Table Of ContentChapter 1
1-1. Define engineering design and elaborate on each important concept in the definition.
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Solution
(Ref. 1.2) Engineering design is an iterative decision-making process that has the objective of creating and
optimizing a new or improved engineering system or device for the fulfillment of a human need or desire, with
regard for conservation of resources and environmental impact.
The essence of engineering (especially mechanical design) is the fulfillment of human needs and desires. Whether a
designer is creating a new device of improving an existing design, the objective is always to provide the “best”, or
“optimum” combination of materials and geometry. Unfortunately, an absolute optimum can rarely be achieved
because the criteria of performance, life, weight, cost, etc. typically place counter-opposing demands upon any
proposed combination of material and geometry.
Designers must not only compete in the marketplace, but must respond to the clear and growing obligation of the
global technical community to conserve resources and preserve the environment.
Finally, iteration, or cut-and-try pervades design methodology. Selection of the best material and geometry are
typically completed through a series of iterations.
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1-2. List several factors that might be used to judge how well a proposed design meets its specified objectives.
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Solution
(Ref. 1.3) The following factors might be used:
(1) Ability of parts to transmit required forces and moments.
(2) Operation without failure for prescribed design life.
(3) Inspectability of potential critical points without disassembly.
(4) Ability of machine to operate without binding or interference between parts.
(5) Ease of manufacture and assembly.
(6) Initial and life-cycle costs.
(7) Weight of device and space occupied.
(8) Ability to service and maintain.
(9) Reliability, safety, and cost competitiveness.
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1.3 Define the term optimum design, and briefly explain why it is difficult to achieve an optimum solution to a
practical design problem.
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Solution
A dictionary definition of adequate is “sufficient for a specified requirement”, and for the word optimum is “greatest
degree attainable under implied or specified conditions”. In a machine design context, adequate design may
therefore be defined as the selection of material and geometry for a machine element that satisfies all of its specified
functional requirements, while keeping any undesirable effects within tolerable ranges. In the same context, optimal
design may be defined as the selection of material and geometry for a machine element with specific the objective of
maximizing the part’s ability to address the most significant functional requirements, making sure that all other
functional requirements are adequately satisfied, and that any undesirable effects are kept within tolerable ranges.
Optimum design of real mechanical elements is complicated by the need to study relationships between and among
functions that embody many variables such as performance, life, weight, cost, and safety. Unfortunately, these
variables place counter-opposing demands upon and selected combination of materials and geometry; design
changes that improve the part’s ability to respond to one significant performance parameter may, at the same time,
degrade its ability to respond to another important parameter. Thus, an absolute optimum design can rarely be
achieved.
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1-4. When to stop calculating and start building is an engineering judgment of critical importance. Write about 250
words discussing your views on what factors are important in making such a judgment.
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Solution
The decision to stop calculating and start building is a crucial engineering responsibility. To meet design objectives,
a designer must model the machine and each of its parts, make appropriate simplifying assumptions where needed,
gather data, select materials, develop mathematical models, perform calculations, determine shapes and sizes,
consider pertinent failure modes, evaluate results, and repeat the loop of actions just listed until a “best” design
configuration is achieved. Questions always arise at each step in the design sequence. For example:
(1) What assumptions should be made, how many, how detailed, how refined?
(2) Are data available on loading spectra, environmental conditions, user practice, or must testing be
conducted?
(3) Are materials data available for the failure modes and operating conditions that pertain, and where are the
data, or must testing be conducted?
(4) What types of modeling and calculation techniques should be used; standard or special, closed-form or
numerical, P-C, workstation, or supercomputer?
(5) How important are reliability, safety, manufacturing, and/or maintainability?
(6) What is the competition in the marketplace for producing this product?
Often, the tendency of an inexperienced new engineer is to model, analyze, calculate, and refine too much, too often,
and too long, loosing market niche or market share as a consequence. On the other hand, the “old-timer” in the
design department often tends to avoid the analysis and build the product “right away”, risking unforeseen problems
in performance, safety, reliability, or manufacturability at high cost. Although dependent upon the product and the
application, the engineering decision to stop calculating and start building is always crucial to success.
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1-5. The stages of design activity have been proposed in 1.6 to include preliminary design, intermediate design,
detail design, and development and field service. Write a two- or three-sentence descriptive summary of the essence
of each of these four stages of design.
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Solution
(1) Preliminary design is primarily concerned with synthesis, evaluation, and comparison of proposed
machine or system concepts. The result of the preliminary design stage is the proposal of a likely-
successful concept to be designed in depth to meet specific criteria of performance, life, weight,
cost, safety, or other aspects of the overall project.
(2) Intermediate design embodies the spectrum of in depth engineering design of individual
components and subsystems for the already pre-selected machine or system. The result of the
intermediate design stage is the establishment of all critical specifications relating to function,
manufacturing, inspection, maintenance, and safety.
