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8/12/2019 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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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)