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Machine design and drawing ch1 and ch2

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    MACHINE DESIGNANDDRAWINGME F24

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    PART 1BASICS

    1. Intro to Mechanical Engineering Design

    2. Materials

    3. Load and Stress Analysis

    4. Deflection and Stiffness

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    1. Introduction to Mechanical

    Engineering Design

    Machine design, machine elements design,

    machine component design, system design

    and fluid power design, internal combustion

    engine design etc are all focused to mechanical

    engineering design.

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    DESIGN

    Formulate a plan for the satisfaction of a humanneed

    The need for the problem has to be identified

    Design problem have no unique answer

    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

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    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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    Steps to Design

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    Design Considerations

    1. Strength2. Stiffness

    3. Wear

    4. Corrosion

    5. Safety6. Reliability

    7. Friction

    8. Usability

    9. Utility10. Cost

    11. Processing

    12. Weight

    13. Life

    14.Noise15.Styling

    16.Shape

    17.Size

    18.Control19.Thermal Properties

    20.Surface

    21.Lubrication

    22.Marketability23.Maintenance

    24.Volume

    25.Liability

    26.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 SteelConstruction (AISC)

    American Iron and Steel

    Institute (AISI) American National Standards

    Institute (ANSI)

    American Society for Metals(ASM)

    American Society ofMechanical Engineers (ASME)

    American Society of TestingMaterials (ASTM)

    American Welding Society(AWS)

    American BearingManufacturers Association(ABMA)

    British Standards Institute (BSI)

    Industrial Fasteners Institute(IFI)

    Institution of MechanicalEngineers (I. Mech. E.)

    International Bureau of Weightsand Measures (BIPM)

    International StandardsOrganization (ISO)

    National Institute for Standardsand Technology (NIST)

    Society of AutomotiveEngineers (SAE)

    American Society ofAgricultural and Biological

    Engineers (ASABE)

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    Economics

    Cost plays an important role in design decisionprocess

    No matter how great the idea may be, if its notprofitable it may never be seen

    The use of standard sizes and large manufacturingtolerances reduce costs

    Evaluating design alternatives with regard to cost Breakeven Points

    Cost Estimates

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    CMPFig- A breakeven point

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    CMPFig-Costvstolerance/mch. process

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    SafetyandProductLiability

    Manufacturerofanarticleisliableforanydamageorharmthat

    resultsbecauseofadefect.

    Analysis and design, quality control and testing procedures areimportant.

    Warnings andinstructionsforuse.

    StressandStrength:Strengthisapropertyofamaterialorofamechanicalelement.

    Variousmetalworkingandheattreatingprocessescausevariations

    instrength.

    Stress isastateofpropertyataspecificpoint withinabodywhichisafunctionofload, geometry,temperatureandmanufacturing

    process.

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    *Uncertainty:

    Examples of uncertainties concerning stress and

    strength include:

    *Composition of material and the effect of 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 fits on stress conditions.

    *Effect of thermomechanical treatment on properties.

    *Intensity and distribution of loading.

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    *Uncertainty:

    *Validity of mathematical models used to represent reality.*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.

    Engineers must accommodate uncertainty.

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    DesignFactor (nd)andFactorofSafety (n):

    parameterallowablemaximum

    parameterfunction-of-loss

    dn

    After the design is completed, the actual design factor

    may change as a result of changes such as rounding up

    to a standard size for a cross section or using off-the-shelf components with higher ratings instead of using

    what is calculated by using the design factor.

    The factor is then referred to as the factor o f safety, n.

    dn

    loadfunction-of-lossloadallowablemax.

