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Chapter 1: Introduction

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Title: Chapter 1: Introduction


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
Design Considerations
  1. Strength
  2. Stiffness
  3. Wear
  4. Corrosion
  5. Safety
  6. Reliability
  7. Friction
  8. Usability
  9. Utility
  10. Cost
  11. Processing
  12. Weight
  13. Life
  • Noise
  • Styling
  • Shape
  • Size
  • Control
  • Thermal Properties
  • Surface
  • Lubrication
  • Marketability
  • Maintenance
  • Volume
  • Liability
  • Recovery

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
Organizations
  • 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)

21
Economics
  • Cost plays an important role in design decision
    process
  • No matter how great the idea may be, if its 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
Service Factors
  • 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

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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