mechanical design basics

[1]

Lecture 1

MECHANICAL ENGINEERING DESIGN REQUIREMENTS AND CONSIDERATIONS

MECHANICAL ENGINEERING DESIGN 2

308803 MECHANICAL DESIGN 337

1.0 INTRODUCTION

Essentially, design is the process of problem solving. The primary objective of any

engineering design project is to fulfil the human need or desire. Professional engineers

are concerned with obtaining solutions to practical problems. Such problems occur in a

wide range of types and their degree of complexity also varies. The real challenge is to

transform the customers needs and expectations into technical specifications in an

efficient and professional manner. This is a complex undertaking, requiring many skills.

The provided solutions must reflect an accurate understanding of customer needs and

the underlying science. Such solutions also require empirical knowledge as well as

engineering judgement. Figure 1.1 shows the basic steps involved in the design process.

Figure 1.1: The design process [2]

Mechanical Design

As shown in Figure 1.2, there are many subfields that are part of the overall domain of

the problem solving process mechanical design is one of those. The field of mechanical

engineering is divided into two broad areas 1) Energy and 2) Structures and motion.

The term mechanical design refers to design in mechanical engineering systems in which

both stems of mechanical engineering can be involved, whereas the field of machine

design is a subset of mechanical design in which the focus is on the structures and

motion stems only.



Figure 1.2: Design Horizons [2]

For example, the design of heat exchangers, air compressors and internal combustion

engines are examples of mechanical design, because those devices depend on the use of

technical material from heat transfer, thermodynamics, and combustion. These topic

areas are related to the energy domain of mechanical engineering. On the other hand,

the designs of a gear box, a V-belt drive system, or a machine structure fall under

machine design category because they draw on technical material from strength of

materials, solid body mechanics, kinematics and dynamics. These technical materials are

all connected to the structures and motion stem of mechanical engineering [2].

2.0 DESIGN CONSIDERATIONS

Most of the design problems in mechanical engineering do not have a single right

answer. Consider, for example, the problem of designing a household washing machine.

There are endless alternatives when it comes to the possible number of workable designs

and none of which could be called an incorrect answer. Obviously, some of the answers

are better than others because they incorporate a more sophisticated knowledge of the

underlying technology, a more creative concept of basic design, a more effective and

economic utilization of existing production technology, a more pleasing aesthetic

appearance, and so on.

Therefore, design engineers are required to carefully review the relevant design factors

before proposing a solution to a particular design problem. These considerations include

issues, such as functionality, reliability and maintainability etc. In addition to the



traditional technological and economic considerations fundamental to the design and

development of mechanical systems, the broader considerations of safety, ecology, and

overall quality of life are also required to be addressed. The following is a list of many

of the important factors, which play a fundamental role in achieving a good design [3].

They are not necessarily in the order of importance.

Functionality Noise

Strength/stress Styling

Distortion/deflection/stiffness Shape

Wear Size

Corrosion Control

Safety Thermal properties

Reliability Surface

Manufacturability Lubrication

Utility Marketability

Cost Maintenance

Friction Volume

Weight Liability

Life Remanufacturing / resource recovery

Most engineering designs involve a huge range of considerations, and it is a challenge to

the engineer to recognize all of them in proper proportion. Following is a summary of

some of the major categories involved.

Traditional

Considerations

Modern

Considerations

Miscellaneous

Considerations

Materials Safety Reliability

Geometry Ecology Maintainability

Operating conditions Quality of life Ergonomics

Cost Aesthetics

Availability

Producibility

Component life

Some of these categories and other design considerations are further discussed in the

following sections [2, 4, 5].



2.1 Material considerations The mathematical relationships used in designing are derived for an idealized material,

which is assumed to posses the following properties:

a) Perfect Elasticity

A perfectly elastic material will return to its original shape immediately upon

removal of the loads.

If a material doesnt have this property, then the mathematical equations, in

many cases, become too complex for practical calculations.

However, it should be noted that there may be a considerable variation

between the actual stresses in the body and the stresses obtained from

equations for an idealized substance.

b) Homogeneity

A homogeneous part/component is one that has the same properties

throughout its entire extent.

c) Isotropy

An isotropic material is one in which the elastic properties are the same in all

directions.

2.2 Safety and liability considerations

The strict liability concept of product liability generally prevails in most of the

developed countries. This concept states that the producers of an article is liable for any

damage or harm that results because of the defect. It doesnt matter whether the

manufacturer knew about the defect, or even could have known about it.

