1. The McGraw-Hill Companies © 2012

2. Chapter Outline We will cover sections 1 through 14 and 17
Shigley’s Mechanical Engineering Design

3. To formulate a plan for the satisfaction of a specified need
Design
To formulate a plan for the satisfaction of a specified need
Process requires innovation, iteration, and decision-making
Communication-intensive
Products should be
Functional
Safe
Reliable
Competitive
Usable
Manufacturable
Marketable
Shigley’s Mechanical Engineering Design

4. Mechanical Engineering Design
Mechanical engineering design involves all the disciplines of mechanical engineering.
Example
Journal bearing: fluid flow, heat transfer, friction, energy transport, material selection, thermomechanical treatments, statistical descriptions, etc.
Shigley’s Mechanical Engineering Design

5. Requires initial estimation, followed by continued refinement
The Design Process
Iterative in nature
Requires initial estimation, followed by continued refinement
Shigley’s Mechanical Engineering Design

6. Design Considerations
Some characteristics that influence the design
Shigley’s Mechanical Engineering Design

7. Computer-Aided Engineering (CAE)
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, procedural programming language, equation solver, etc.
Shigley’s Mechanical Engineering Design

8. Acquiring Technical Information
Libraries
Engineering handbooks, textbooks, journals, patents, etc.
Government sources
Government agencies, U.S. Patent and Trademark, National Institute for Standards and Technology, etc.
Professional Societies (conferences, publications, etc.)
American Society of Mechanical Engineers, Society of Manufacturing Engineers, Society of Automotive Engineers, etc.
Commercial vendors
Catalogs, technical literature, test data, etc.
Internet
Access to much of the above information
Shigley’s Mechanical Engineering Design

9. A Few Useful Internet Sites
Shigley’s Mechanical Engineering Design

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

11. Ethical Guidelines for Professional Practice
National Society of Professional Engineers (NSPE) publishes a Code of Ethics for Engineers and an Engineers’ Creed.
Six Fundamental Canons
Engineers, in the fulfillment of their professional duties, shall:
Hold paramount the safety, health, and welfare of the public.
Perform services only in areas of their competence.
Issue public statements only in an objective and truthful manner.
Act for each employer or client as faithful agents or trustees.
Avoid deceptive acts.
Conduct themselves honorably, responsibly, ethically, and lawfully so as to enhance the honor, reputation, and usefulness of the profession.
Shigley’s Mechanical Engineering Design

12. A set of specifications for parts, materials, or processes
Standards and Codes
Standard
A set of specifications for parts, materials, or processes
Intended to achieve uniformity, efficiency, and a specified quality
Limits the multitude of variations
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
Shigley’s Mechanical Engineering Design

13. Standards and Codes
Some organizations that establish standards and codes of particular interest to mechanical engineers:
Shigley’s Mechanical Engineering Design

14. Cost is almost always an important factor in engineering design.
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.
Shigley’s Mechanical Engineering Design

15. Close tolerances generally increase cost
Require additional processing steps
Require additional inspection
Require machines with lower production rates
Shigley’s Mechanical Engineering Design

16. A cost comparison between two possible production methods
Breakeven Points
A cost comparison between two possible production methods
Often there is a breakeven point on quantity of production
EXAMPLE
Automatic screw machine
25 parts/hr
3 hr setup
$20/hr labor cost
Hand screw machine
10 parts/hr
Minimal setup
Breakeven at 50 units
Shigley’s Mechanical Engineering Design

17. Safety and Product Liability
Strict Liability concept generally prevails in U.S.
Manufacturer is liable for damage or harm that results because of a defect.
Negligence need not be proved.
Calls for good engineering in analysis and design, quality control, and comprehensive testing.
Shigley’s Mechanical Engineering Design

18. An inherent property of a material or of a mechanical element
Stress and Strength
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
Stress
A state property at a specific point within a body
Primarily a function of load and geometry
Sometimes also a function of temperature and processing
Shigley’s Mechanical Engineering Design

