Chapter 1.pptx design of machine element ppt
- 1. By Biruk Girma Machine element I
- 2. Introduction;- Machine element deals with Mechanical design of machine parts and components. Mechanical design is a complex undertaking, requiring many skills. Extensive relationships need to be subdivided into a series of simple tasks. The complexity of the subject requires a sequence in which ideas are introduced and iterated. Design ;- Design is an iterative process with many interactive phases. To design is either to formulate a plan for the satisfaction of a specified need or to solve a problem. If the plan results in the creation of something having a physical reality, then the product must be functional, safe, reliable, competitive, usable, manufacturable, and marketable. Design is an innovative and highly iterative process. It is also a decision-making process. Design is a communication-intensive activity in which both words and pictures are used, and written and oral forms are employed. Chapter one
- 3. By Biruk.G 3 *Mechanical Engineering Design Mechanical engineers are associated with the production and processing of energy and with providing the means of production, the tools of transportation, and the techniques of automation. Phases and Interactions of the Design Process What is the design process? How does it begin? Does the engineer simply sit down at a desk with a blank sheet of paper and job down some ideas? What happens next? What factors influence or control the decisions that have to be made? Finally, how does the design process end?
- 4. By Biruk.G 4 The complete design process, from start to finish, is often outlined as in Fig beside Identification of need generally starts the design process. Recognition of the need and phrasing the need often constitute a highly creative act. There is a distinct difference between the statement of the need and the definition of the problem. The definition of problem is more specific and must include all the specifications for the object that is to be designed. The specifications are the input and output quantities, the characteristics and dimensions of the space the object must occupy, and all the limitations on these quantities Design process
- 5. By Biruk.G 5 *Cont… The synthesis of a scheme connecting possible system elements is sometimes called the invention of the concept or concept design. Both analysis and optimization require that we construct or devise abstract models of the system that will admit some form of mathematical analysis. We call these models mathematical models Evaluation is a significant phase of the total design process. Evaluation is the final proof of a successful design and usually involves the testing of a prototype in the laboratory. Communicating the design to others is the final, vital presentation step in the design process
- 6. By Biruk.G 6 *Design Considerations When we use the expression design consideration, we are referring to some characteristic that influences the design of the element or, perhaps, the entire system. Usually quite a number of such characteristics must be considered and prioritized in a given design situation * Acquiring Technical Information Some sources of information are:
- 7. By Biruk.G 7 *Standards and Codes A standard is a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency, and a specified quality. One of the important purposes of a standard is to place a limit on the number of items in the specifications so as to provide a reasonable inventory of tooling, sizes, shapes, and varieties. A code is a set of specifications for the analysis, design, manufacture, and construction of something. The purpose of a code is to achieve a specified degree of safety, efficiency, and performance or quality. Some of the standards and codes, as well as addresses, can be obtained in most technical libraries. The organizations of interest to mechanical engineers are:
- 8. By Biruk.G 8 *Stress and Strength The survival of many products depends on how the designer adjusts the maximum stresses in a component to be less than the component’s strength at specific locations of interest. The designer must allow the maximum stress to be less than the strength by a sufficient margin so that despite the uncertainties, failure is rare. *Strengths are the magnitudes of stresses at which something of interest occurs, such as the proportional limit, 0.2 percent-offset yielding, or fracture. In many cases, such events represent the stress level at which loss of function occurs.
- 9. 9 Various metalworking and heat-treating processes, such as forging, rolling, and cold forming, cause variations in the strength from point to point throughout a part * Stress is a state property at a specific point within a body, which is a function of load, geometry, temperature, and manufacturing processing * From stress strain curve we can see that the pointes 1. proportional limit where hooks law permits( O-A) 2. The points C and D are called the upper and lower yield points respectively 3. At E, the stress, which attains its maximum value is known as ultimate stress 4. The stress corresponding to point F is known as breaking stress By Biruk.G
- 10. By Biruk.G 10 *Factor of Safety When designing machine parts, it is desirable to keep the stress lower than the maximum or ultimate stress at which failure of the material takes place. This stress is known as the working stress or design stress. It is also known as safe or allowable stress. Factor of Safety It is defined, in general, as the ratio of the maximum stress to the working stress. Mathematically, In case of ductile materials e.g. mild steel, where the yield point is clearly defined, the factor of safety is based upon the yield point stress. In such cases, In case of brittle materials e.g. cast iron, the yield point is not well defined as for ductile materials. Therefore, the factor of safety for brittle materials is based on ultimate stress.
- 11. By Biruk.G 11 Before selecting a proper factor of safety, a design engineer should consider the following points : *Reliability The reliability R can be expressed by a number having the range 0 ≤ R ≤ 1. A reliability of R = 0.90 means that there is a 90 percent chance that the part will perform its proper function without failure. The failure of 6 parts out of every 1000 manufactured might be considered an acceptable failure rate for a certain class of products. This represents a reliability of
- 12. By Biruk.G 12 *Dimensions and Tolerances
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- 14. By Biruk.G 14 *Engineering Materials The selection of a material for a machine part or a structural member is one of the most important decisions the designer is to make. The decision is usually made before the dimensions of the part are established The knowledge of materials and their properties is of great significance for a design engineer. The machine elements should be made of such a material which has properties suitable for the conditions of operation.
- 15. By Biruk.G 15 *Classification of Engineering Materials The engineering materials are mainly classified as : I. Metals and their alloys, such as iron, steel, copper, aluminum, etc. II. Non-metals, such as glass, rubber, plastic, etc. The metals may be further classified as : (a) Ferrous metals, and (b) Non-ferrous metals. oThe *ferrous metals are those which have the iron as their main constituent, such as cast iron, wrought iron and steel. oThe non-ferrous metals are those which have a metal other than iron as their main constituent, such as copper, aluminum, brass, tin, zinc, etc.
- 16. By Biruk.G 16 * Selection of Materials for Engineering Purposes The following factors should be considered while selecting the material : 1. Availability of the materials. 2. Suitability of the materials for the working conditions in service. 3. The cost of the materials. 4. The important properties, which determine the utility of the material are physical, chemical and mechanical properties Physical Properties of Metals The physical properties of the metals include luster, color, size and shape, density, electric and thermal conductivity, and melting point. Mechanical Properties of Metals The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load.
- 17. By Biruk.G 17 These mechanical properties of the metal include strength
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- 19. By Biruk.G 19 Called resistance of a material to penetration by a pointed
- 20. By Biruk.G 20 Thank you for your attention