\chapter{Prerequisits}
 Before beginning  the discussion on the
design process, it is important to state some of the prequisits that should be
held in mind during the design process.  The word prequisit was chosen for this
chapter to state that the designer need to keep in mind during the design
process.  The prequists include, scheduling, material selction, human factors
and safty issues.  
\section{Scheduling}
Developing a new producut following the design process is always limited by the
amount of time available for the entire process.  Over the years several
methods and procedures have been proposed to manage and plane for the
production of an artifact.   Many projects have failed through the lack of
attention to such detail. Historically the engineering projects were a one
engineer kind of project, the designer, the drafter and the planner is the same
person.  In recent years with the advancement of the products and technology
the products requirs teems of engineers and technical helpers beside the
reguilar workers to be accomplished.  In the following we will discuss some of
the existing techniques for scheduling.
\subsection{Gant Chart}
 This chart was introduced by Henry L. Gant and Frederick Taylor in the early
 1900s to aid mangement of  projects.  The Gant chart is taken the form of a
 bar charts.  The steps to establish a Gantt chart are as follows:
 \begin{enumerate}
 \item list all events or milestones of the project in an ordered list
 \item estimate the time required to establish each event
 \item list the starting time and end time for each event
 \item represnt the information in a bar chart
 \end{enumerate}
 An example of the Gantt chart is shown in figure \ref{gantt}
 \begin{figure}
 \vspace{4in}
 \caption{Gantt Chart}
 \label{gantt}
 \end{figure}
 \subsection{CPM/PERT}
 The critical path method (CPM) and the program evaluation and review technique
 (PERT) which were origionally developed during the same time period, are the
 two most widely used approaches. 
 PERT was developed under the guidance of the U.S. Navey Special Projects
 Office by a team  that included members from the Lockheed Missile Systems
 Division and the consulting firm Booz, Allen and Hamilton.  The technique was
 developed to monitor the effort of 250 main conrstactors and 9,000
 subcontractors concerened with the Polaris missile project.  CPM was the
 result of the efforts of the Integrated Engineering Control Group of E.I.
 duPont de Nemours \& Coompany and was developed to monitor activities related
 to design and construction.  
 
 Asurvey of manufacturing companies in the United States revealed that CPM/PERT
 are used for over 65\% among all other methods.  A CPM/PERT project generally
 has the following characteristics:
 \begin{enumerate}
 \item There are clearly defined activities or jobs whose accomplishement
 results in project completion
 \item Once started, the activity or job continues without interruption
 \item The activities or tasks are independent, which means they may be
 strated, stopped, and performed individually in a prescribed sequence
 \item The activities or jobs are orederd and they are follow each other in a
 specified manner
 \end{enumerate}
 Even though the CPM and PERT techniques were developed independently, the
 basic theory and symbology in both techniques are essentialy the same.
 \subsection{Definitions}
 Several symbols, terms and definitions are used in developing the CPM/PERT
 networks and they are:
 \begin{enumerate}
 \item Event (node) :This represents a point in time in the life of a project. 
 An enevnt can be the beginning or the end of an activity.  A circle is used to
 represent an event.  Generally, each network event is identified with a
 number.
 \item Activity: This is an effort needed to carry out certain portion of the
 project.  AN activity consumes money, labor, time and materials and is denoted by
 a continous arrow [$\longrightarrow$].  the direction of progress on the project
 is indicated by the arrowhead.  Further, an activity both begins and ends at a
 circle.
 \item Dummy activity: This is an imaginary activity that takes zero time and no
 resources to perform.  Its purpose is to show appropriate relationships between
 activities.  A doted arrow is use to present such activity.
 \item Network paths.  These are the paths used or needed to reach the project
 termination point or event from the project starting point or event.
 \item Critical path: This is the longest path with respect to time duration
 through the PERT/CPM network.  In other words the crtical path is the largest sum
 of activity duration of all individual network paths.
 \item Earlist event time (EET) : This is the earlist time at which an event
 occurs, providing all proceeding activities are accomplished within their
 estimated time.
 \item Latest event time (LET): This is the latest time at which an event could be
 reached without delaying the predicted project completion date.
 \item Total float: This is the latest time of an event minus the earlist time of
 the preceding event and the duration time of the in-between activity.
 \end{enumerate}
 \subsection{Network Developmenet}
 Steps involved in constructing a CPM network ar as follows:
 \begin{enumerate}
 \item Break down the design into indvidual activities and identify each activity
 \item Estimate the time required for each activity
 \item Determine the activity sequence
 \item Construct the CPM network, using the defined symbols
 \item Determine the critical path of the network
 \end{enumerate}
 Similarly, the steps involved in developing a PERT network are
 \begin{enumerate}
 \item Break down the design into indvidual activities and identify each activity
 \item Determine the activity sequence
 \item Construct the PERT network, using the defined symbols
 \item Obtain the expected time to perform each activity, using the following
 formula
 \[T_{e}=\frac{x+4y+z}{6}\]
 where\\
 $T_{e}$ is the expected time for the activity\\
 $x$ is the optimistic time estimate for the activity\\
 $y$ is the most likely time estimate for the activity\\
 $z$ is the pessimistic time estimate for the activity\\
 \item Determine the network critical path
 \item Compute the variance asscoiated  with the estimated expected time of $T_{e}$
 of each activity using the following formula
 \[S=\left(\frac{z-x}{6}\right)^{2}\]
 \item Obtain the probability of accomplishing the design project on the stated
 date using the following formula:
 \[w=\frac{T-T_{L}}{\left[ \sum{ S_{cr}^{2}}\right]^{\frac{1}{2}}}\]
 where\\
 $S_{cr}^{2}$ is the variance of activities on the critical path\\
 $T_{L}$ is the last activity's earliest expected completion time, as calculated
 through the network.\\
 $T$ is the design project due date, expressed in time units.
 
