Institution of Engineers, Dhaka - 1000 May 24, 2015
MECHANICAL DESIGN PART - 01 Dr. Mohammad Motalab
BUET
Assistant Professor Department of Mechanical Engineering Bangladesh University of Engineering and Technology BUET, Dhaka - 1000
Department of Mechanical Engineering
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MACHINE DESIGN Introduction to Mechanical Engineering Design • Mechanical Engineering Design refers to the decision making processes which are used byfor thethe mechanical engineers in order to formulate plans physical realization of machines, devices, and systems. • The systematic and creative application of scientific and mathematical principles to practical ends such as the design, manufacture, and operation of efficient and economical structures, machines, processes, and systems is called Design. • The purpose of this course to understand these decision making processes and for applying them to practical situations.
BUET
Department of Mechanical Engineering
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.) • The ‘design’ is to formulate a plan for the satisfaction of human need. • An example illustrating need for design – Failure of a microprocessor component due to the thermal cycling operation. Example of using underfill material in microprocessor in order to minimize solder strains.
• Invention of GPS during world war-II.
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.) • Mechanical Design means the design of things and systems of devices, a mechanical nature – machines, products, structures, and instruments. • Mechanical Engineering Design includes all disciplines of mechanical engineering, such as the thermal and fluid sciences, too. • The first studies in mechanical engineering design are in mechanical design, which is the contents of the courses Machine Design-I and Machine Design-II.
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.) Design Consideration • The characteristics that influence the design of the machine or the entire system are referred as the design consideration. Some typical design considerations are strength, reliability, mechanical and thermal properties, life of the parts, weight, size and shape, etc. Standards Used • A standard is a set of specification of parts, materials, or processes intended to achieve uniformity, efficiency, and specified quality. BUET Department of Mechanical Engineering
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.) • Better control of inventory of tooling. • ASME, ASTM, AISC, AISI, BSI, etc. Strength • Strength is a property of a material or mechanical element. It depends upon the choice, the treatment of the material (forging, rolling, cold forming etc.), and the processing of the material. Example: Different strength of solder material for different aging, testing temperatures. • Strength is usually denoted as the S. S u is ultimate strength and Sy is yield strength. Example of a stressstrain curve of solder material. BUET Department of Mechanical Engineering
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.) Stress • Normal stress is denoted as and shear stress is denoted as . • Stress components: x , 1 etc. • Allowable Stress refers to the reduced value of strength that is used in design to determine the geometrical dimensions of parts in mechanical systems. The strength reduction is based on the specified standards. For example, according to AISC, for a member in tension, 0.45 0.60 BUET Department of Mechanical Engineering
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.) • The dimensions of Stress components are selected such as:
Factor of Safety • A design factor of safety n d, or n, sometimes called simply design factor, is defined by the relation: • Material internal flaw, size effect (example of standard deviation), load effect, computed stress, environmental concern (corrosive agent), for complicated can’t be accurately determined, accidental part load,stress danger to life or property, price class etc. BUET Department of Mechanical Engineering
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MACHINE DESIGN Introduction to Mechanical Engineering Design (Contd.)
• Factor of safety for brittle material
• Factor of safety for ductile material
Margin of safety, 1
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MACHINE DESIGN Factor of Safety for Different Load Kind of Load
Dead Load
Ductile Material
Cast Iron
(Steel) Based Based on Su on Sy
Based on ultimate strength
3-4
1.5-2
5-6
Timber
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Leather
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Repeated Load, one direction, gradual (mild shock)
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3
7-8
10
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Repeated, Reverse, Gradual (mild shock)
8
4
10-12
15
12
Shock
10-15
5-7
15-20
20
15
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MACHINE DESIGN Stress Analysis Pure Tension/ Simple Tension Characteristics of Simple Tension • Bar is straight and made of homogeneous material • Line of action of the force coincides with the center (axis) of the bar. • The section is taken much away from any discontinuity or abrupt change in the geometry. • The bar that fulfills these conditions is said to be in simple tension/ BUET Department of Mechanical Engineering
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MACHINE DESIGN Stress Analysis Engineering Stress/ Nominal Stress • The value may only be the average stress at a section. This is called ‘Engineering Stress’.
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MACHINE DESIGN Stress Analysis Pure Shear/ Simple Shear
• Normal Strain and Shear Strain
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MACHINE DESIGN Stress Analysis Isotropic Stress • The stress across any imaginary internal surface turns out to be equal in magnitude and always directed perpendicularly to the surface.
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MACHINE DESIGN Stress Analysis Stress-Strain Diagram • Proportional limit, yield stress, ultimate strength.
• Modulus of elasticity. BUET Department of Mechanical Engineering
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MACHINE DESIGN Three Dimensional Stress State at a Point
1st subscript in a stress notation Identifies the axis
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2nd subscript identifies the Direction of the stress component 16
MACHINE DESIGN Three Dimensional Stress State at a Point
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MACHINE DESIGN Stress-Strain Relation
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MACHINE DESIGN Stress-Strain Relation
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MACHINE DESIGN Stress-Strain Relation
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Design of Knuckle joint
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Design of Knuckle joint Two or more rods subjected to tensile and compressive forces are fastened together
Their axes are not in alignments but meet in a point
The joint allows a small angular momen of one rod relative to another
It can be easily connected and disconnected
Applications: Elevator chains, valve rods, etc
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2 1 ∗
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2
1 ∗
2 1 ∗
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Fatigue Failure
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Endurance Strength
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Endurance Strength Aluminum Alloy
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Endurance Strength
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Endurance Strength Surface Effects
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Endurance Strength Size Effects
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Endurance Strength Load Effects
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Endurance Strength Temperature Effects
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Endurance Strength Reliability Effects
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Endurance Strength Stress Concentration Effects
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Fluctuating Load
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Design for Fatigue Loading
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Design for Fatigue Loading
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Design of Shaft
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Design of Shaft
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Design of Shaft
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Design of Shaft
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Design of Shaft
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Design of Shaft
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Design of Shaft Torque Calculation
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Design of Shaft
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Design of Shaft
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Design of Shaft (Combined Load) Bending Stress
Torsional Stress
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Design of Shaft (Combined Load)
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Design of Shaft (Combined Load) ASME Design Formula for Fully Reversed Bending and Steady Torsion
For Bending, Axial, and Torsional Load: DE-ASME
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Design of Shaft Torsional Rigidity
Lateral Rigidity
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Design of Shaft
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Limits and Fits Related Terms
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Stress Concentration
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Stress Concentration Reducing the Stress Concentration Factor
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Stress Concentration Determining Stress Concentration Factor
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Stress Intensity Factor
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Stress Intensity Factor Determining Stress Intensity Factor
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Stress Intensity Factor Fracture Toughness
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Compression Members - Column
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Critical Load of Column
Euler formula Parabolic Expression
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Critical Load of Column
Euler formula Parabolic Expression
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Critical Load of Column Effects of End Conditions
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Critical Load of Column
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Eccentric Column Secant Formula for Eccentric Column
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Eccentric Column
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