Design of Machine Elements

Chapter 1: Introduction
What is a machine?
A machine is a device that transforms or transmits energy to perform a specific
task, typically involving mechanical motion. (But, ML?!)

Key characteristics:

Composed of multiple components (machine elements).

Involves energy conversion or transmission

Designed to achieve specific function(s) (e.g., lifting, cutting, transporting).

Examples:

Simple machines: Lever, pulley, screw .

Complex machines: Car engine, wind turbine, CNC machine.
Cut-Section of an Engine

https://www.howacarworks.com/basics/the-engine

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What are machine elements?

Machine elements are the fundamental components that make

Examples:

Shafts

Screws, Fasteners

Weldments

Springs

Bearings

Gears

Clutches

Brakes

Couplings

Flywheels

Belts 4

Chains
Gearbox

https://sunwayautoparts.com/understanding-car-transmission-system-and-transmission-parts/

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 Design
To formulate a plan for the satisfaction of a specified need
● Process requires innovation, iteration, and decision-making
● Communication-intensive
● Engineering Design ≠ Invention
Products should be
• Functional
• Safe
• Reliable
• Competitive
• Usable
• Manufacturable
• Marketable

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 Mechanical Engineering Design

Mechanical engineering design involves all the disciplines of

Example
• Journal bearing: fluid flow, heat transfer, friction, energy
transport, material selection, thermomechanical treatments,
statistical descriptions, etc.
• Almost everything involves mechanics of solids and, certainly,
mechanics.

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 The Design Process

• Iterative in nature
• Requires initial estimation,
followed by continued
refinement

• Earlier subjects
• (Mechanics of Solids,
• Fluid Mechanics)
• focussed primarily on
Analysis and Evaluation
Fig. 1‒1

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 Design Considerations
Some characteristics that influence the design
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. Remanufacturing/resource recovery
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 Computational Tools
Computer-Aided Engineering (CAE)
• Any use of the computer and software to aid in the
engineering process
• Includes
• Computer-Aided Design (CAD)
• Drafting, 3-D solid modeling, etc.
• Computer-Aided Manufacturing (CAM)
• CNC toolpath, rapid prototyping, etc.
• Engineering analysis and simulation
• Finite element, fluid flow, dynamic analysis, motion, etc.
• Math solvers
• Spreadsheet, programming language, equation solver, etc.

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 A Few Useful Internet Sites
 www.globalspec.com
 www.engnetglobal.com
 www.efunda.com
 www.thomasnet.com
 www.uspto.gov
 www.machinedesign.com
 www.powertransmission.com

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 The Design Engineer’s Professional Responsibilities
Satisfy the needs of the customer in a competent, responsible,
ethical, and professional manner.
Some key advice for a professional engineer
• Be competent
• Keep current in field of practice
• Keep good documentation
• Ensure good and timely communication
• Act professionally and ethically

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 Standards and Codes
Standards
• A set of specifications for parts, materials, or processes
• Intended to achieve uniformity, efficiency, and a specified
quality
• Defines a recognized good practice, or an agreed upon
uniformity, or a minimum level of acceptability
• May be generated within a company, across an industry,
within a country, or internationally.
• Limits the multitude of variations

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 Standards and Codes (continued)
Design Code
• A set of specifications for the analysis, design, manufacture,
and construction of something
• To achieve a specified degree of safety, efficiency, and
performance or quality
• Does not imply absolute safety
Various organizations establish and publish standards and codes
for common and/or critical industries