 Table \ref{tab1} presents the probabilities for selective values of $w$.
 \begin{table}
 \begin{tabular}{|l|l|}
 $w$&Probability\\ \hline
 -3&0.0013\\
 -2.5&0.006\\
 -2&0.023\\
 -1.5&0.067\\
 -1&.159\\
 -0.5&0.309\\
 0&.5\\
 0.5&0.69\\
 1&0.84\\
 1.5&0.933\\
 2&0.977\\
 2.5&0.994\\
 3&0.999\\
 
 \end{tabular}
 \label{tab1}
 \end{table}
 \end{enumerate}
 \subsubsection*{Example}
 A mechanical design project was broken down into a number of major jobs or
 activities, as shown in table \ref{tab2}.  Develop a CPM network using the data
 given in the table and determine the critical path of the network, along with the
 expected project duration time period.
 
 \begin{table}
 \begin{tabular}{cccc}
 Activity&ACtivity&Immediate Predecessor&Activity Duration\\
 Description&identification&activity or activities&week or weeks \\ \hline
Group formation&A&--&1\\
Literature search and&&&\\
design idea generation&B&A&2\\
Idea evaluation&C&B&1\\
Design and evaluation&D&C&4\\
Part manufacture&E&D&3\\
Part procurement&F&D&4\\
Design assembly&G&E,F&2\\
Testing&H&G&2\\
Cost estimation &I&G&4\\
Report and presentation&J&H,I,3\\

