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ME260 Mechanical Engineering
Design II
Instructor notes
Definition of Design
Many available definitionsOne definition: Design is the process of inventing artifacts that display a new physical order, organization, and form in response to functionAnother definition: Design is a conscious effort directed towards the ordering of the functional, material, and visual requirements of a problem
Design Example
Problem: Build a tool/device that is capable of opening metal cansDesign Response:
Regular, manualcan opener
Manual safe-operation can opener
Electric safecan opener
Principles of Design
BalanceRhythmProportion/ScaleEmphasisHarmony
Apply to design in generalbut not necessarily all important to “mechanical design”
Also, some of these involve aesthetics which may or may not be important from a mechanical point of view
Important/Often Encountered Mechanical Design Principles
Balance
Proportion/Scale
Physical balance often involving geometric symmetry
Pertains to ergonomics (thestudy of human factors in design)
Size of door and inside space must accommodate people/merchandise to be elevated. Also, location of buttonsmust be convenient
Important/Often Encountered Mechanical Design Principles
Harmony Integration of components in a systemto work seamlessly together
This pertains toDesign for Assembly (DFA) concepts,
i.e. the ease with which one can assemble and disassemble parts
Design Guidelines
Functional RequirementsMaterial/Manufacturing/Cost RequirementsVisual Requirements
Design Guidelines
Functional Requirements
A can/bottle opener must be able to open cans/bottles, otherwise it is a dysfunctional can/bottle opener
Design Guidelines Material/Manufacturing/Cost Requirements
More material used in a design means more costMore material normally means stronger design
(i.e. less chance of breaking/failure)More material also normally means higher
manufacturing costThe type of material also affects both cost and
likelihood of failure. It affects performance in generalMore material/manufacturing also typically means more
environmental pollutionFinally, the material for your part should be amenable
to manufacturing techniques/processes available to you
Dilemma
Design Guidelines Material/Manufacturing/Cost RequirementsExamples: 1- You can not create a perfect
2- A bigger diameter car axle is less likely to break but costs more
3- A car axle made from diamond is both prohibitive in cost as well as can easily fracture/break, i.e. is not tough towithstand a hit. Steel, however, is a good choice material here
This pertains to Design for Manufacture (DFM) concepts,i.e. the design process needs to integrate manufacturing feasibility into it
An example of CAD (Computer-Aided Design)
Design Guidelines
Visual RequirementsMany times you want your product to be either:
1- Appealing to the human eye for marketability2- Of certain color to serve a certain purpose
Example 1: Car manufacturers compete to make visually appealing carsExample 2: Protective coats/pants for firefighters are typically made of heat reflective colors, not black for example.
Mechanical Properties of Materials
Force is not an objective measure of loadingStress = = Force/Area (F/Ao) is
Why? To answer this answer first:If a force of 1 lb is applied to a rubber band and a force of 100 lb is applied to another, which rubber band will break first?Answer: depends on their cross-sectional area, i.e. the stress that they are subjected to
F
F
Area = Ao
lo
F
F
Area = A
l
(left) Before deformation, and (right) after deformation
Mechanical Properties of Materials
Deformation is simply change in dimensions or geometry/shape of a material under loadingThe change in length, l =l – lo is not an objective measure of deformation. This is positive change if material is loaded in tension and negative change if loaded in compression.Strain (the relative change in length) = e = l / lo is. Strain sometimes is expressed as a percentage, i.e. as 100×l / lo.
If a rubber band is extended by 1 cm and another by 1 m, which one will break first?Answer: depends on how much they stretched (l) relative/compared to their original length (lo ), i.e. depends on how much they strained.
F
F
Area = Ao
lo
F
F
Area = A
l
(left) Before deformation, and (right) after deformation
Mechanical Properties of Materials
Mechanical Properties of Materials
Material Types Ferrous metals/alloysNonferrous metals/alloysPolymersCeramics
GlassDiamond, Graphite
WoodComposites
Iron-based materials, e.g. steels
Both made from pure carbonbut have different atomic structure
Plastics and rubber are prime examples
Compounds of metallic & nonmetallic elements, e.g. chinaware
Natural and organic material
e.g. nickel, silver, etc.
A combination of two/more material types
Solid with a random atomic structure like a fluid