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 Standards and Codes (continued)
• Some organizations that establish standards and codes of
particular interest to mechanical engineers:
Aluminum Association (AA)
American Bearing Manufacturers Association (ABMA)
American Gear Manufacturers Association (A GMA)
American Institute of Steel Construction (AISC)
American Iron and Steel Institute (AISI)
American National Standards Institute (A NSI)
American Society of Heating, Refrigerating and Air-Conditioning Engineers (A SHRAE)
American Society of Mechanical Engineers (ASME)
American Society of Testing and Materials (ASTM)
American Welding Society (AWS)
ASM International
British Standards Institution (BSI)
Industrial Fasteners Institute (IFI)
Institute of Transportation Engineers (I TE)
Institution of Mechanical Engineers (IMechE)
International Bureau of Weights and Measures (BIPM)
International Federation of Robotics (IFR)
International Standards Organization (ISO)
National Association of Power Engineers (N APE)
National Institute for Standards and Technology (N IST)
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Society of Automotive Engineers (SAE)
 In India

Bureau of Indian Standards: https://www.bis.gov.in/

Example:
Boilers and Pressure Vessels Code in India: IS 2825

https://law.resource.org/pub/in/bis/manifest.med.1.html

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 Economics
• Cost is almost always an important factor in
engineering design.
• Use of standard sizes is a first principle of cost
reduction.
• Table A‒17 lists some typical preferred sizes.
• Certain common components may be less expensive in
stocked sizes.

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18
 Stress and Strength
Stress
• A state property at a specific point within a body
• Primarily a function of load and geometry (e.g., F∕A, My∕I)
• Sometimes also a function of temperature and processing

Strength
• An inherent property of a material or of a mechanical element
• Depends on treatment and processing
• May or may not be uniform throughout the part
• Examples: Ultimate strength, yield strength

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 Uncertainty
Common sources of uncertainty in stress or strength
• Composition of material and the effect of variation on properties.
• Variations in properties from place to place within a bar of stock.
• Effect of processing locally, or nearby, on properties.
• Effect of nearby assemblies such as weldments and shrink fits on stress
conditions.
• Effect of thermomechanical treatment on properties.
• Intensity and distribution of loading.
• Validity of mathematical models used to represent reality.
• Intensity of stress concentrations.
• Influence of time on strength and geometry.
• Effect of corrosion.
• Effect of wear.
• Uncertainty as to the length of any list of uncertainties.

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 Modeling Uncertainty
Deterministic method (factor of safety method)
• Determines values for a “strength” parameter and a “stress”
parameter.
• Provides sufficient margin between the two to predict
successful functionality.
• Often used when statistical data is not available

•
Stochastic method (reliability method)
• Based on statistical nature of the design parameters
• Focus on the probability of survival of the design’s function
(reliability)
• Often limited by availability of statistical data
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 Factor of Safety
• The factor of safety is the ratio of a loss-of-function parameter
(e.g. yield strength, allowable load, allowable deflection, etc.) to
an applied parameter (e.g. stress, load, deflection, etc.).
loss­of­function parameter
factor of safety n  (1‒1)
applied parameter

• One of the most common parameters for factor of safety is stress

(1‒2)

• Stress and strength terms must be of the same type and units.
• Stress and strength must apply to the same critical location in
the part.
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 Factor of Safety (continued)
• A factor of safety less than unity predicts failure.
• A factor of safety greater than unity predicts a successful design.
• Neither case provides any information about statistical
percentage of failures.
• A higher factor of safety may increase confidence in a greater
reliability, but there is no data to quantify that confidence.
• All loss-of-function modes must be analyzed, and the mode with
the smallest design factor governs.

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 Factor of Safety and Design Factor
• A design process may start with a factor of safety that it is
desired to achieve.
• The desired goal is usually called a design factor, nd .
• The design factor may be used to solve for the maximum
allowable value of the applied parameter.
loss­of­function parameter
Maximum allowable parameter  (1‒3)
nd

• A realized design, with its use of standard sizes and rounded

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 Selection of Design Factor
Selection of a design factor should include consideration of
• Accuracy of the prediction of the situation, including loads,
strengths, wear, environment, manufacturing quality,
maintenance, etc.
• Cost of overachieving the requirements
• Consequences of failure
Guidance comes from
• Experience within the industry
• Applicable codes and standards

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Example

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