Chapter 1: Introduction BAE 417-Design of Machine Systems

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Chapter 1 Introduction

• BAE 417-Design of Machine Systems
• Fall 2005

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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

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• To design is either to formulate a plan for the
  satisfaction of a specified need or to solve a
  problem.

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Design Process Actions

• Conceive alternative solutions
• Analyze, test, simulate, or predict performance
  of alternatives
• Choose the best solution
• Implement design

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Design is

• An innovative and iterative process
• A communication intensive activity
• Subject to constraints

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Steps to Design
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Design Considerations

• 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

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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

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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)

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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

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Product Liability

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

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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)

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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

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Measures of Strength

• S Strength
• Ss Shear Strength
• Sy Yield Strength
• Su Ultimate Strength
• - Mean Strength

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Measures of Stress

• t Shear Stress
• s Normal Stress
• s1 Principal Stress
• sy Stress in y-direction
• sr Radial Stress
• st Tangential Stress

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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

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Loads Used to Obtain Stresses

• Where
• Wd- dead loads
• Wl- live loads
• k- service factor
• Fw- wind load
• Fmisc- locality effects (earthquakes)

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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

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Factor of Safety

• Design factors (nd) are defined as
• and
• where
• ns-accounts for uncertainty of strength
• nd-accounts for uncertainty of loads

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Realized Factor of Safety
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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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Homework Assignment

• Do problems1-6, 1-9, and 1-14 for next Tuesday.