Introduction, Slide 1ME 422 – Machine Design I

Glen Prater, Jr.

Associate Professor and ChairmanSH 200, 588-6331, gprater@louisville.edu

Office hours: M, W, F, 10:00-10:50, or by appointment

Mechanical Engineering DepartmentUniversity of Louisville

Louisville, Kentucky

Fall Semester 2001

ME 422 – Machine Design I

Introduction, Slide 2ME 422 – Machine Design I

Course Overview

Topics

• The engineering design process

• Fundamental concepts related to the designof mechanical components and machines

• Design for strength and reliability

• Machine component design (fasteners,weldments, springs)

• Open-ended design projects

Textbook

Mechanical Engineering Design, Shigley and Mischke, 6th Edition

Stress distributions in an axially loaded rectangular bar with different stress raisers:

filleted shoulder, central circular hole, U-notches

Introduction, Slide 3ME 422 – Machine Design I

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

Introduction, Slide 4ME 422 – Machine Design I

Grading

Course grades will be based on selectively graded homework, four quizzes, two midterm examinations, three design projects, and a final examination:

Homework 10%

Quizzes 3@5%

Midterm Exams 2@15%

Design Projects 2@12.5%

Final Exam 20%

The scale below will be used to assign letter grades. These percentages may be lowered, depending upon the class score distribution.

90-100% A

80-89% B

65-79% C

50-64% D

Introduction, Slide 5ME 422 – Machine Design I

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

Introduction, Slide 6ME 422 – Machine Design I

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

Introduction, Slide 7ME 422 – Machine Design I

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.

• 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.”

• The primary objective of machine design is synthesis, or creation, not analysis. Analysis is a tool that serves as a means toward an end.

Increasing Stress

Finite element model of a pickup truck floorpan assembly

Introduction, Slide 8ME 422 – Machine Design I

The Traditional Design Process

Introduction, Slide 9ME 422 – Machine Design I

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

• Perform supporting analyses (thermodynamic, heat transfer, fluid mechanics, kinematics, force, stress, life, cost, compatibility with special constraints)

• Select the best mechanism

• Document the designAlternative design concepts for cross members

in a light-duty truck floorpan assembly

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.

Concept 6 Two integrated, split transverse cross members, rear transverse member similar to original, reduced sheet thickness in cross members.

Introduction, Slide 10ME 422 – Machine Design I

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)

• Select the best component designs

• Document component designs; prepare a layout drawing

A-pillar component geometries

FrontReinforcement

CornerReinforcement

Introduction, Slide 11ME 422 – Machine Design I

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

Introduction, Slide 12ME 422 – Machine Design I

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 - Cost - Thermal properties- Distortion/stiffness - Processing requirements - Surface finish- Wear - Weight - Lubrication- Corrosion - Life - Marketability- Safety - Noise - Maintenance- Reliability - Aesthetic considerations - Volume- Friction - Shape - Liability- Usability/utility - Size - Scrapping/recyclability

Introduction, Slide 13ME 422 – Machine Design I

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.”

Introduction, Slide 14ME 422 – Machine Design I

ASME Standards and Codes Related to Standardization

A112 Plumbing Materials and Equipment B1 Screw Threads B5 Machine Tools - Components, Elements, Performance, and Equipment B18 Standardization of Fasteners B29 Chains, Attachments and Sprockets for Power Transmission and Conveying

B32 Metal and Metal Alloy Wrought Mill Product Nominal Sizes B40 Standards for Pressure and Temperature Instruments and Accessories B46 Classification and Designation of Surface Qualities B47 Gage Blanks B73 Chemical Standard Pumps B89 Dimensional Metrology B94 Cutting Tools, Drivers, and Bushings B107 Hand Tools and Accessories B133 Gas Turbine Procurement HST Overhead Hoists MFC Measurement of Fluid Flow in Closed Conduits MH1 Pallets, Slip Sheets, and Other Bases For Unit Loads SRB Slew Ring Bearing STS Steel Stacks Y14 Engineering Drawing and Related Documentation Practices

Introduction, Slide 15ME 422 – Machine Design I

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

Introduction, Slide 16ME 422 – Machine Design I

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

Introduction, Slide 17ME 422 – Machine Design I

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

Introduction, Slide 18ME 422 – Machine Design I

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

Introduction, Slide 19ME 422 – Machine Design I

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.

Introduction, Slide 20ME 422 – Machine Design I

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.

Introduction, Slide 21ME 422 – Machine Design I

Dealing With Uncertainty (1)

stress

iprelationshstrengthtoloadtheinexponentm

factordesignn

n

strengthmd

all

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:

Introduction, Slide 22ME 422 – Machine Design I

Dealing With Uncertainty (2)

stress allowablemean

strengthmeanS

factor designmeann

n

S

all

dall

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:

Stochastic Method

• Does not use a design factor. Based upon the concept of reliability, R:

0.1R0