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CHAPTER 1

Introduction:1.1 Design1.2 Mechanical Engineering Design1.3 The Phases of Design (Design Process Elements)1.4 Design Considerations1.5 Design Tools and Resources1.6 The Design Engineer’s Professional Responsibilities1.7 Codes and Standards1.8 Economics1.9 Safety and Product Liability1.10 Stress and Strength1.11 Design Factor and Factor of Safety1.12 Reliability

1.1 Design To design is either to formulate a plan for the satisfaction of a human need or to solve a

problem.

If the plan results in the creation of something having a physical reality, the product must

be:

i. Functional: product must perform to fill its intended need and customer

expectation.

ii. Safe: product is not hazardous to the user, by standers, or surrounding property.

iii. Reliable: product will perform its intended function satisfactorily or without failure

at a given age.

iv. Competitive: product is a contender in its market.

v. Usable: Easy to use. Accommodating to human size, strength, posture, reach, force,

power, and control.

vi. Manufacturability: product has been reduced to a minimum number of parts,

suited to mass production, with dimensions, distortion, and strength under control.

vii. Marketable: product can be bought, and service.

It is important that the designer begin by identifying exactly how he will recognize a

satisfactory alternative, and how to distinguish between two satisfactory alternatives in

order to identify the better.

From this kernel (most important part), optimization strategies can be formed or selected.

Then the following tasks unfold (open out):

1. Invent alternative solutions.

2. Establish key performance metrics.

3. Through analysis and test, simulate and predict the performance of each alternative,

retain satisfactory alternatives, and discard unsatisfactory ones.

4. Choose the best satisfactory alternative discovered as an approximation to

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optimality.

5. Implement (put into operation) the design.

1.2 Mechanical Engineering Design: Mechanical Design: Transformation of concepts and ideas into useful machinery. Or the

design of components and systems of a mechanical nature-machines, structure, devices, and

instruments.

Machine: Combination of mechanisms and other components that transforms, transmit or

uses energy, load or motion for a specific purpose.

It utilizes: mathematics, materials sciences, and engineering mechanics sciences.

It involves: all the disciplines (rules) of mechanical engineering.

Its ultimate goal: is to size and shape the element and choose appropriate materials and

manufacturing processes so that the resulting system can be expected to perform its

intended function without failure.

Example: design of journal bearing involves: fluid flow, heat transfer, friction, energy

transport, material selection, and thermo-mechanical treatments.

1.3 The Phases of Design (Interaction between Design Process Elements)The process of design is basically an

exercise in creativity.

It can be outlined by design flow

diagrams with feedback loops as shown

in next figure.

Note that the process is neither

exhaustive nor rigid and will probably

be modified to suit individual problems.

Identification of Need: Design process begins with

recognition of the need, real or

imagined, and a decision to do

something about it.

Need is often not evident at all

Sensitive person, one who is easily disturbed by things, is more likely to recognize a

need and more likely to do something about it.

Example: The need for cleaner air or present equipment requires improving

durability.

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Definition of the Problem:It include all the specifications for the object that is to be design.

Specifications which include all forms of input and output quantities must be spelled

out.

Once the specifications have been prepared, relevant design information is collected

to make a feasibility study.

As a result of this study, changes are made in the Specifications and Requirements.

When some idea as to the amount of space needed or available for a project has been

determined, to-scale layout drawings may be started.

Synthesis: Putting together of the solution represents may be the most challenging and

interesting part of design.

Ideation and invention phase (where the largest possible number of creative solutions

is originated).

The designer combines separate parts to form a complex whole of various new and

old ideas and concepts to produce an overall new idea or concept.

Analysis and optimization: It has as its objective satisfactory performance, as well as durability with minimum

weight and competitive cost.

Synthesis cannot take place without both analysis or resolution and optimization,

because the product under design must be analyzed to determine whether the

performance compiles with the specifications.

If the design fails, the synthesis procedure must begin again.

Designer must: specify the dimensions, select the components and materials, and

consider the manufacturing, cost, reliability, serviceability, and safety.

Testing (Evaluation): It is the final proof of a successful design and usually involves the testing of a

prototype in the laboratory.

Is it really satisfying the need?

Is it reliable?

Will it compete successfully with similar product?

Can a profit be made from this product?

Is it easy to maintained and adjusted?

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Presentation: It is a selling job.

The engineer, when presenting a new solution to administrative, management, or

supervisory persons, is attempting to sell or to prove to them that this solution is better one.

Unless this can be done successfully, the time and effort spent on obtaining the solution have

been largely wasted.

