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DI:2. BASIC DESIGN
•Before we begin,
1.Design is the satisfaction of need.
2.Design is never an exact process, and each design will differ.
3.Try to do it right the first time.
4.Most design methods try to cut the problems into smaller problems.
•One of the common problems encountered by designers is the overwhelming number of details. Most design methods focus on dealing with detail overload. The challenges a designer faces are,
- multiple technologies require arbitrary decisions
- a design will have many components that interact, and the effects of changes can be widespread
- economics
- other competitive designs
•Design is typically referred to as having certain stages,
-Conceptual
-Synthesis
-Detailed
-Analysis
•The typical stages of design include,
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Need
Specifications
feasibility
conceptual
detailed
analysis/testing
review
•Design factors commonly considered are,
-functional requirements
-physical constraints
-specifications
-aesthetics
-usability/user interface
-cost
-manufacturing
-evaluation/testing/analysis
-maintenance
-retirement
•A more detailed design sequence is shown below,
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START: select a needed design
e.g. a support cable |
determine |
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functional elements |
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major changes e.g. |
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replace cable with beam |
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e.g. maximum tension |
pick members |
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geometry, etc. |
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e.g 1/2” steel cable
e.g. find stress and compare to ultimate strength
e.g. use factor of safety
select values
and properties small changes e.g. diameter
analyze for suitability
decide on deficiencies
no deficiencies
DONE: Approve design
•Conceptual - The selection of general components to go into a system. At this point the exact form of final point is inexact. At this point we might be deciding to put wheels on a car.
•Synthesis - The selection of components or devices for the system. At this point the general geometry, and components for the system are selected.
•Detailed - Exact dimensions are finally assigned to parts in the system.
•Analysis - The review of design details to determine suitability. This is done after the exact design is complete. It may lead to redesign.
•The activity of design creates a dilemma for management in that it adds to the overall cost of a product, but it can also reduce the final cost of a product.
•We can draw graphs that illustrate the total amount committed in the final cost from the first concept, to the final product. Most of the final cost is determined by decisions early in the design phase.
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product costs
product starts shipping
costs committed |
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money spent |
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time |
design only |
preparation for manufacturing |
• By planning for design, and then committing fully, we can obtain a better product.
design |
quality design process |
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changes |
(do it right the first time) |
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rush to design |
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(just do it and fix it later) |
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project |
time |
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start |
production |
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release |
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•Over-the-wall is an engineering approach that has developed because of management pressures. It helps split designs into clean stages and responsibilities. This approach does simplify management up front, but requires fire fighting as problems arise.
•A product life cycle has four phases,
1.Identify needs, plan and design
2.Manufacture and deliver
3.Use, maintain, repair
4.Retire
DI:2.1 REFERENCES
Ullman, D.G., The Mechanical Design Process, McGraw-Hill, 1997.