Introduction

Machine Element Design

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• Walid Khraisat

What Is Design?

• Engineering design is a systematic process by

which solutions to the needs of humankind are

obtained

– Examples

– Lightweight,compact wireless communication devices

– High-temperature resistance material for reentry

vehicle

Machine Design

Machine Design is a process of selecting the

proper machine components for a predetermined

function.

To clarify this definition, machine is defined as a

system of machine elements selected or

designed for a specific function.

What Is Design?; cont’

• Teamwork with engineers, scientists,

environmentalists, economists, sociologists, legal

personnel, marketing personnel, etc

• Public sentiments as executed through

government regulations and political influence

(e.g. new transportation vehicle banned in

downtown, etc)

• Engineers must be beware of the impact of our

actions on society and the environment

What Is Design? • Object’s aesthetic appearance; Like clothes

• The process of applying the various techniques and scientific principles for the purpose of designing a device, a process or a system in sufficient detail to permit its realization

• Define, calculate motions, forces, and changes in energy sizes, shapes, materials

• Failure—stress and deflection analysis needed – Applied, inertial loads; Parts geometry; Analysis of the

forces, moments, and torques; Dynamics of the system

Engineering Design • Designare; To designate, mark out

– To outline, plan or plot, as action or work; To

conceive, invent, contrive

• Mechanism

– A device which transforms motion to some desirable

pattern and typically develops low forces and

transmits little power

• Machine

– Contains mechanism which are designed to provide

significant forces and transmit power

Machine Elements

MACHINE ELEMENTS

Gears, Springs, Bolts, ...

Electric motors, Electric switches, ...

Optical sensors, Sonic sensors, ...

Control circuits, ...

Microprocessors, ...

Creativity

Among the many factors which effect creativity

in design are; designer’s experience, designer’s ability, economic factors, and designer’s own preference.

A product should be designed to function under all of the possible severe

environmental conditions.

*10-step Design Process.

1. Identification of a need

2. Problem definition

3. Search

4. Constraints

5. Criteria

6. Alternative solutions

7. Analysis

8. Decision

9. Specification

10. Communication

1.Identify the

need

2. Define

problem

3. search

4. Constraints

5. criteria 6.

Alternative

solutions

7. Analysis

8. Decision

9.

Specification

10.

communication

Iterative Design Process

Two distinct and separate approaches 1. The deterministic, or factor-of-safety approach. In this

method, the maximum stress or stresses in a part are kept

below the minimum strength by a suitable design factor or

margin of safety, to ensure that the part will not fail.

2. The stochastic, or reliability approach. This method involves

the selection of materials, processing and dimensions such that

the probability of failure is always less than a pre selected value

Factor of Safety Method

The stochastic method doesnot use a design factor at all

rather attention is focused on the probability of survival –

Reliability-

Probability Density Function

Probability-Based Design

For any Normally distributed variable:

68.3% of all values will lie between μ −σ and μ + σ (i.e. μ ± σ ) 95.45% of all values will lie within μ ± 2 σ

99.73% of all values will lie within μ ± 3 σ

Reliability R

10 R

fPR 1

Pf Probability of Failure

Reliability Method of Design

2-2 Statistical Parameters 2-3 Probability Density Function

and Cumulative Distribution Function

R(z) is the integral of the standardized normal

distribution from -∞ to z0 (in other words, the

area under the curve to the left of z0)

Pf(z) =1-R

( the area under the curve to the right of z0)

Probability of failure is 0.1587 reliability is

0.8413

Answer:

FOS is equivalent to 1.33 gives a reliability of

84.13%.

Which is lower than 100% Reliability needed so it

is insufficient for the present design, therefore

there is a need to increase this factor

Ex:A round 1018 steel rod having yield strength (540,

40) Mpa is subjected to tensile load (220, 18) kN.

Determine the diameter of rod that results in a

reliability of 0.999 (z = -3.09). Table A10 0.001

Readings:

1-11

1-12

2-2

20-3