fundamental law, only two are in general use: the cycloidal and involute profiles. The involute has important advantages: it is easy to manufacture and the center distance between a pair of involute gears can be varied without changing the velocity ratio. Thus chose tolerances between shafts are not required when utilizing the involute profile.There are several standard gear types. For applications with parallel shafts, straight spur gear, parallel helical, or herringbone gears are usually used. In the case of intersecting shafts, straight bevel of spiral bevel gears are employed. For nonintersecting and nonparallel shafts, crossed helical, worm, face, skew bevel or hypoid gears would be acceptable choices. For spur gears, the pitch circles of mating gears are tangent to each other. They roll on one another without sliding. The addendum is the height by which a tooth projects beyond the pitch circle (also the radial distance between the pitch circle and the addendum circle). The clearance is the amount by which the addendum (tooth height below the pitch circle) in a given gears exceeds the addendum of its mating gear. The tooth thickness is the distance across the tooth along the arc of the pitch circle while the tooth space is the distance between adjacent teeth along the arc of the pitch circle. The backlash is the amount by which the width of the tooth space exceeds the thickness of the engaging tooth at the pitch circle.中文：运动的综合，凸轮和齿轮机构是形成许多机械装置的基本几何结构单元，这些机械装置包括自动包装机、打印机、机械玩具、纺织机械和其他机械等。