When designers sell a new idea, they also sell themselves.

1.4 Design Considerations: It means the characteristic that influences the design of the element or the entire system.

Many of the important ones (not necessarily in order of importance) are as follows:

Some of these characteristics have to do directly with the dimensions, the material, the

processing, and the joining of the elements of the system. Several characteristics may be

interrelated, which affects the configuration of the total system.

1.5 Design Tools and Resources: Computational Tools:

o Computer-aided design (CAD): software allows the development of 3-D design from

which conventional 2-D orthographic views with automatic dimensioning can be

produced. Manufacturing tool paths can be generated from the 3-D models from a 3-

D database. This database can rapidly helps in calculation of mass properties.

Geometric properties are also easy to find. Examples of such software are: AutoCAD,

I-Deas, ProEngineer……etc.

o Computer-aided engineering (CAE): It is applied to all engineering application. With

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this definition, CAD can be considered as a subset of CAE. Some example of

engineering based software for mechanical engineering application ( software that

might be integrated within a CAD system) include:

1) Finite Element Analysis: Algor, Ansys, Abaqus.

2) Program for Simulation: Adams, Working Model.

o Computer-aided applications: word processing (e.g. Excel, Lotus) and mathematical

solvers (e.g. Maple, Matlab, MathCad).

Acquiring (obtaining) Technical Information: Libraries: such as Engineering dictionaries, handbooks, journals

Government sources: such as ministries, Institutions

Professional societies: such as American Society of Mechanical Engineering

(ASME).

Commercial vendors: such as Catalogs, samples, cost information

Internet: The computer network gateway to websites associated with most of the

categories listed above.

Etc...

1.6 The Design Engineer’s Professional Responsibilities: When you are working on a design problem, it is important to develop a systematic approach.

The following steps will help you organize your solution processing technique:

Understand the problem.

Identify the known.

Identify the unknown and formulate the solution strategy.

State all assumptions and decisions.

Analyze the problem.

Evaluate your solution.

Present your solution.

1.7 Codes and Standards: A Standard: is a set of specifications for parts, materials, or processes intended to achieve

uniformity, efficiency, and specified quality.

A code: is a set of specifications for the analysis, design, manufacture, and construction of

something.

Purpose of Code: to achieve a specified degree of safety, efficiency, and performance

or quality.

Some organization and societies that have established specifications for standards and safety

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or design code. Such as:

1. American Institute of Steel Construction (AISC).

2. American Society of Mechanical Engineers (ASME).

3. International Standards Organization (ISO).

1.8 Economics:The cost plays an important role in the design decision process that we could easily spend as

much as time in studying the cost factor as in the study of the entire subject of design.

Few general approaches and simple rules that will help reduce the cost in design:

Standard Sizes: Using the standard stock and size is the first principle of cost reduction. In

design something; there are many purchased parts, such as motors, pumps, bearings, and

fasteners. In this case, it is important for designers to make a special effort to specify parts

that are readily available. Parts that are made and sold in large quantities usually are least in

cost.

Large Tolerances: close tolerances need additional steps in manufacturing which means

additional cost. In this case, parts with large tolerances can often be reduced by machines

with higher production rates and at the same time low cost.

Cost Estimates: there are many ways of obtaining relative cost figures; cost so that two or

more designs can be roughly compared. For example, to compare the cost of one design with

another is simply to count the number of parts or the steps to make the design.

Breakeven Points: when two or more design approaches are compared for cost, the choice

between them depends upon a set of conditions such as the quantity of production, the speed

of the assembly lines, or some other condition.

Example: Consider a situation in which a certain part can be manufactured at the rate of 25

parts per hour on an automatic screw machine or 10 parts per hour on a hand screw machine.

Let us suppose, too, that the setup time for the automatic is 3 h and that the labor cost for

either machine is $20 per hour, including overhead. Figure 1–3 is a graph of cost versus

production by the two methods. The breakeven point for this example corresponds to 50

parts. If the desired production is greater than 50 parts, the automatic machine should be

used.

1.9 Safety and Product Liability: A concept states that the manufacturer of an article is liable for any damage or harms that

result because of a defect. And it does not matter whether the manufacturer knew about the

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defect, or even could have known about it.

Example: a product was manufactured say 10 years ago, the manufacturer is still liable for

any damage or harm even if the product could not have been considered defective on the

basis of all technological knowledge that are available at that time.

1.10 Stress and Strength: Strength: is a property of a material or of a mechanical element. The strength of an element

depends upon the choice, the treatment, and the processing of the material.

