DESIGNDESIGN Design is a procedure either to formulate a Design is a procedure either to formulate a

plan for the satisfaction of a specified need plan for the satisfaction of a specified need or to solve a problem.or to solve a problem.

If the plan can create something having a If the plan can create something having a physical reality, then the product must be of physical reality, then the product must be of following characteristics:following characteristics:

a)a) Functional:Functional: The product must satisfy the The product must satisfy the intended need and customer expectationintended need and customer expectation

b)b) Safe:Safe: It should not be hazardous to the user, It should not be hazardous to the user, bystanders, or surrounding property.bystanders, or surrounding property.

c) Reliable:c) Reliable: The product should perform its The product should perform its intended function satisfactorily or without failure intended function satisfactorily or without failure at a given age.at a given age.

d) Competitive: d) Competitive: The product must be a The product must be a contender/contestant in its market.contender/contestant in its market.

e) Usable: e) Usable: It should be “user friendly”. It must It should be “user friendly”. It must accommodate human size, strength, reach, force, accommodate human size, strength, reach, force, power, and control.power, and control.

f) Manufacturable: f) Manufacturable: It should consist of minimum It should consist of minimum number of parts. It must be suitable to mass number of parts. It must be suitable to mass production, with controlled dimensions, distortion production, with controlled dimensions, distortion and strength.and strength.

g) Marketable: g) Marketable: It can be bought and repaired very It can be bought and repaired very easily.easily.

Mechanical Engineering Design (MED)Mechanical Engineering Design (MED)

A design needing the skill or knowledge involved in A design needing the skill or knowledge involved in all the disciplines of Mechanical Engineering i.e. all the disciplines of Mechanical Engineering i.e. Mechanics of solids and fluids, mass and Mechanics of solids and fluids, mass and momentum transport, manufacturing processes, momentum transport, manufacturing processes, processing and production of energy, tools of processing and production of energy, tools of transportation, techniques of automation, etc.transportation, techniques of automation, etc.

Some phrases as examples of MEDSome phrases as examples of MED Machine design, machine-element design, Machine design, machine-element design,

machine-component design, systems design, machine-component design, systems design, and fluid-power design.and fluid-power design.

Some special examples of MEDSome special examples of MED Internal combustion engine design, turbo-machinery Internal combustion engine design, turbo-machinery

design, jet engine design, HCAC system design, etc.design, jet engine design, HCAC system design, etc.

Interaction between Design Process Interaction between Design Process ElementsElements

The complete design process is shown here.The complete design process is shown here. Depending on the nature of the design task, several Depending on the nature of the design task, several

design phases may be repeated through the life of design phases may be repeated through the life of

the productthe product..

Some important steps used in design processSome important steps used in design process

Recognition and IdentificationRecognition and Identification Usually design begins when someone recognizes a Usually design begins when someone recognizes a

need, and then decides to do something about it.need, and then decides to do something about it. It is considered as a highly creative act.It is considered as a highly creative act. A need is easily recognized after someone else has A need is easily recognized after someone else has

stated it.stated it. There is a distinct difference between the statement There is a distinct difference between the statement

of the need and the identification of the problem.of the need and the identification of the problem. The problem is more specific.The problem is more specific.

If the need is for cleaner air, the problem may If the need is for cleaner air, the problem may be that of reducing the dust discharge from be that of reducing the dust discharge from power plant stacks.power plant stacks.

Definition of problem must include all the specifications for the object needs to be designed.

The specifications include input and output quantities, the characteristics and dimensions of the space required for the object, all the limitations on these quantities.

The synthesis of a scheme connecting possible system elements is also called the invention of the concept.

It is basically the selection of possible mechanism or combination of mechanisms to accomplish the desired task.

Analyses are required to be performed to evaluated the system performance i.e. better or satisfactory.

Synthesis schemes that do not survive analysis are revised, improved, or discarded.

Schemes with potential are optimized for best performance.

Evaluation is a significant phase of the total design process.

It is the final proof of a successful design, and involves the testing of a prototype in the laboratory.

Communicating the design to others is the final, vital presentation step in the design process.

Design ConsiderationsDesign Considerations Design consideration is referred to as a

characteristic that influences the design of the element or the entire system

For example, strength required for an element is an important factor in the determination of geometry and dimensions of the element

Thus strength is an important design consideration.

Following are important characteristics considered in a given design situation:

1 Functionality 14 Noise

2 Strength/stress 15 Styling

3 Distortion/deflection/stiffness 16 Shape

4 Wear 17 Size

5 Corrosion 18 Control

6 Safety 19 Thermal properties

7 Reliability 20 Surface

8 Manufacturability 21 Lubrication

9 Utility 22 Marketability

10 Cost 23 Maintenance

11 Friction 24 Volume

12 Weight 25 Liability

13 life 26 Resource recovery

Codes and StandardsCodes and Standards A A standardstandard is a set of specifications for parts, is a set of specifications for parts,

materials, or processes, required to achieve materials, or processes, required to achieve uniformity, efficiency, and a specified quality.uniformity, efficiency, and a specified quality.

