LECTURE# 01: Introduction

Danish HameedDepartment of Mechanical EngineeringMirpur University of Science & Technology, Mirpur A.K.Pakistan

04/03/2015

Danish Hameed

BREAKUP OF LECTURE # 01

MachineDesignEngineering DesignMechanical Engineering DesignStep to DesignDesign ConsiderationCodes & StandardsOrganizationsEconomicsProduct LiabilityUncertaintyMeasure of StrengthMeasure of StressStress Allowance

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Load Use to Obtained StressesService FactorsFOSRealized Factor of SafetyReliability

Danish Hameed

WHAT IS A MACHINE

MACHINE : A device for transforming or transfering energy

An apparatus consisting of inter-related units(machine elements)

A device that modifies force and motion

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A machine receives energy in some available form and uses it to do some particular kind of work e.g. 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

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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

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STRESS : Internal reacting force per unit area due to the effects of external applied forces

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DESIGN

Formulate a plan for the satisfaction of a human need

Design problem have no unique answer

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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

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Design has an authentic purpose

the creation of an end result by takingdefinite action, or

the creation of something having physicalreality

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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

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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

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The ultimate goal in machine design is to

a)b)c)

size and shape the partschoose appropriate material andchoose manufacturing process

So that resulting machine can be expected toperform its intended/signified functionwithout failure

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An engineer/ designer 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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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

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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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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

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Steps to Design

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Design Considerations

1.2.3.4.5.6.7.8.9.10.11.12.13.

StrengthStiffnessWearCorrosionSafetyReliabilityFrictionUsabilityUtilityCostProcessingWeightLife

14. Noise15. Styling16. Shape17. Size18. Control19. Thermal Properties20. Surface21. Lubrication22. Marketability23. Maintenance24. Volume25. Liability26. 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

Loss of Functionnd = Impressed Load

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

ndAverage Strength Average Stress

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Measures of Strength

S StrengthSs Shear StrengthSy Yield StrengthSu Ultimate Strength S - Mean Strength

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Measures of Stress

t Shear Stresss Normal Stresss1 Principal Stresssy Stress in y-directionsr Radial Stressst Tangential Stress

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Stress Allowable (AISC)

Tension: 0.45 Sy ? sall ? 0.60 SyShear: tall = 0.40 SyBending: 0.60 Sy ? sall ? 0.75 SyBearing: sall = 0.90 Sy

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Engr.Uqab Munir Butt

Loads Used to Obtain Stresses

F = Wd + Wl + kFl + Fw + Fmisc

Where:

Wd- dead loadsWl- live loadsk- service factorFw- wind loadFmisc- locality effects (earthquakes)

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Service Factors

ApplicationsElevatorsTraveling Crane SupportsLight Machinery SupportsReciprocating Machinery SupportsFloor and Balcony Supports

k 21.251.201.50

1.33

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Factor of Safety

Design factors (nd) are defined as:

and

strengthnd = stress

nd = n sn z

where ns-accounts for uncertainty of strength nd-accounts for uncertainty of loads

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Realized Factor of Safety

nr =Ss

nr =Ss t

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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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