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