The best way to prevent the product liability problems is to adopt good engineering in

analysis and design, quality control, and comprehensive testing procedures. The

followings are some of the techniques to improve product safety [4]:

a). Safety awareness

The important first step in developing engineering competence in the safety area is

cultivating an awareness of its importance. All engineers and technicians, who are

involved in the design process, must be aware of the significance of the safety of the

products they are delivering.



b). Imagination and Ingenuity

The design engineer must be imaginative and ingenious to anticipate potentially

hazardous situations relating to a product. The old saying that anything that can happen

probably will happen sooner or later is relevant.

c). Techniques and guidelines

The following techniques can be used to improve the safety of a product.

- Review the total life cycle of the product from initial production to final disposal,

with an eye toward uncovering significant hazards. Various stages of product life

cycle, such as manufacturing, transporting, storing, installing, using, and

servicing, should be kept in mind when analysing the product safety attributes.

- Safety provisions should represent a balanced approach.

- Safety should be regarded as an integral feature of the basic design.

- Where possible, a fail safe design should be used.

- Adherence to government and industry standards should be ensured.

- Warnings of all significant hazards that remain after the design has been made as

safe as reasonable possible should be provided.

2.3 Ecological considerations

Making a product environmentally-friendly is another very important design aspect that

needs to be considered right at the early stages of product development. The basic

ecological objectives of mechanical engineering are:

- to utilize materials so that they are economically recyclable within reasonable

time periods without causing objectionable air and water pollution

- to minimise the rate of consumption of non-recycled energy resources (such as

fossil fuels) both to conserve these resources and to minimise thermal pollution

Ecological factors are much more difficult for the design engineer to tie down than are

such matters as stress and deflection. The following suggestions are useful to be

considered.

I. Consider all aspects of the basic design objective involved, to be sure that it is

environmentally safe.

II. Consider design for recycling.



III. Select environmentally-friendly materials

IV. Select green manufacturing processes.

V. Where possible, use reusable packaging

2.4 Ecological assessment and analysis

There are several tools, such as life cycle assessment (LCA), available for the

environmental performance evaluation of products and services. Life cycle assessment

(LCA) is a tool that can be used to evaluate the environmental impact of a product,

service, or activity throughout its life cycle. It can be employed to identify environmental

hot spots in a product's life cycle and to select new environmentally optimised

solutions for new products. The LCA consists of the following four major steps:

Goal definition and scope

Inventory analysis

Impact assessment

Interpretation of results

Figure 1.3 shows a generalized arrangement of the four phases of LCA.

Figure 1.3: Generalized framework of LCA [6]

Goal

Definition and

Scope

Inventory

Analysis

Impact

Assessment

Interpretation

2.4.1 Goal definition and scope

This phase is aimed at defining and describing processes, activities, materials, new and old

parts used in the manufacturing, packaging, transportation, distribution, use, maintenance

and end-of-life treatment of a product. As shown in Figure 1.4, the inputs and

environmental impact associated with each of the product life cycle are identified.

Measurement units, key assumptions, boundaries and likely limitations are also defined for

each of the identified processes and activities.

Figure 1.4: Environmental assessment inputs and outs

2.4.2 Inventory analysis

In this stage of LCA, detailed information and data on all the direct and indirect

environmental inputs and outputs are gathered. This includes:

raw materials (virgin / recycled)

energy consumed

emissions to air and water

waterborne wastes

co-products

solid waste (from processes and products) and other environmental releases

Materials

Use/ Maintenance

Manufacturing

End-of-life

Assembly

Packaging

Distribution

Transportation

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MECHANICAL ENGINEERING DESIGN 1 - 9


2.4.3 Impact assessment

In this phase of the assessment, the inventory results are interpreted into potential impacts.

Basically, these interpretations reflect the entities, which are to be protected by the impact

assessment study. These entities include human health, ecosystem health and the resource

base. For the impact assessment phase, the following four steps are recommended [6-8]:

a) Classification: defining the impact categories

b) Characterization: quantifying the environmental impacts and impact

categories

c) Normalization: expressing the results of characterization on a

common scale to facilitate comparison

d) Weighting: reflecting the relative significance of impact categories

2.4.4 Interpretation

This is the final step of the life cycle assessment process. The impact assessment results are

interpreted along the lines of the defined goal and scope of the study.

2.5 Factor of safety (SF)

The quality of a design can be measured by many criteria. It is always required to calculate

one or more factors of safety to estimate the likelihood of failure. There may be legislated, or

generally accepted, design codes which must be adhered to as well.

A factor of safety or safety factor can be expressed in many ways. It is typically a ratio of two

quantities that have the same units, such as strength/stress, critical load/applied load, load to

fail part/expected service overload, maximum cycles/applied cycles, or maximum safe

speed/operating speed. A safety factor is always unitless and is denoted by SF.

a). Value of Safety Factor

As a machine or product may have more than one potential mode of failure, therefore, it can

have more than one value of safety factor. The smallest value of SF for any component is of

greatest concern, since it predicts the most likely mode of failure.

