ME 422 – Machine Design I
Glen Prater, Jr.
Associate Professor and Chairman
SH 200, 588-6331, gprater@louisville.edu
Office hours: M, W, F, 10:00-10:50, or by appointment
Mechanical Engineering Department
University of Louisville
Louisville, Kentucky
Fall Semester 2001
ME 422 – Machine Design I
Introduction, Slide 1
Course Overview
Topics
• The engineering design process
• Fundamental concepts related to the design
of mechanical components and machines
• Design for strength and reliability
• Machine component design (fasteners,
weldments, springs)
• Open-ended design projects
Textbook
Stress distributions in an axially loaded
rectangular bar with different stress raisers:
filleted shoulder, central circular hole, U-notches
Mechanical Engineering Design, Shigley and Mischke, 6th Edition
ME 422 – Machine Design I
Introduction, Slide 2
Course Outcomes
• Ability to apply knowledge of mathematics, science, and engineering in the field of
mechanical engineering
• Ability to design a system, component, or process to meet desired needs in the
field of mechanical engineering
• Ability to identify, formulate and solve mechanical engineering problems
• Understanding of professional and ethical responsibility in the field of mechanical
engineering
• Ability to communicate effectively
• Recognition of the need for, and an ability to engage in, life-long learning in the
field of mechanical engineering
• Ability to use the techniques, skills, and modern tools necessary for the practice of
mechanical engineering
ME 422 – Machine Design I
Introduction, Slide 3
Grading
Course grades will be based on selectively graded homework, four quizzes,
two midterm examinations, three design projects, and a final examination:
Homework
Quizzes
Midterm Exams
Design Projects
Final Exam
10%
3@5%
2@15%
2@12.5%
20%
The scale below will be used to assign letter grades. These percentages may
be lowered, depending upon the class score distribution.
90-100%
80-89%
65-79%
50-64%
ME 422 – Machine Design I
A
B
C
D
Introduction, Slide 4
Code of Ethics for Engineers (1)
Fundamental Principles
Engineers uphold and advance the integrity, honor, and dignity of the
Engineering profession by:
• using their knowledge and skill for the enhancement of human welfare;
• being honest and impartial, and serving with fidelity the public, their
employers and clients, and
• striving to increase the competence and prestige of the engineering
profession.
The American Society of Mechanical Engineers
ME 422 – Machine Design I
Introduction, Slide 5
Code of Ethics for Engineers (2)
Fundamental Canons
• Engineers shall hold paramount the safety, health and welfare of the
public in the performance of their professional duties.
• Engineers shall perform services only in the areas of their competence.
• Engineers shall continue their professional development throughout their
careers and shall provide opportunities for the professional development
of those engineers under their supervision.
• Engineers shall act in professional matters for each employer or client as
faithful agents or trustees, and shall avoid conflicts of interest.
• Engineers shall build their professional reputations on the merit of their
services and shall not compete unfairly with others.
• Engineers shall associate only with reputable persons or organizations.
• Engineers shall issue public statements only in an objective and truthful
manner.
The American Society of Mechanical Engineers
ME 422 – Machine Design I
Introduction, Slide 6
Machine Design
Machine Design…
…is an iterative process that has as its primary objective
the synthesis of machines in which the critical problems
are based upon material sciences and engineering
mechanics sciences.
This synthesis involves the creative conception of
mechanisms, and optimization with respect to
performance, reliability and cost.
Increasing Stress
• Machine design does not encompass the entire field
of mechanical engineering. Design where the critical
problems involve the thermal/fluid sciences fall under
the broader category of “mechanical engineering
design.”
Finite element model of a pickup
truck floorpan assembly
• The primary objective of machine design is synthesis,
or creation, not analysis. Analysis is a tool that serves
as a means toward an end.
ME 422 – Machine Design I
Introduction, Slide 7
The Traditional Design Process
ME 422 – Machine Design I
Introduction, Slide 8
Preliminary Design Phase
Often the first step in which a designer becomes involved,
and may not involve intense iteration. In this phase, we
deal with the entire machine:
• Define function
• Identify constraints involving cost, size, etc.
• Develop alternative conceptions of
mechanism/process combinations that can satisfy the
constraints
Concept 1 Two longitudinal members, one trans-verse
split-end cross member, small transverse member in
transmission tunnel, rear transverse member similar to
original, gauge reduction.
• Perform supporting analyses (thermodynamic, heat
transfer, fluid mechanics, kinematics, force, stress,
life, cost, compatibility with special constraints)
• Select the best mechanism
Concept 6 Two integrated, split transverse cross
members, rear transverse member similar to original,
reduced sheet thickness in cross members.