The major purpose of standard is to place a The major purpose of standard is to place a limit on the number of items in the limit on the number of items in the specifications in order to provide a reasonable specifications in order to provide a reasonable inventory of tooling, sizes, shapes, and inventory of tooling, sizes, shapes, and varieties.varieties.

A A codecode is a set of specifications for the is a set of specifications for the analysis, design, manufacture, and analysis, design, manufacture, and construction of something.construction of something.

The purpose of a code is to achieve a specified The purpose of a code is to achieve a specified degree of safety, efficiency, and performance or degree of safety, efficiency, and performance or quality.quality.

Safety codes do not give absolute safety.Safety codes do not give absolute safety. Absolute safety is impossible to obtain.Absolute safety is impossible to obtain. All of the organizations and societies listed All of the organizations and societies listed

below have established specifications for below have established specifications for standards and safety or design codes:standards and safety or design codes:

Aluminum Association (AA)Aluminum Association (AA) American Gear Manufacturers Associations (AGMA)American Gear Manufacturers Associations (AGMA) American Institute of Steel Construction (AISC)American Institute of Steel Construction (AISC) American Iron and Steel Institute (AISI)American Iron and Steel Institute (AISI) American National Standards Institute (ANSI)American National Standards Institute (ANSI) American Society for Metals (ASM)American Society for Metals (ASM)

American Society of Mechanical Engineers (ASME)American Society of Mechanical Engineers (ASME) American Society of Testing and Materials (ASTM)American Society of Testing and Materials (ASTM) American Welding Society (AWS)American Welding Society (AWS) American Bearing Manufacturers Association American Bearing Manufacturers Association

(ABMA) (ABMA) British Standards Institution (BSI)British Standards Institution (BSI) Industrial Fasteners Institute (IFI)Industrial Fasteners Institute (IFI) Institution of Mechanical Engineers (I. Mech. E.)Institution of Mechanical Engineers (I. Mech. E.) International Bureau of weights and measures International Bureau of weights and measures

(BIPM)(BIPM) International Standards Organization (ISO)International Standards Organization (ISO)

Material strength and StiffnessMaterial strength and Stiffness

The standard tensile test is used to obtain a variety of material characteristics and strength for the design.

A typical tension-test specimen with original dia. d0 and gauge length L0 is shown in Fig. 3.1.

The specimen is then brought under tension by load P to get the deflection.

The stress produced is: σ = P/A0 ; where A0 = π/4. d0

2

If the deflection, or extension of gauge length is L - L0, the strain is given as:

ε = (L – L0)/L0

The results are plotted in Fig. 3.2 for ductile and brittle materials. Ductile materials deform much more than brittle materials.

Point pl in Fig. 3.2a is called the proportional limit. Up to this point σ = Eε (Hook’s law) where E is constant of proportionality known as Young’s modulus

E is a measure of the stiffness of a material.

Point el in the figure is called elastic limit. If the specimen is loaded beyond this

point, the deformation is known as plastic. Between pl and el the diagram is not a

perfectly straight line. Many materials reach a point where strain

increases rapidly without corresponding increase in stress. This point is called yield point.

Some materials, especially brittle materials do not have an obvious yield point.

For this reason, yield strength Sy is defined by an offset method as shown in Fig. 3.2, where line ay is drawn at slope E.

The ultimate, or tensile, strength (Su or Sul) corresponding to point u in figure is the maximum stress reached on stress-strain diagram.

Some materials exhibit downward trend after the maximum stress is reached and fractured at point f.

However, materials such as cast irons and high-strength steels fracture earlier such that points u and f are identical as shown in Fig. 3.2b.

Strength is a built-in property of a material, or of a mechanical member because of the selection of a particular material or process or both.

Stress is something that occurs in a part, usually as a result of its being assembled into a machine and loaded.

The stress calculated by the formula σ = P/A0 is not true stress but is engineering stress because it is based on the original area before the load is applied.

Actually, true stress is larger than engineering stress because area reduces when the load is applied.

To get the true stress area and load must be measured simultaneously during the test.

In ductile materials, as shown in Fig. 3.2a, stress decreases from point u to f. Beyond point u the specimen begins to “neck”.

So, the true stress is much higher than engineering stress at the necked section.

Contrary to engineering stress-strain diagram, true stress-strain diagram is based on true stress as well as on true strain.