(3) Detail design is concerned mainly with configuration, arrangement, form, dimensional
compatibility and completeness, fits and tolerances, meeting specifications, joints, attachment and
retention details, fabrication methods, assemblability, productibility, inspectability, maintainability,
safety, and estaqblishing bills of material and purchased parts. The result of the detail design stage
is a complete set of working drawings and specifications, approved for production of a prototype
machine.
(4) Development and field service activities include development of a prototype into a production
model, and following the product into the field, maintaining and analyzing records of failure,
maintenance procedures, safety problems, or other performance problems.
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1-6. What conditions must be met to guarantee a reliability of 100 percent?
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Solution
A designer must recognize at the outset that there is no way to specify a set of conditions that will guarantee a
reliability of 100%. There will always be a finite probability of failure.
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1-7. Distinguish between fail safe design and safe life design, and explain the concept of inspectability, upon which
they both depend.
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Solution
(Ref 1.5) Fail safe design is implemented by providing redundant load paths in a structure so that if failure of a
primary structural member occurs, a secondary member is capable of carrying the load on an emergency basis until
the primary structural failure is detected and repaired.
Safe life design is implemented by carefully selecting a large enough safety factor and establishing inspection
intervals to assure that the stress levels, the potential flaw size, and the governing failure strength levels combine to
give crack growth rate slow enough to assure crack detection before the crack reaches its critical size.
Both fail safe and safe life design depend on regularly scheduled inspections of all potential critical points. This
implies that critical point locations must be identified, unfettered inspection access to the critical points must be
designed into the structure from the beginning (inspectability), appropriate inspection intervals must be established
(usually on a statistical basis), and a schedule must be established and executed to assure proper and timely
inspections.
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1-8. Iteration often plays a very important role in determining the material, shape, and size of a proposed machine
part. Briefly explain the concept of iteration, and give an example of a design scenario that may require an iterative
process to find a solution.
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Solution
A dictionary definition of iteration is “to do again and again.” In he mechanical design context, this may imply the
initial selection of a material, shape, and size for a machine part, with the “hope” that functional performance
specifications can be met and that strength, life, and safety goals will, at the same time be achieved. Then,
examining the “hope” through the use of applicable engineering models, make changes in the initial selection of
material, shape or size that will improve the part’s ability to meet the specified goals, and repeat the process (iterate)
until the goals are met.
For example, assume a stepped shaft needs to be designed for a newly proposed machine. Neither the material, the
shape, nor the size are known at the outset. The loads, torques, speed, and bearing support locations are initially
known. The iteration steps for such a case might include:
(1) Select (assume) a potential material.
(2) Establish a coordinate system and make a stick-sketch free-body diagram of the shaft,
showing all known forces and moment and their locations.
(3) Make a first-iteration conceptual sketch of the proposed shaft.
(4) Using appropriate shaft design equations, calculate tentative diameters for each stepped
section of the shaft.
(5) By incorporating basic guidelines for creating shape and size, transform the first-iteration
sketch into a more detailed second-iteration sketch that includes transition geometry from one
step to another, shoulders, fillets, and other features.
(6) Analyze the second-iteration shaft making appropriate changes (iterations) in material (to
meet specified strength, stiffness, or corrosion resistance specifications), changes in shape (to
alleviate stress concentrations, reduce weight, or provide for component retention), and
changes in size (to reduce stress or deflection, or eliminate interference).
(7) Continue iterations until a satisfactory design configuration has been achieved
A more specific example of the design iteration process is discussed in Example 8-1.
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1-9. Write a short paragraph defining the term “simultaneous engineering” or “concurrent engineering”.
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Solution
“Simultaneous” , or “concurrent” engineering is a technique for organizing and displaying information and
knowledge about all design-related issues during the life cycle of a product, from the time marketing goals are
established to the time the product is shipped. The technique depends upon an iterative computer system that allows
on-line review and rapid update of the current design configuration by any member of the product design team, at
any time, giving “simultaneous” access to the most current design configuration to all members. Properly executed,
this approach prevents the need for costly “re-designs” by incorporating requirements of down-stream processes
early in the preliminary design stage.
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1-10. Briefly describe the nature of codes and standards, and summarize the circumstances under which their use
should be considered by a designer.
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Solution
(Ref. 1.9) Codes are usually legally binding documents, compiled by a governing agency, that are aimed at
protecting the general welfare of its constituents and preventing loss of life, injury, or property damage. Codes tell
the user what to do and when to do it.
Standards are consensus-based documents, formulated through a cooperative effort among industrial organizations
and other interested parties, that define good practices in a particular field. Standards are usually regarded as
recommendations to the user for how to do the task covered by the standard.
A designer should consider using applicable codes and standards in every case. If codes are not adhered to, a
designer and their company may be exposed to litigation. If standards are not used, cost penalties, lack of
interchangeability, and loss of market share may result and overall performance may be compromised as well.
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