One of the basic problems in dealing with stress and strength is how to relate the two in order

to develop a safe, economic, and efficient design.

The AISC : is developing the permissible-stress method that defined the allowable stress and

possible loads.

1.11 Design Factor and Factor of Safety:

Engineers employ a safety factor to ensure against foregoing unknown uncertainties involving

strength and loading.

Uncertainty: Uncertainties in machinery design abound. Examples of uncertainties concerning

stress and strength include:

o Composition of material and the effect of variation on properties.

o Variations in properties from place to place within a bar of stock.

o Effect of processing locally, or nearby, on properties.

o Effect of nearby assemblies such as weldments and shrink fits on stress conditions.

o Effect of thermomechanical treatment on properties.

o Intensity and distribution of loading.

o Validity of mathematical models used to represent reality.

o Intensity of stress concentrations.

o Influence of time on strength and geometry.

o Effect of corrosion.

o Effect of wear.

o Uncertainty as to the length of any list of uncertainties.

This factor is used to provide assurance that the load applied to a member dose not exceed the

largest load it can carry

The factor of safety can be defined as:

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ns=Failure load

allowable load

If it is defined in terms of strength design:

ns=material strengthallowable stress

In this case, the material strength represents either static or dynamic properties. If loading is static, the

material strength is either the yield strength or the ultimate strength. For fatigue loading, the material

strength is based on the endurance limit (will take it later). Allowable stress is also called Design

Stress.

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 employing what is calculated by using the design factor. The factor is

then referred to as the factor of safety, ns. The factor of safety has the same definition as the

design factor, but it generally differs numerically.

1.12 Reliability:

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The statistical measure of the probability that a mechanical element will not fail in use.

It is expressed by a number having the range:

0<R<1

For example: R=0.9 , means that there is a 90 percent chance that the part will perform its

proper function without failure.

Suppose we have 6 parts fail out of 1000 parts manufactured, then R=1− 61000

=0.994.

In the reliability method of design, the designer’s task is to make a judicious selection of

materials, processes, and geometry so as to achieve a reliability goal.

1.13 Dimensions and Tolerances:

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The following terms are used generally in dimensioning:

Nominal size :

The size we use in speaking of an element. For example, we may specify a 1.5 in pipe

or a 0.5 in bolt.

Either the theoretical size or the actual measured size may be quite different.

The theoretical size of a 1.5 in is 1.490 in for the outside diameter & the diameter of the 1.5 in

bolt, say, may actually measure 1.492 in.

Limits:

The stated maximum and minimum dimensions

Tolerance:

The difference between the two limits.

Bilateral tolerance:

The variation in both directions from the basic dimension.

That is, the basic size is between the two limits, for example, 1.005 ± 0.002 in.

The two parts of the tolerance need not be equal.

Unilateral tolerance:

The basic dimension is taken as one of the limits, and variation is permitted in only one

direction, for example, 1.005 (+0.004/−0.000) in.

Clearance:

A general term that refers to the mating of cylindrical parts such as a bolt and a hole.

The word clearance is used only when the internal member is smaller than the

external member.

The diametral clearance is the measured difference in the two diameters. The radial

clearance is the difference in the two radii.

Interference:

The opposite of clearance, for mating cylindrical parts in which the internal member is

larger than the external member.

Allowance:

The minimum stated clearance or the maximum stated interference for mating parts.

When several parts are assembled, the gap (or interference) depends on the dimensions and

tolerances of the individual parts.

Materials Must always make “things “out of materials

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Must be able to manufacture this “thing” Topics first introduced in Materials Science course How do we determine the properties of a material?

Tables How were these values determined?

Generally via destructive testing Listed in tables Statistical variation Values listed are minimums Best data from testing of prototypes under intended loading conditions Parameters of interest in material selection for design?

Strength Stiffness Weight Toughness Conductivity Thermal Corrosion resistance

Primary parameters of interest in material selection Strength : Amount of load (or weight, or force) a part can take before

breaking or bending Stiffness: Amount of deflection or deformation for a given load Weight All of these depend on geometry: EXTENSIVE values We would like to derive results that are independent of size

(geometry) : INTENSIVE values

Extensive vs. Intensive values:Extensive Intensive

Weight (kg) Strength (N) Stiffness (N/m)

Density (kg/m3) Yield strength or Ultimate Strength (N/m2) Modulus of Elasticity (N/m2)

How do we determine these values?

Types of quasi static material testing: Tension Compression Bending Torsion