NUST School of Mechanical and Manufacturing Engineering ME-311 Machine Design Lecture 1 Instructor: Dr. Jawad Aslam • Introduction: – – – – – – – – – – Meaning of Design Mechanical Engineering Design Design Phases Design Considerations Design Tools and Resources The Design Engineer’s Professional Responsibilities Codes & Standards Economics Stress and Strength Uncertainty Page 2 • Meaning of Design To design is to formulate a plan or strategy for satisfaction of specified need or solve a problem Design is a highly innovative and iterative process Design is also a decision making process Sometimes, when information is limited, decisions are made tentatively, with the provision to adjust when more information is available A designer will have to effectively communicate and work with people from different fields and specialisations Page 3 • Meaning of Design Engineering tools (such as mathematics, statistics, computers, graphics, languages) are combined to produce a plan that , when carried out, produces a product that is functional, safe, reliable, competitive, usable, manufacturable, and marketable, regardless of who builds or uses it Page 4 • Mechanical Engineering Design Mechanical engineers are associated with the production and processing of energy and with providing means of production, the tools of transportation, and the techniques of automation The skills and knowledge base are extensive Among the disciplinary bases are mechanics of solids and fluids, mass and momentum transport, manufacturing processes, and electrical and information theory/technology Mechanical engineering design involves all the disciplines of mechanical engineering Example: A simple journal bearing involves fluid flow, heat transfer, friction, energy transport, material selection, thermo-mechanical elements, statistical descriptions and so on Page 5 • Design Phases The complete design process, from start to finish, is often as outlined in the figure on the right Identification of need generally starts the design process The definition of problem is more specific and must include all the specifications for the object that is to be designed Identification of need Definition of problem Synthesis Analysis and optimisation Evaluation Iteration Presentation Page 6 • Design Phases The synthesis of a scheme connecting possible system elements is sometimes called the invention of concept or concept design Synthesis and analysis and optimisation are intimately and iteratively related. Emphasize that design is an iterative process in which we proceed though several steps, evaluate the results, and then return to an earlier phase of the procedure Evaluation is a significant phase of the total design process. Evaluation is the final proof of a successful design and usually involves testing the prototype in a lab Communicating the design to others is the final, vital presentation step in the design process Page 7 • Design Considerations 1. Functionality 2. Strength / Stress 3. Distortion / Deflection / Stiffness 4. Wear 5. Corrosion 6. Safety 7. Reliability 8. Manufacturability 9. Utility 10. Cost 11. Friction 12. Weight 13. Life 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. Noise Styling Shape size Control Thermal properties Surface Lubrification Marketability Maintenance Volume Liability Remanufacturing/ Resource recovery Page 8 • Design Tools and Resources Computational Tools CAD CAE CFD FEA Acquiring Technical Information Libraries Government Sources Professional Societies Commercial Vendors Internet Page 9 • The Design Engineer’s Professional Responsibilities In general, the design engineer is required to satisfy the needs of customers (management, clients, consumers, etc.) It is expected to do so in a competent, responsible, ethical and professional manner To start on the road to success, you need to cultivate your professional work ethic and process skills before graduation, so that when you begin your formal engineering career, you will be prepared to meet the challenges Development of communication skills Page 10 • Codes and Standards A code is a set of specifications for the analysis, design, manufacture and construction of something A standard is a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency and a specified quality Aluminium Association (AA) American Gear Manufacturers Association (AGMA) American Institute of Steel Construction (AISC) American Iron and Steel Institute (AISI) American National Standards Institute (ANSI) ASM International (formerly American Society for Metals) American Society of Mechanical Engineers (ASME) American Society for Testing and Materials (ASTM) American Welding Society (AWS) American Bearing Manufacturers Association (ABMA) British Standards Institute (BSI) Industrial Fasteners Institute (IFI) Institute of Mechanical Engineers (I.Mech.E) International Bureau of Weights and Measures (BIPM) International Standards Organisation (ISO) National Institute for Standards and Technology (NIST) Society of Automotive Engineers (SAE) Page 11 • Economics The consideration of cost plays an immensely important role in the design decision process We could easily spend as much time in studying the cost factor as in the study of the entire subject of design The use of standard or stock sizes is the first principle of cost reduction Among the effects of design specifications on costs, tolerances are perhaps the most significant. Tolerances, manufacturing processes and surface finish are interrelated and influence the producibility of the end product in many ways. Page 12 • Economics Close tolerances may necessitate additional steps in processing and inspection or even render a part completely impractical to produce economically At times, it happens that when two or more design approaches are compared for cost, the choice between the two depends on a set of conditions such as the quantity of production, the speed of the assembly lines, or some other condition There then occurs a point corresponding to equal cost, called the breakeven point Page 13 • Economics There are many ways of obtaining relative cost figures so that two or more designs can be roughly compared Comparing the monetary cost per unit weight Compare the number of parts Other cost estimators such as area, volume, horsepower, torque, capacity, speed and various other performance ratios Page 14 • Stress and Strength The survival of many products depends on how the designer adjusts the maximum stress 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 percentoffset yielding, or fracture Strength is a property of a material or of a mechanical element Page 15 • Stress and Strength The strength of an element depends on the choice, the treatment and the processing of the material Remember that strength is an inherent property of a part, a property built into the part because of the use of a particular material and process Various metalworking and heat-treating processes, such as forging, cold forming, cause variations in strength from point to point throughout a part Page 16 • Uncertainty Examples of uncertainties concerning stress and strength include Composition of material and the effect of its variation on properties Variations in properties from place to place within a bar of stock Effect of processing locally, or nearby, on properties Effect of nearby assemblies such as weldments and shrink fit on stress conditions Effect of thermomechanical treatment on properties Intensity and distribution of loading Validity of mathematical models used to present reality Page 17 • Uncertainty Intensity of stress concentrations Influence of time on strength and geometry Effect of corrosion Effect of wear Uncertainty as to the length of any list of uncertainties Page 18 Thank you Page 19
