机械工程科技英语

机械工程科技英语 Lesson 1 Overview of Engineering Mechanics As we look around us we see a world full of “things”: machines, devices, tools, things that we have designed, built, and used; things made of wood, metals, ceramics, and plastic. We know from experience that some things are better than others; they last longer, cost less, are quieter, look better, or are easier to use. Ideally, however, every such item has been designed according to some set of “functional requirements” as perceived by the designers-that is, it has been designed so as to answer the question, “Exactly what function should it perform?” In the world of engineering, the major function frequently is to support some type of loading due to weight, inertia, pressure, etc. From the beams in our homes to the wings of an airplane, there must be an appropriate melding of materials, dimensions, and fastenings to produce structures that will perform their functions reliably for a reasonable cost over a reasonable lifetime. The goal of this text is to provide the background, analyses, methods, and data required to consider many important quantitative aspects of the mechanics of structures. In practice, these quantitative methods are used in two quite different ways: (1) The development of any new device requires an interactive, iterative consideration of form, size materials, loads, durability, safety, and cost. (2) When a device fails (unexpectedly) it is often necessary to carry out a study to pinpoint the cause of failure and to identify potential corrective measures. Our best designs often evolve through a successive elimination of weak points. To many engineers, both of the above processes can prove to be absolutely fascinating and enjoyable, not to mention (at times) lucrative. In any “real” problem there is never sufficient good, useful information; we seldom know the actual loads and operating conditions with any precision, and the analyses are seldom exact. While our mathematics may be precise, the overall analysis is generally only approximate, and different skilled people can obtain different solutions. In this book most of the problems will be sufficiently “idealized” to permit unique solutions, but is should be clear that the “real world” is far less idealized, and that you usually will have to perform some idealization in order to obtain solution. The technical areas are will consider are frequently called “statics” and “strength of materials”, “statics” referring to the study of forces acting on stationary devices, and “strength of materials” referring to the effects of those forces on the structure (deformations, load limits, etc.). While a great many devices are not, in fact, static, the methods developed here are perfectly account (we shall briefly mention how this is done). Whenever the dynamic forces are small relative to the static loadings, the system is usually considered to be static. As we proceed, you will begin to appreciate the various types of approximations that are inherent in any real problem: Primarily, we will be discussing things that are in “equilibrium”, i.e., not accelerating. However, if we look closely enough, everything is accelerating. We will consider many structural 批注 [zx1]: 工程机构概要 批注 [zx2]: 惯性 批注 [zx3]: 紧固件、连接 批注 [zx4]: 反复 批注 [zx5]: 耐久力 批注 [zx6]: 查明 members to be “weightless”-but they never are. We will deal with forces that act at a “point”-but all forces act over an area. We will consider some parts to be “rigid”-but all bodies will deform under load. We will make many assumptions that clearly are false. But these assumptions should always render the problem easier, more tractable. You will discover that the goal is to make as many simplifying assumptions as possible without seriously degrading the result. Generally there is no clear method to determine how completely, or how precisely, to treat a problem: If our analysis is too simple, we may not get a pertinent answer; if our analysis is too detailed, we may not be able to obtain any answer. It is usually preferable to start with a relatively simple analysis and then add more detail as required to obtain a practical solution. During the past two decades, there has been a tremendous growth in the availability of computerized methods for solving problems that previously were beyond solution because the time required to solve them would have been prohibitive. At the same time the cost of computer capability and use has decreased by orders of magnitude. In addition, we are beginning to experience an influx of “personal computers” on campus, in the home, and in business. Accordingly, we will begin to introduce computer methods in this text. Lesson 2 Fundamentals of Mechanical Design Mechanical design means the design of things and systems of a mechanical nature –machines, produces, structures, devices, and instruments. For the most part mechanical design utilizes mathematics, the materials sciences, and the engineering – mechanics science. The total design process is of interest to us. How does it benign? Does the engineer simply sit down at this desk with a blank sheet of paper? And, as he jots down some ideas, what happens next? What factors influence or control the decisions which have to be made? Finally, then, how does this design process end? Sometimes, but not always, design begins when an engineer recognizes a need and decides to do something about it. Recognition of the need and phrasing it in so many words often constitute a highly creative act because the need may be only a vague discontent, a feeling of uneasiness, or a sensing that something is not right. The need is usually not evident at all. For example, the need to do something about a food-packing machine may be indicated by the noise level, by the variation in package weight, and by slight but perceptible variations in the quality of the packing or wrap. There is a distinct difference between the statement of the need and the identification of the problem which follows this statement. The problem is more specific. If the need is for cleaner air, the problem might be that of reducing the dust discharge from power-plant stacks, or reducing the quantity of irritants from automotive exhausts. Definition of the problem must include all the specifications for the thing that is to be designed. The specifications are the input and output quantities, the characteristics and dimensions of the space the thing must occupy and all the limitations on these quantities. We can regard the thing to be designed as something in a black box. In this case we must specify the inputs and outputs of the box together with their characteristics and limitations. The specifications define the cost, the number to be manufactured, the expected life, the range, the operating temperature, and the reliability. There are many implied specifications which result either from the designer’s particular environment or from the nature of the problem itself. The manufacturing processes which are available, together with the facilities of a certain plant, constitute restrictions on a designer’s freedom, and hence are a part of the implied specifications. A small plant, for instance, may not own cold-working machinery. Knowing this, the designer selects other metal-processing methods which can be performed in the plant. The labor skills available and the competitive situation also constitute implied specifications. After the problem has been defined and a set of written and implied specifications has been obtained, the next step in design is the synthesis of an optimum solution. Now synthesis can not take place without both analysis and optimization because the system under design must be analyzed to determine whether the performance complies with the specifications. The design is an iterative process in which we proceed through several steps, evaluate the results, and then return to an earlier phase of the procedure. Thus we may synthesize several components of a system, analyze and optimization require that we construct or devise abstract models. In creating them it is our hope that we can find one that will simulate the real physical system very well. Evaluation is a significant phase of the total design process. Evaluation is the final proof of a successful design, which usually involves the testing of a prototype in the laboratory. Here we wish to discover if the design really satisfies the need or needs. Is it reliable? Will it compete successfully with similar produces? Is it economical to manufacture and to use? Is it easily maintained and adjusted? Can a profit be made from its sale or use? Communicating the design to others is the final, vital step in the design process. Undoubtedly many great designs, inventions, and creative works have been lost to mankind simply because the originators were unable or unwilling to explain their accomplishments to others. Presentation is a selling job. The engineer, when presenting a new solution to administrative, management, or supervisory persons, is attempting to sell or to prove to them that this solution is a better one. Unless this can be done successfully, the time and effort spent on obtaining the solution have been largely wasted. Basically, there are only three means of communication available to us. There are the written, the oral, and the graphical forms. Therefore the successful engineer will be technically competent and versatile in all three forms of communication. A technically competent person who lacks ability in any one of these forms is severely handicapped. If ability in all three forms is lacking, no one will ever know how competent that person is! The competent engineer should not be afraid of the possibility of not succeeding in a presentation. In fact, occasional failure should be expected because failure or criticism seems to accompany every really creative idea. There is a great deal to be learned from a failure, and the greatest gains are obtained by those willing to risk defeat. In the final analysis, the real failure would lie in deciding not to make the presentation at all. 批注 [zx7]: 综合 批注 [zx8]: 至关重要的一步 批注 [zx9]: 多功能