• Document the design
Alternative design concepts for cross members
in a light-duty truck floorpan assembly
ME 422 – Machine Design I
Introduction, Slide 9
Intermediate Design Phase
Generally occurs after preliminary design, but the two
phases may overlap. Intermediate design always involves
iterations. In this phase, we deal with individual
components of the machine:
• Identify components
• Define component functions
• Identify constraints involving cost, size, etc.
• Develop tentative conceptions of the components
mechanism/process combinations using good form
synthesis principles
• Perform supporting analyses (including analyses at
each critical point in each component)
Front
Reinforcement
Corner
Reinforcement
A-pillar component geometries
• Select the best component designs
• Document component designs; prepare a layout
drawing
ME 422 – Machine Design I
Introduction, Slide 10
Detail Design Phase
Subsequent to intermediate and. In this phase,
we deal with individual components of the
machine and the machine as a whole:
• Select manufacturing and assembly
processes
• Specify dimensions and tolerances
• Prepare component detail drawings
• Prepare assembly drawings
Line rendering of a pickup box assembly showing
geometric details such as wheel well openings,
cross members, and bed corrugation
Lecture material in this course focuses on the preliminary and intermediate
phases. The design projects will involve elements of detail design
ME 422 – Machine Design I
Introduction, Slide 11
Design Considerations
The design of a component or system may be influenced by a number of requirements.
If a requirement affects design, it is called a design consideration. For example, if the
ability to carry large loads without failure is important, we say that strength is a design
consideration. Most product development projects involve a number of design
considerations:
- Strength/stress
- Distortion/stiffness
- Wear
- Corrosion
- Safety
- Reliability
- Friction
- Usability/utility
- Cost
- Processing requirements
- Weight
- Life
- Noise
- Aesthetic considerations
- Shape
- Size
ME 422 – Machine Design I
- Thermal properties
- Surface finish
- Lubrication
- Marketability
- Maintenance
- Volume
- Liability
- Scrapping/recyclability
Introduction, Slide 12
Standards and Codes
Standards and codes represent a prescriptive approach to design that may be
incorporated into a design process.
Standards
A set of technical definitions and guidelines for designers and manufacturers.
Standards are written by “experts” and are considered voluntary. ASME
groups develops and maintains standards using committees.
Code
A set of standards that has been adopted by one or more governmental
bodies or incorporated into a contract. Essentially, a code is a set of standards
with the force of law behind it.
According to its web site, ASME “maintains and distributes 600 codes and
standards used around the world for the design, manufacturing and installation of
mechanical devices.”
ME 422 – Machine Design I
Introduction, Slide 13
ASME Standards and Codes Related to Standardization
A112
B1
B5
B18
B29
B32
B40
B46
B47
B73
B89
B94
B107
B133
HST
MFC
MH1
SRB
STS
Y14
Plumbing Materials and Equipment
Screw Threads
Machine Tools - Components, Elements, Performance, and Equipment
Standardization of Fasteners
Chains, Attachments and Sprockets for Power Transmission and Conveying
Metal and Metal Alloy Wrought Mill Product Nominal Sizes
Standards for Pressure and Temperature Instruments and Accessories
Classification and Designation of Surface Qualities
Gage Blanks
Chemical Standard Pumps
Dimensional Metrology
Cutting Tools, Drivers, and Bushings
Hand Tools and Accessories
Gas Turbine Procurement
Overhead Hoists
Measurement of Fluid Flow in Closed Conduits
Pallets, Slip Sheets, and Other Bases For Unit Loads
Slew Ring Bearing
Steel Stacks
Engineering Drawing and Related Documentation Practices
ME 422 – Machine Design I
Introduction, Slide 14
ASME Standards for Screw Threads (1)
B1.1-1989 Unified Inch Screw Threads (UN and UNR Thread Form)
B1.2-1983 (R1991) Gages and Gaging for Unified Inch Screw Threads
B1.3-1992 Screw Thread Gaging Systems for Dimensional Acceptability – Inch and Metric
Screw Threads (UN, UNR, UNJ, M, and MJ)
B1.5-1997 Acme Screw Threads
B1.7M-1984 (R1992) Nomenclature, Definitions and Letter Symbols for Screw Threads
B1.8-1988 (R1994) Stub Acme Screw Threads
B1.11-1958 (R1994) Microscope Objective Thread
B1.12-1987 (R1998) Class 5 Interference-Fit Thread
B1.13M-1995 Metric Screw Threads – M Profile
B1.15-1995 Unified Inch Screw Threads
B1.16M-1984 (R1992) Gages and Gaging for Metric M Screw Threads
B1.20.1-1983 (R1992) Pipe Threads, General Purpose (Inch)
B1.20.7-1991 (R1998) Hose Coupling Screw Threads (Inch)
B1.21M-1997 Metric Screw Threads – MJ Profile
B1.22M-1985 (1992) Gages And Gaging Practice For "MJ" Series Metric Screw Threads
B1.30M-1992 Screw Threads – Standard Practice for Calculating and Rounding Dimensions
ME 422 – Machine Design I
Introduction, Slide 15
ASME Standards for Screw Threads (2)
B1.1-1989 Unified Inch Screw Threads (UN and UNR Thread Form)
Scope: This Standard specifies the thread form, series, class, allowance, tolerance,
and designation for unified screw threads. (In order to emphasize that unified screw
threads are based on inch modules, they may be denoted unified inch screw threads.)
