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Chapter 1: Introduction

Chapter 1: Introduction. WHAT IS A MACHINE. MACHINE : A device for transforming or transfering energy An apparatus consisting of interrelated units (machine elements) A device that modifies force and motion.

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Chapter 1: Introduction

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  1. Chapter 1: Introduction

  2. WHAT IS A MACHINE • MACHINE : A device for transforming or transfering energy An apparatus consisting of interrelated units (machine elements) A device that modifies force and motion

  3. A machine receives energy in some available form and uses it to do some particular kind of work • A petrol engine is a machine, which may use the heat energy derived from the combustion of the fuel to propel a vehicle along the road

  4. A lathe is a machine which receives mechanical energy from the line shaft through the belt or gears and uses that energy to remove metal from a bar or other piece of work • LINK OR ELEMENT : Each part of a machine which has motion relative to some other part • STRUCTURES : Made up of series of members of regular shape that have a particular function for load carrying

  5. SYNTHESIS : Concerned with the problem of selecting the size of the mechanism to perform a given function • STRESS : Internal reacting force per unit area due to the effects of external applied forces

  6. DESIGN • Formulate a plan for the satisfaction of a human need • The need for the problem has to be identified • Design problem have no unique answer

  7. A good answer today may well turn out to be a poor answer tomorrow, if there is a growth of knowledge during the period • A design is always subject to certain problem-solving constraints • A design problem is not a hypothetical problem

  8. Design has an authentic purpose the creation of an end result by taking definite action, or the creation of something having physical reality

  9. ENGINEERING DESIGN • The process in which scientific principles and the tools of engineering mathematics, computers, graphics and English are used to produce a plan which, when carried out, will satisfy a human need

  10. MECHANICAL ENGINEERING DESIGN • Design of things and systems of mechanical nature, machines, products, structures, devices, and instruments • For the most part, mechanical design utilizes mathematics, the materials sciences, and the engineering mechanics sciences

  11. The ultimate goal in machine design is to size and shape the parts choose appropriate material and choose manufacturing process So that resulting machine can be expected to perform its intended function without failure

  12. An engineer should be able to calculate and predict the mode and conditions of failure for each element and then design it to prevent that failure • This requires stress and deflection analysis for each part

  13. Stresses are functions of applied and inertial loads • An analysis of the forces, moments, torques and dynamics of system must be done before stresses and deflections can be completely calculated

  14. Design • A design must be: • Functional- fill a need or customer expectation • Safe- not hazardous to users or bystanders • Reliable- conditional probability that product will perform its intended function without failure to a certain age. • Competitive- contender in the market • Usable- accommodates human size and strength • Manufacturable- minimal number of parts and suitable for production • Marketable- product can be sold and serviced

  15. Design Process Actions • Conceive alternative solutions • Analyze, test, simulate, or predict performance of alternatives • Choose the “best” solution • Implement design

  16. Design is… • An innovative and iterative process • A communication intensive activity • Subject to constraints

  17. Steps to Design

  18. Strength Stiffness Wear Corrosion Safety Reliability Friction Usability Utility Cost Processing Weight Life Noise Styling Shape Size Control Thermal Properties Surface Lubrication Marketability Maintenance Volume Liability Recovery Design Considerations

  19. Codes and Standards • Code- a set of specifications for the analysis, design, manufacture, and construction of something • Standard- a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency, and a specified quality

  20. Aluminum Association (AA) American Gear Manufacturers Association (AGMA) American Institute of Steel Construction (AISC) American Iron and Steel Institute (AISI) American National Standards Institute (ANSI) American Society for Metals (ASM) American Society of Mechanical Engineers (ASME) American Society of Testing Materials (ASTM) American Welding Society (AWS) American Bearing Manufacturers Association (ABMA) British Standards Institute (BSI) Industrial Fasteners Institute (IFI) Institution of Mechanical Engineers (I. Mech. E.) International Bureau of Weights and Measures (BIPM) International Standards Organization (ISO) National Institute for Standards and Technology (NIST) Society of Automotive Engineers (SAE) American Society of Agricultural and Biological Engineers (ASABE) Organizations

  21. Economics • Cost plays an important role in design decision process • No matter how great the idea may be, if it’s not profitable it may never be seen • The use of standard sizes and large manufacturing tolerances reduce costs • Evaluating design alternatives with regard to cost • Breakeven Points • Cost Estimates

  22. Product Liability • “Strict liability” concept prevails in the U.S. • Manufacturers are liable for any damage or harm that results from a defect.

  23. Uncertainty • Roman Method- repeat designs that are proven • Factor of Safety Method of Philon- separate the loss-of-function load and the impressed load using a ratio • Permissible Stress- fraction of significant material property (i.e., strength)

  24. Uncertainty • Design Factor Method- factor of safety is increased with rounding error to achieve nominal size (5.3 mm designed bolt size is increased to 6.0 mm) • Stochastic Design Factor Method- uncertainty in stress and strength is quantified for linearly proportional loads

  25. Measures of Strength • S – Strength • Ss – Shear Strength • Sy – Yield Strength • Su– Ultimate Strength • - Mean Strength

  26. Measures of Stress • t – Shear Stress • s –Normal Stress • s1 –Principal Stress • sy– Stress in y-direction • sr– Radial Stress • st– Tangential Stress

  27. Stress Allowable(AISC) • Tension: 0.45 Sy ≤ sall ≤ 0.60 Sy • Shear: tall = 0.40 Sy • Bending: 0.60 Sy ≤ sall ≤ 0.75 Sy • Bearing: sall = 0.90 Sy

  28. Loads Used to Obtain Stresses • Where: Wd- dead loads Wl- live loads k- service factor Fw- wind load Fmisc- locality effects (earthquakes)

  29. Applications Elevators Traveling Crane Supports Light Machinery Supports Reciprocating Machinery Supports Floor and Balcony Supports k 2 1.25 1.20 1.50 1.33 Service Factors

  30. Factor of Safety • Design factors (nd) are defined as: and where ns-accounts for uncertainty of strength nd-accounts for uncertainty of loads

  31. Realized Factor of Safety

  32. Reliability • Probability that a mechanical element will not fail in use 0 ≤ R ≤ 1 • Reliability approach to design: judicious selection of material, processes, and geometry to achieve reliability goal • Factor of Safety Method- time proven, widely accepted • Reliability Approach- new, requires data

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