\end{tabular}
\label{tab2}
\end{table}


Using the defined symbols and the data given in table \ref{tab2}, the CPM network
is shown in figure \ref{cpm}.  The paths originated at event 1 and terminated at
event 11 are as follows
\begin{enumerate}
\item A-B-C-D-E-G-I-J time=20
\item A-B-C-D-F-G-I-J  time=21
\item A-B-C-D-E-G-H-J  time =18
\item A-B-C-D-F-G-H-J  time=19
\end{enumerate}
these results indicate that the longest path is (A-B-C-D-F-G-I-J)and the predicted
completion time for the design project is 21 weeks.
\section{Material Selction}
The design even of the simplest element requires the selection of a suitable material and
a desion regarding the methods of manufacturing which are to be used in producing the
element.  These two factors are closely related and the choice made will affect the shape,
size, appearance, cost and so on, and may make the difference between a commercial success
or a commercial failure.  As the design becomes more complex and involves more elements,
so the selection of suitable materials and methods of production becomes more difficult. 
The design engineer must be sufficiently familiar with the charactersitics and properties
of materials, and the way in which they can be shaped to ensure that his decisions are
good.
\subsection{Material Classifications and Preperties}
The various types of materials that may be used in a product design are classified as:
\begin{enumerate}
\item Metals: This important materials classification can be further divided into two
catagories; ferrous and nonferrous alloys.  Ferrous alloys are based on iron, which has
good mechanical properties. Nonferrous alloys are based on materials other than iron, such
as copper, tin, aluminum and lead.
\item Ceramics and glass: These are the result of the combination of metallic and
nonmetallic elements.  They are good insulators, brittle, thermally stable, more wear
resisitant compared to metals, harder and lower in thermal expansion than most metals.
\item Woods and organics: These are obtained from trees and plants.  Their major advantage
is that they are renewable resource.  However, some of their drawbacks are that they
absorb water, require special treatment to prevent rotting and are more flammable.
\item Polymers or plastics: These materials change viscosity with variation in
temperature; therfore they are easy to mold into a given shape. There are several benefits
of polymers: good insulators; resistant to chemicals and water; smooth surface finish;
available in many colors; eliminating the need for paintaing.  There are several drawbacks
of polymers: low strength, deterioration in ultraviolet light, and excessive creep at all
temperatures.
\end{enumerate}
The properties of material can be divided into six catagories: Mechanical, thermal,
physical, chemical, and fabrication.  Mechnaical properties include: fatigue, strength,
elasticity, wear, hardness and plsticity.  Thermal properteis include, absorptivity,
conductivity, fire resistance and expansion coefficient.  Physical properties are:
density, permeability, viscosity, crystal sturcture, and porosity.  Chemical properties
include: corrosion, oxidation and hydraulic permeability. Four important electrical
properties are: hysteresis, conductivity, coercive force and dielectric constant.  There
are several fabrication properties, some of which are: weldability, castability,
machinability and heat treatability.
\subsection{Material Selction Process}
Several systematic approaches have been developed for selecting materials, one of these
procedures is listed here:
\begin{enumerate}
\item Perform a material requirements analysis: This step is concerened with determining
the environmental and service condition at which the product will have to operate under.
\item List the suitable material: This step calls for filtering through the avialable
material and obtain several suitable candidates.
\item Choose most suitable material: This step involves analysis of the material listed
with respect to such factors as cost, performance, availability and fabrication ability
and then select the most suitable material.
\item Obtain the necessary test data: this step calls for determining the important
properties of the selected material experemintally under real lif operational conditions.
\item Product specification fulfillment:  The selected material must satisfy the stated
specifications
\item Cost: This important factor plays a dominant role in marketing of the end product.
\item Material availability: The selected material must be available at a reasonable cost.
\item Material Joining approach: Under real life conditions, it may be impractical to
produce an elemnet using a single piece of material.  This may call for manufacturing the
components with different pieces of material joined together to form a single unit.
\item Fabrication: Engineering products usually require some level of fabrication, and
many differnt fabrication techniques are available.  Factors affecting the fabrication
method selected include: time constraints, material type, product application, cost,
quantity to be manufactured.
\item Technical issues: Technical factors mainly concern with th material mechanical
properties.  Examples include: strength compared to anticipated load, sfety factors,
weight, temperature variation and potential loading changes.
\end{enumerate}
\subsection{Primary Manufacturing Methods}
The primary manufacturing methods which are used to convert a material into the basic
shape required are
\begin{enumerate}
\item Casting: Casting is widely used first step in manufacturing process.  During 
casting, an item takes on its initial usable shape.  Inthe casting process a solid is
meted down, heated to a desirable temperature level.  The molten material is then poured
into a mold made in the required shape.  The cast items may range in size and weight from
a fraction of an inch to several yards.  Typical examples are a zipper's individual teeth,
propellers, and the stern frams of ships.
\item Forging: Forging which is amongthe most important methods of manufacturing items of
high performance uses, involves changing the shape of the piece of material by exerting 
force on that peice.  The methods of applying pressure include: mechanical press,
hydraulic press and drop hammer. Products such as cranckshafts, wernches and connecting
rods are the result of forging.  The materila to be forged may be hot or cold.
\item Machining: machining involves removing unwanted material from a block of material
according to a given specifications, such as size, shape, and finish to produce a final
product. There are many machining processes such as milling, boring, grinding and
drilling.
\item Welding: Welding is a versatile production process which is used for combining items
produced by some other means of manufacturing.  Welding is the process of permanently
joining two materials through coalescence, wich involves a combination of pressure,
temperature and surface conditions.
\end{enumerate}
\section{Human Fctors}

The science that considers human and his reaction to familiar as well as strange
environment is known by several terms including human engineering, human
factors, human performance and ergonomics.  To adapt humman to fit the environment
is almost impossible, so means must be deviced to adapt the environment to suit
the human.  Human behvaior is critical to the success of a engineering system. 
Therefore it is important to take typical human behaviors into consideration
during the design phase.  Examples of typical human behaviors are
\begin{enumerate}
\item Human are often reluctant to admit errors
\item Human usually perform tasks while thinking about other things
\item People frequently misread or overlook instructions and labels
\item People often respond irrationally in emergency situations
\item A significant perecntage of people become complacent after successfully
handeling dangerous items over a long period of time
\item Most people fail to recheck outlined procedures for errors
\item People are generally poor estimators of speed, clearance, or distance. 
They frequently overestimate short distance and underestimate large distance
\item People are generally too impatient to take the time needed to observe
precaustions
\item People are often reluctant to admit that they cannot see objects well
enough, due either to poor eyesight or to inadequate illumination.
People generally use their hands for examining or testing.
\end{enumerate}