Several variations in thread form have been developed for unified threads; however,
this Standard covers only UN and UNR thread forms.
For easy reference, a metric translation of this Standard has been incorporated as
Appendix C. Appendices A through C contain useful information that is supplementary
to the sections of this Standard.
Order No. M020889 $55.00
ME 422 – Machine Design I
Introduction, Slide 16
ASME Standards for Screw Threads (3)
B1.7M-1984 (R1992) Nomenclature, Definitions and Letter Symbols for Screw Threads
Scope: The purpose of this Standard is to establish uniform practices for standard
screw threads with regard to the following:
a. Screw thread nomenclature, and
b. Letter symbols for designating features of screw threads for use on drawings, in
tables of dimensions which set forth dimensional standards and in other records,
and for expressing mathematical relationship.
This Standard consists of a glossary of terms, and illustrated table showing the
application of symbols, and a table of thread series designations. Many of the terms
and symbols specified in this Standard vary considerably from those of previous issues
because ISO terms and symbols have been adopted where the intended definition is
the same.
Order No. L00011 $32.00
ME 422 – Machine Design I
Introduction, Slide 17
Economics
Strength, safety, reliability, and cost are perhaps the most important design
considera-tions. In general the design alternative that satisfies other design
considerations at the lowest costs is to be preferred. Issues affecting the
“cost” of a design include:
•
•
•
•
•
•
Product development costs
Material choice
Manufacturing processes involved
Economies of scale
Tolerances specified
Use of standard sizes and
components
Breakeven point for two different screw manufacturing processes
ME 422 – Machine Design I
Introduction, Slide 18
Safety and Reliability
Safety is paramount, most importantly because it is an ethical issue. Safety is
also related to function. Safe designs tend to function well and perform
reliably.
The United States law recognizes the concept of strict reliability. The
manufacturer of a product is responsible for any damage or harm that arises
due to a defect in the product. It doesn’t matter how long after manufacture
the damage occurs, or if the defect is due to a design flaw or manufacturing
error. Negligence does not have to be proven. A plaintiff only has to establish
that the product was defective and that the defect caused damage or harm.
ME 422 – Machine Design I
Introduction, Slide 19
Uncertainty – Inherent in Engineering Design
Sources of Uncertainty
• Random variables associated with material processing result in strength
distributions that vary from sample to sample. Some samples will have
strengths greater than the specified value. Others – hopefully a very few
– will have strengths lower than the specified value.
• Statistical scatter in critical dimensions specified into the design during
the detail design phase due to imperfections in manufacturing processes.
• Approximations used in the analytical expressions used to perform
design calculations.
• Inexact knowledge of the magnitude and tie history of external loads.
• Effect of corrosion and wear on strengths.
ME 422 – Machine Design I
Introduction, Slide 20
Dealing With Uncertainty (1)
Permissible Stress Method
• Permissible stress in a design is based upon a fraction of material
strength. The actual fraction is based upon experience with successful
designs. Still used by civil engineers and for the design of weldments.
Design Factor Method
• There is a difference between a design goal, which may be based upon
experience (often involving load) and design realization which is based
upon a specific failure criterion (often involving stress) quantified by a
strength value:
all 
strength
nm
d
n  design factor
m  exponent in the load to strength relationsh ip
  stress
ME 422 – Machine Design I
Introduction, Slide 21
Dealing With Uncertainty (2)
Stochastic Design Factor Method
• Stochastic means involving random variables, and uncertainty in strength
and stress can be statistically quantified.
• The design factor equation can then be adapted to use to determine a
mean design factor. For a linear load stress relationship:
all 
S
nd
n  mean design factor
S  mean strength
all  mean allowable stress
Stochastic Method
• Does not use a design factor. Based upon the concept of reliability, R:
0  R  1.0
ME 422 – Machine Design I
Introduction, Slide 22