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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)
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