True strain or logarithmic strain is the sum of the incremental elongations divided by the current gauge length at load P:

ε = ln (L/Lo)

HardnessHardness The resistance of a material to penetration

by pointed tool is called hardness. Following are the two important methods

used to measure the hardness:Rockwell hardness test: It has good

reproducibility, and thus measurements are easily and quickly made by it.

Hardness number is read directly from a dial.

The indenters are described as a diamond, a 1/16 in-diameter ball.

The Brinell hardness test: is used for general purpose in which a force is applied through an indenting tool.

The hardness number (HB) is found as a number equal to the applied load divided by the area (spherical surface) of indentation.

The units of HB are the same as those of stress.

Both methods are nondestructive in most of the cases which is a clear advantage.

Hot-Working ProcessesHot-Working Processes

A process in which a metal is heated above its recrystallation temperature. e.g. hot rolling, forging, hot extrusion, and hot pressing.

Hot rolling: is used to produce particular shapes and dimensions of a material bar of steel, aluminum, magnesium, and copper (See Fig. 3-10).

Tubing can be manufactured by hot-rolling strip or plate Seamless tubing is manufactured by roll-piercing a solid

heated rod with a piercing mandrel. Extrusion: is the process by which great pressure is

applied to a heated metal billet or blank, causing it to flow through a restricted orifice.

This process is more common with materials of low melting point such as Al, Cu, Mg, Pb, Zn.

Forging: is the hot working of metal by hammers, presses, or forging machines.

It produces a refined grain structure that results in increased strength and ductility.

Comparing with castings, forgings have greater strength for the same weight.

Drop forgings can be made smoother and more accurate than sand castings.

However, the initial cost of the forging dies is greater than the cost of patterns used in castings.

Cold-Working ProcessesCold-Working Processes

Cold working is defined as the forming of the metal at low temperature (usually room temperature).

Cold-worked parts require less machining, are more accurate, and have a bright new finish relative to hot-worked parts.

Cold-finished bars and shafts are produced by rolling, drawing, turning, grinding, and polishing.

The largest percentage of products are made by the cold-rolling and cold-drawing processes.

Cold-rolling is now used mostly for the production of wide flats and sheets.

Both cold-rolling and cold drawing have the same effect upon the mechanical properties.

A cold-working process does not change the grain size, however distorts it.

Cold-working causes an increase in yield strength, ultimate strength and hardness, but a decrease in ductility (See Fig. 3-11).

Some other cold-working processes are heading, roll threading, spinning, stamping, blanking, coining, forming, and shallow drawing.

The heat treatment of steelThe heat treatment of steel

The time and temperature controlled processes that relieve residual stresses and/or modifies material properties of steel are called heat treatment of steel.

The material properties of steel include hardness (i.e. strength), ductility, and toughness.

The common heat treatment operations are as follows:

Annealing: The heating of a material to a temperature that is approximately 100°F above the critical temperature is called annealing.

The material is held at this temperature for a time that is sufficient for the carbon to dissolve and diffuse through it.

Then the material is cooled slowly in the same furnace till the complete transformation is achieved to have a full anneal.

Annealing is used to soften a material and make it more ductile, to relieve residual stresses, and to refine the grain structure.

Normalizing is a process included in annealing. In this process, parts are heated to a slightly higher

temperature than in full annealing. Parts are then cooled in still air at room

temperature. This cooling is more rapid than that used in full annealing.

Quenching: A controlled cooling rate of the material through water and oil media to get the desired hardness is called quenching.

Normalizing (cooling in still air) is an example of mild quench.

The oil quench is quite slow but avoids the cracks caused by the rapid expansion of objects.

Water quenching is used for carbon steel, medium carbon, and low alloy steels.

The effectiveness of quenching depends upon the fact that a cooled austenite structure does not transform into pearlite instantaneously. But it requires time to initiate and to complete the process.

When the material is cooled rapidly to 400°F or less, the austenite is transformed into a structure called martensite.

Martensite is a supersaturated solid solution of carbon in ferrite. It is the hardest and strongest form of steel.

The rapid cooling of steel between 400 and 800°F and then holding it for a sufficient time results in the transformation of austenite into bainite.

Bainite is an intermediate structure of pearlite and martensite.

Tempering: The relieving of a steel specimen from the internal stresses by a modest heating process is called tempering or drawing.

It is a combination of stress relieving and softening of specimen.

After the hardening of a specimen during quenching process, it is reheated to some temperature below the critical temperature for a certain period of time. Then allowed to cool in still air.

The temperature to which it is reheated depends upon the composition and the degree of hardness and toughness required.

This reheating process releases the carbon held in the martensite, forming carbide crystals.

The structure obtained is called tempered martensite. It is a superfine dispersion of iron carbide in fine-grained

ferrite.