19. Common sources of uncertainty in stress or strength
Shigley’s Mechanical Engineering Design

20. Based on statistical nature of the design parameters
Uncertainty
Stochastic 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
Shigley’s Mechanical Engineering Design

21. Establishes a design factor, nd
Uncertainty
Deterministic method
Establishes a design factor, nd
Based on absolute uncertainties of a loss-of-function parameter and a maximum allowable parameter
If, for example, the parameter is load, then
Shigley’s Mechanical Engineering Design

22. Example 1-1 Solution Answer Answer
Shigley’s Mechanical Engineering Design

23. Often used when statistical data is not available
Design Factor Method
Often used when statistical data is not available
Since stress may not vary linearly with load, it is more common to express the design factor in terms of strength and stress.
All loss-of-function modes must be analyzed, and the mode with the smallest design factor governs.
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.
The factor of safety is the realized design factor of the final design, including rounding up to standard size or available components.
Shigley’s Mechanical Engineering Design

24. Example 1-2 Solution Answer Answer
Shigley’s Mechanical Engineering Design

25. Reliability
Reliability, R – The statistical measure of the probability that a mechanical element will not fail in use
Probability of Failure, pf – the number of instances of failures per total number of possible instances
Example: If 1000 parts are manufactured, with 6 of the parts failing, the reliability is
or 99.4 %
Shigley’s Mechanical Engineering Design

26. Reliability
Series System – a system that is deemed to have failed if any component within the system fails
The overall reliability of a series system is the product of the reliabilities of the individual components.
Example: A shaft with two bearings having reliabilities of 95% and 98% has an overall reliability of
R = R1 R2 = 0.95 (0.98) = or 93%
Shigley’s Mechanical Engineering Design

27. Power Transmission Case Study Specifications
Shigley’s Mechanical Engineering Design

28. Power Transmission Case Study Specifications
Shigley’s Mechanical Engineering Design

29. Power Transmission Case Study Specifications
Shigley’s Mechanical Engineering Design

30. Chapter Outline
Shigley’s Mechanical Engineering Design

31. Example 1-2 Solution Answer Answer
Shigley’s Mechanical Engineering Design

32. Used to obtain material characteristics and strengths
Standard Tensile Test
Fig. 2–1
Used to obtain material characteristics and strengths
Loaded in tension with slowly increasing P
Load and deflection are recorded
Shigley’s Mechanical Engineering Design

33. The stress is calculated from
Stress and Strain
The stress is calculated from
where
is the original cross-sectional area.
The normal strain is calculated from
where l0 is the original gauge length and l is the current length corresponding to the current P.
Shigley’s Mechanical Engineering Design

34. Stress-Strain Diagram
Plot stress vs. normal strain
Typically linear relation until the proportional limit, pl
No permanent deformation until the elastic limit, el
Yield strength, Sy , defined at point where significant plastic deformation begins, or where permanent set reaches a fixed amount, usually 0.2% of the original gauge length
Ultimate strength, Su , defined as the maximum stress on the diagram
Ductile material
Brittle material
Fig. 2–2
Shigley’s Mechanical Engineering Design

35. Elastic Relationship of Stress and Strain
Slope of linear section is Young’s Modulus, or modulus of elasticity, E
Hooke’s law
E is relatively constant for a given type of material (e.g. steel, copper, aluminum)
See Table A-5 for typical values
Usually independent of heat treatment, carbon content, or alloying
Fig. 2–2 (a)
Shigley’s Mechanical Engineering Design

36. True Stress-Strain Diagram
Engineering stress-strain diagrams (commonly used) are based on original area.
Area typically reduces under load, particularly during “necking” after point u.
True stress is based on actual area corresponding to current P.
True strain is the sum of the incremental elongations divided by the current gauge length at load P.
Note that true stress continually increases all the way to fracture.
Engineering stress-strain
True Stress-strain
(2-4)
Shigley’s Mechanical Engineering Design