When SF = 1, the stress in the part is equal to the strength of the material and failure occurs.

Therefore, the factor of safety should always be greater than 1.

b). Choosing a Safety Factor

Choosing an appropriate safety factor is very important and requires a thorough

understanding and assessment of the related factors. The safety factor can be thought of as a



measure of the designers uncertainty in the available data, analytical models, failure

theories and the material property data. How much greater than one SF must be depends on

many factors including:

- the level of confidence in the model on which the calculations are based

- the knowledge of the range of possible in-service loading conditions

- the level of confidence in the available material strength information

- consequence of failure human safety and economics

- cost of providing a large safety factor

Table 1.1 provide guidelines for the choice of a safety factor for ductile materials.

Table 1.1: Factors used to determine a safety factor for ductile materials [5]

Information Quality of Information Factor

Material test data

The actual material used was tested 1.3

F1 Representative material test data are available 2

Fairly representative material test data are available 3

Poorly representative material test data are available 5+

Operating conditions

in which the product

will be used

Are identical to material test conditions 1.3

F2 Essentially room-ambient environment 2

Moderately challenging environment 3

Extremely challenging environment 5+

Analytical models

used for analysing

loading and stress

Models have been tested against experiments 1.3

F3 Models precisely represent system 2

Models approximately represent system 3

Models are crude approximations 5+

The overall safety factor is taken as the largest of the three factors chosen. Due to the

uncertainties involved, a safety factor typically should not be taken to more than one

decimal place accuracy.

SF ductile = MAX (F1, F2, F3)



As brittle materials are designed against the ultimate strength, so failure means fracture

(without visible warning of failure before fracture), therefore the safety factor for brittle

materials is often made twice that which would be used for ductile material in the same

conditions.

SF brittle = 2 x MAX (F1, F2, F3)

Table 1.2 provides more information on the recommended values for a safety factor. The

method of determining a safety factor are only guidelines to obtain a starting point and is

obviously subject to the judgment of the design engineer in selecting factors in each

category.

Table 1.2: Recommended values for a safety factors [4]

Quality / Nature of the Available Information Safety

Factor

1

Materials are exceptionally reliable,

The product is used under controllable conditions

Loads and stresses can be determined with certainty

(This scenario is more suited to situations where low weight is a particularly important

consideration).

1.25 1.5

2

Materials used are well-known

The product is used under reasonably constant environmental conditions

Loads and stresses that can be determined readily.

1.5 2

3

Average materials

Ordinary environments

Loads and stresses that can be determined.

2 2.5

4 Rarely used (less tried) materials or for brittle materials under average conditions

of environment, load, and stresses. 2.5 3

5 The materials that havent been used before will be used under average conditions

of environment, load, and stresses. 3 4

6 Materials that are better-known are to be used in uncertain environments or

subjected to uncertain stresses. 3 4

7

Repeated loads: These loads test the fatigue strength of materials. Therefore, the

above values of safety factor must be applied to the endurance limit (not the yield

strength).

8 Impact factors: For applications involving impact loads, an impact factor should be

included when determining the safety factor.

9 Brittle materials: In situations where the ultimate strength is used as the theoretical

maximum, the above factors should be approximately doubled.

10 A more detailed analysis should be performed for applications requiring higher

safety factors.



2.6 System of units

There are three most commonly used systems of units, as shown in Table 1.3.

Table 1.3: English, British and SI Units [4]

3.0 BASIC RELATIONSHIPS

a). Work and Energy

Work done W = Force x distance

= FS

Where s is the distance through which force is applied.

Figure 1.5 shows a wheel being turned by a tangential force F acting at radius R. Let the

wheel rotate through q revolutions. Then the work done, W, is given by

W = F (2R) (q) = FS

Figure 1.5: Wheel being turned by a tangential force F [4]

The torque produce by the force F is give by

T = F x R



Suppose the wheel is rotated through an angle by applying the torque T. Then the work

done, W, is given by

W = T

OR

W = FR

In SI system, the unit for work is newton.meter (Nm), called the Joule. The work done is also expressed as Kinetic Energy, Potential Energy or Internal Energy. The

total amount of energy is conserved in all transfers.

b). Power The rate of energy transfer by work is called power and is denoted by . It is given by

= F V In SI units, the unit for power is Watt (Joule/s), which is the same as 1 N.m/s. Furthermore:

1 Revolution = 2 radians 60 sec = 1 minute 1000 W = 1 KW Then the power in kilowatt is determined by the relationship as shown in Table 1.4

Table 1.4: Power in Kilowatts [4]

c). Conservation of Energy When there is no mass transfer across the boundaries of the system, the conservation of energy would be represented by the relationship, as shown in Table 1.5.