Every time  man dials a telephone, derives a car, studies an oscilloscope,
operates a computer or forms a part on a lathe he has joined his sensing,
decison-making, and muscular powers to an engineering system.  Figure \ref{erg}
shows the coupling of components necessary to consider man as an integral part
of an efficient man-machine system.
\begin{figure}
\vspace{4in}
\caption{Man-machine System}
\label{erg}
\end{figure}
\subsection{Human Sensory Capabilities}
 Human sensory capabilities include;
  
 \begin{enumerate}
 \item Sight: Human eyes see differently from different angles or positions. 
 For example, looking straight ahead, the eyes can preceive all colors. 
 However, with an increase in the viewing angle, human preception begins to
 decrease.  The limits of color vison are given in table \ref{tabc}
\begin{table} 
 \begin{tabular}{ccccccc}
 Situation&Green&Blue&Yellow&White&Green-Red&Red\\ \hline
 Vertical&40$^{o}$&80$^{o}$&95$^{o}$&130$^{o}$&--&45$^{o}$\\
 Horizontal&--&100$^{o}$&120$^{o}$&180$^{o}$&60$^{o}$&--\\
 \end{tabular}
\label{tabc}
\end{table}
Aslo at night or in the dark, small-sized orange, blue, green and yellow
warning lights are impossible to distinguish from a distance.
\item Noise: The performance quality of a task requiring instense concentration
may be affected by noise.  It is established fact that noise contribute to
peoples' feelings such as irritability, boredom, and well being.  A noise level
below 90 DB (decibels) is considered hramless and above 100 DB is unsafe. 
Noise levels in excess of 130 dB are considered unpleasant and may actually be
painful.  
\item Touch: Touch adds to, or may be even replace, the information transmitted
to the brain by eays and ears.  For example, it is possible to ditinguish
control knob shapes by touch alone, which could be valuable if, say, power goes
out and there is no light.
\item Vibration and motion: It is now accepted fact that the poor performance
ofphysical and mental tasks by equipment operators vould be partially or fully
to vibrations. For example, eye strain, headaches and motion sickness could
result from low frequency, large amplitude vibration.  Useful guidlines for
reducing the effcts of vibration and motion are as folows:
\begin{enumerate}
\item Eliminate vibration with an amplitude greater than 0.08 millimeter
\item Use such devices as shock absorbers and springs wherever possible
\item Use damping materials or cushioned seats to reduce vibration wherever
possible
\item Since seated humans are most affected by vertical vibration, use this
information to reduce vertical vibration.
\item The resonant frequency of the Human vertical trunk, in the seated
position, is between 3 and 4 cycles per second.  Therfore, avoid any seating
design that would result in or would transmit vibration of 3 to 4 cycles per 
second.
\end{enumerate}
\end{enumerate}
\subsection{Anthropometric Data}
Anthropometry is the scince dealing with dimensions of the human body.  These
dimensions are divided into statistical groups known as percentiles.  If 100
men are lined up from smallest to the largest in anay given respect they would
be classified from the 1 percentile to the 100 percentile.  The 2.5 percentile
male means that designs based on this series of dimensions would allow up to
2.5\% of the population to use the system and the remaining 97.5\%  would be
excluded.  Figure \ref{adult} shows the dimesnions of a standing adult male.
\begin{figure}
\vspace{7in}
\caption{Standing adult male}
\label{adult}
\end{figure}
\section{Safety Considerations}
One important consideration in engineering design is that the resulting product
must be safe for humans.  In recent years, the liability decisons by the courts
have furher increased the importance of safety. According to the National
Safety Concil, over 105,000 Americans wre accedently killed and over 10 million
suffered a disabiling injuury in 1980 which translted into \$83.2 billion. The
concern for saftey is not new it has been there for thousands of years. 

There are many  saftey functions during the design process including:
\begin{enumerate}
\item Developing accedent prevention requirements for the basic desgin
\item Participating in the design reviews
\item Performing hazard analyses during the product design cycle

\end{enumerate}
\subsection{Saftey Analysis Techniques}
the two techniques, failure modes and effect analysis (FMEA) and fault trees,
which  are used for design stage reliability analysis can also be applied for
saftey analyses.
\subsubsection{Failure modes and effect analysis (FMEA)} 
This method was origionally developed for use in the design and evelopment of
flight control systems. The method can also be used to evaluste design at the
initial stage from the point view of safety.  Basically the technique calls for
listing the potential failure modes of each part, as well as the effects on the
listed parts and on humans.  The technique may be broken down into seven steps
\begin{enumerate}
\item Defining system boundaries and requirements
\item Listing all items
\item Identifying each component and its associated failure modes
\item Assigning an occurrence probability or failure reate to each failure mode
\item Listing the effects of each failure mode on concerend items and people
\item Entering remarks for each possible failure mode
\item Reviewing and initiating appropriate corrective measures
\end{enumerate}
\subsubsection{Fault trees}
This technique uses various symbols.  It starts by identifying an undesirable
event, called the top event, and then successively asking the question "How
could this event occure?" This process continues until the fault events need
not be developed further.  If occurrence data are known for the basic or
primary fault event, the occurence measure for the top event can be calculated.