37. Compression tests are used to obtain compressive strengths.
Compression Strength
Compression tests are used to obtain compressive strengths.
Buckling and bulging can be problematic.
For ductile materials, compressive strengths are usually about the same as tensile strengths, Suc = Sut .
For brittle materials, compressive strengths, Suc , are often greater than tensile strengths, Sut .
Shigley’s Mechanical Engineering Design

38. Torsional Strengths
Torsional strengths are found by twisting solid circular bars.
Results are plotted as a torque-twist diagram.
Shear stresses in the specimen are linear with respect to the radial location – zero at the center and maximum at the outer radius.
Maximum shear stress is related to the angle of twist by
q is the angle of twist (in radians)
r is the radius of the bar
l0 is the gauge length
G is the material stiffness property called the shear modulus or modulus of rigidity.
Shigley’s Mechanical Engineering Design

39. Maximum shear stress is related to the applied torque by
Torsional Strengths
Maximum shear stress is related to the applied torque by
J is the polar second moment of area of the cross section
For round cross section,
Torsional yield strength, Ssy corresponds to the maximum shear stress at the point where the torque-twist diagram becomes significantly non-linear
Modulus of rupture, Ssu corresponds to the torque Tu at the maximum point on the torque-twist diagram
Shigley’s Mechanical Engineering Design

40. Modulus of resilience, uR
Resilience – Capacity of a material to absorb energy within its elastic range
Modulus of resilience, uR
Energy absorbed per unit volume without permanent deformation
Equals the area under the stress- strain curve up to the elastic limit
Elastic limit often approximated by yield point
Shigley’s Mechanical Engineering Design

41. Area under curve to yield point gives approximation
Resilience
Area under curve to yield point gives approximation
If elastic region is linear,
For two materials with the same yield strength, the less stiff material (lower E) has greater resilience
Shigley’s Mechanical Engineering Design

42. Toughness – capacity of a material to absorb energy without fracture
Modulus of toughness, uT
Energy absorbed per unit volume without fracture
Equals area under the stress-strain curve up to the fracture point
Shigley’s Mechanical Engineering Design

43. Area under curve up to fracture point
Toughness
Area under curve up to fracture point
Often estimated graphically from stress-strain data
Approximated by using the average of yield and ultimate strengths and the strain at fracture
Shigley’s Mechanical Engineering Design

44. Resilience and Toughness
Measures of energy absorbing characteristics of a material
Units are energy per unit volume
lbf·in/in3 or J/m3
Assumes low strain rates
For higher strain rates, use impact methods (See Sec. 2-5)
Shigley’s Mechanical Engineering Design

45. Two most common hardness-measuring systems Rockwell A, B, and C scales
Hardness – The resistance of a material to penetration by a pointed tool
Two most common hardness-measuring systems
Rockwell
A, B, and C scales
Specified indenters and loads for each scale
Hardness numbers are relative
Brinell
Hardness number HB is the applied load divided by the spherical surface area of the indentation
Shigley’s Mechanical Engineering Design

46. Strength and Hardness
For many materials, relationship between ultimate strength and Brinell hardness number is roughly linear
For steels
For cast iron
Shigley’s Mechanical Engineering Design

47. Example 2-2
Shigley’s Mechanical Engineering Design

48. Material Numbering Systems
Common numbering systems
Society of Automotive Engineers (SAE)
American Iron and Steel Institute (AISI)
Unified Numbering System (UNS)
American Society for Testing and Materials (ASTM) for cast irons
Shigley’s Mechanical Engineering Design

49. UNS system established by SAE in 1975
UNS Numbering System
UNS system established by SAE in 1975
Letter prefix followed by 5 digit number
Letter prefix designates material class
G – carbon and alloy steel
A – Aluminum alloy
C – Copper-based alloy
S – Stainless or corrosion-resistant steel
Shigley’s Mechanical Engineering Design

50. For steel, letter prefix is G
UNS for Steels
For steel, letter prefix is G
First two numbers indicate composition, excluding carbon content
Second pair of numbers indicates carbon content in hundredths of a percent by weight
Fifth number is used for special situations
Example: G52986 is chromium alloy with 0.98% carbon
Shigley’s Mechanical Engineering Design