Table 1.5: Conversation of energy [4]



4.0 DESIGN TESTING AND VALIDATION

The traditional methods and techniques of stress and deflection analysis are primarily

applicable to parts that are made up of simple geometric shapes, such as cylinders,

rectangular or triangular prisms. However, many real machine parts have more complicated

geometric forms; making accurate calculations of stress and deflection are difficult and even

impossible with classical techniques. For example, analysing the stress and deflection in a

part like the crankshaft, as shown in Figure 1.6, becomes difficult because of the highly

intricate nature of the part. Such problems make the conventional methods highly laborious,

inefficient and difficult to apply.

Figure 1.6: Crankshaft of a diesel-truck engine [5]

These types of objects can be divided into finite number of contiguous and discrete

elements, as shown in Figure 1.7. Then a large set of equations is developed, each of which is

applied to an element and to the nodes that connect the elements. These equations are

subsequently solved simultaneously to analyse the stresses and deflections. This method is

known as Finite Element Analysis (FEA).

Figure 1.7: Finite element method of an engine piston, connecting rod, and crankshaft [5]



The mathematical theory behind FEA is beyond the scope of this unit, and is covered in a

number of books. This topic is also covered in detail in the unit titled Finite Element

Analysis 431.

THE FINITE ELEMENT METHOD

Finite element analysis is a numerical technique and is well suited to digital computing

machines. The FEA is based on the formation of a simultaneous set of algebraic equations

relating forces to corresponding displacements at discrete preselected points (called nodes)

on the structure. These mathematical equations, also referred to as force displacement

relations, are expressed in matrix notation.

As stress varies throughout the continuum of any part, dividing the part into a finite

number of discrete elements connected together at their nodes (called a mesh) provides an

approximation of the stress and strain within the part for any given set of boundary

conditions and load applied at various nodes in the structure. The approximation can be

improved by using more elements of smaller size at the expense of increased computation

time. The computation time has been reduced remarkably because of the development of

very high speed computing machines.

An important part of the designers work is to choose an appropriate type, number and

distribution of elements to optimize the trade-off between accuracy and computation time.

Large elements can be used in regions of the part where stress gradient varies slowly. In

regions where the stress changes rapidly, such as near stress concentrations or applied loads

and boundary conditions, a finer mesh is needed. This is shown in Figure 1.8, in which the

mesh density varies in different regions of the part.

Figure 1.8: High density of elements near regions of high stress concentrations [3]



The basic procedure for stress analysis using FE method include the following steps [4]:

I. Dividing the part into discrete elements

II. Defining the properties of each element

III. Assembling the element stiffness matrix

IV. Applying known external loads at nodes

V. Specifying part support conditions

VI. Solving the system of simultaneous algebraic equations

VII. Calculating stresses in each element

5.0 REFERENCES

1. Earle, J.H., Engineering Design Graphics. 12th ed. 2007: Pearson Prentice Hall.

2. Spotts, M.F., T.E. Shoup, and L.E. Hornberger, Design of Machine Elements. Eighth ed.

2004: Pearson Prentice Hall.

3. Budynas, R.G. and J.K. Nisbett, Shigley's Mechanical Engineering Design. Eighth ed.

2008: McGraw Hill.

4. Juvinall, R.C. and K.M. Marshek, Fundamentals of Machine Component Design. Fourth

ed. 2006: John Wiley & Sons, Inc.

5. Norton, R.L., Machine Design: An Integrated Approach. Third ed. 2006: Pearson Prentice

Hall.

6. Westkmper, E., L. Alting, and G. Arndt, Life cycle management and assessment:

approaches and visions towards sustainable manufacturing. Proceedings of the Institution

of Mechanical Engineers Part B-Journal of Engineering Manufacture, 2001. 215(B5 ):

p. 599 - 626.

7. Craighill, A.L. and C.J. Powell, A life cycle assessment and economic evaluation of

recycling : a case study. 1995, Centre for Social and Economic Research on the Global

Environment (CSERGE). p. 1 - 28.

8. Rebitzer, G., et al., Life cycle assessment: Part 1: Framework, goal and scope definition,

inventory analysis, and applications Environment International, 2004. 30(5): p. 701 - 720.



PRACTICE QUESTIONS

Q1: What are the steps involved in the design process of a product? Draw a sketch to

illustrate the relationship between these steps.

Q2: Differentiate between Mechanical Design and Machine Design.

Q3: Define Homogeneity and Isotropy.

Q4: What is LCA? Draw a sketch to demonstrate the relationship between different

phases of LCA?

Q5: What is the purpose of Inventory analysis in LCA?

Q6: Define factor of safety.

Q7: Usually, the FoS for brittle materials is made twice that would be used for ductile

materials in the same conditions. Why?

Q8: What is FEA?