    (Eq.1.1)

    (Eq.1.2)

    EXAMPLE 1 1

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    EXAMPLE 1-1:

    Consider the maximum load on the structure is known

    with an uncertainity of 20 %, the load causing failure is

    known within 15 %. If the load causing failure isnominal ly9 kN, determine the design factor and the

    maximum allowable load that will offset the absolute

    uncertainties.

    nd=1 / 0. 851/1.2

    =1.4

    From Eq 1-2 the maximum allowable load is found to be

    Maximum allowable load= 9/1.4 = 6.4 kN

    Solution: To account for its uncertainty the loss offunction load must increase to 1/0.85, whereas the

    maximum allowable load must decrease to1/1.2. thus

    to offset the absolute uncertainties the design factor

    should be:

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    EXAMPLE 1-2:A solid circular rod of diameter d undergoes a bending

    moment M = 100 N-m inducing a stress = 16M/(d3).

    Using a material strength of 170 MPa and a design factor of

    2.5, determine the minimum diameter of the rod. Usingtable A-17, select a preferred fractional diameter and

    determine the resulting factor of safety.

    )(or

    S

    stressallowable

    strengthfunction-of-lossnd

    (Eq.1.3)

    Solution:

    From Eq. (1.3), = S/nd

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    55.3)100(16

    )022.0()10)(170(16

    n,safetyoffactorwithnReplacing

    mm.22issizepreferredhighernextthe17,-AtableFrom

    02111.0

    )5.2()10(170

    5.2)100(1616

    16

    363

    d

    3/1

    6

    3/1

    3

    M

    Sdn

    S

    Mnd

    n

    S

    d

    M

    d

    d

    mm21.11m

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    Reliability:

    Is the statistical measure of probability that amechanical element will not fail in use.

    The failure of 6 parts out of every 1000 manufactured

    parts might be considered as an acceptable failure

    rate for a certain class of products.

    This represents a reliability of

    R=1-(6/1000) = 0.994 or 99.4%

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    *Dimension and Tolerances.

    The following terms are used generally in dimensioning:

    *Nom inal size.

    The size we use in speaking of an element.

    For example, we may specify a 40 mm pipe or a in. bolt.

    Either the theoretical size or the actual measured size may

    be quite different. The theoretical size of a 40 mm pipe is

    47.5 mm for the out-side diameter. And the diameter of the

    in bolt, say, may actually measure 0.492 in.

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    *Limi ts.

    The stated maximum and minimum dimensions.

    *Tolerance.

    The difference between the two limits.

    *Bilateral to lerance.

    25 0.05 mm

    *Unilateral tolerance.

    mm

    25

    05.0

    000.0

    mm2505.0

    05.0

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    *MaterialStrengthandStiffness

    Fig-Stress strain diagram obtained through a standard tensile test;

    (a) ductile materials, (b) brittle materials

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    ASTM A 582/A 582M-95b (2000), Grade 303Se -Free-

    Machining Stainless Steel Bars:

    A describes a ferrous metal, but does not sub classify it as

    cast iron, carbon steel, alloy steel, tool steel, or stainless steel;582is a sequential number without any relationship to the

    metals properties;

    M indicates that the standard A582M is written in rationalized

    SI units (the M comes from the word Metric), hence together582/A582M includes both inch-pound and SI units;

    95 indicates the year of adoption or last revision and a letter b

    following the year indicates the third revision of the standard

    in1995;

    (2000), a number in parentheses, indicates the year of last re-

    approval;

    Grade 300Seindicates the grade of the steel, and in this case,

    it has a Se (selenium) addition.American Society of Testing Materials (ASTM)

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    AISI/SAE No. 1020

    The first digit indicates that this is plain carbon

    steel.

    The second digit indicates there are no alloying

    elements.

    The last two digits indicates that the steel contains

    approximately 0.20 percent carbon.

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    Measures of Strength

    SStrength

    SsShear Strength

    SyYield Strength

    SuUltimate Strength

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    Measures of Stress

    Shear StressNormal StressPrincipal StressyStress in y-directionrRadial StresstTangential Stress

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    Stress Allowable

    (AISC)

    Tension: 0.45 Sy all 0.60 Sy

    Shear: all= 0.40 Sy Bending: 0.60 Sy all 0.75 Sy Bearing: all= 0.90 Sy

    American Institute of Steel Construction (AISC)


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