engineering design ge121 design for ‘x’

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GE 121 – Engineering Design - 2009

Engineering DesignGE121

Design for ‘X’

Lecture 12B

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Design for ‘X’

Design for X is a key element of successful design

Design for:Manufacturing and AssemblyAffordability (engineering economics)Reliability and MaintainabilitySustainability (i.e., environmental issues)

All these issues are tied together in terms of design for quality, quality function deployment, and the house of quality

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Design for Manufacturing / Assembly: Can This Design be Made?

Design for manufacturing and assembly is an important issue

Most companies are very concerned with how to get high quality products to market quickly

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Design for Manufacturing (DFM)

Based on minimizing costs of production and/or time to market for a product, while maintaining an appropriate level of quality

Requires that the design team include experts in manufacturing, marketing, and engineering

One basic methodology consists of six steps:1. Estimate the manufacturing costs for a given design

alternative2. Reduce the cost of the components where possible3. Reduce the cost of assembly4. Reduce the cost of supporting production5. Consider the effects of DFM on other objectives6. If the results are not acceptable, revise the design

and try again

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Design for Assembly (DFA)

Related to DFM, but focuses specifically on the aspect of manufacturing in which the artifact/system is put together

A number of design choices can impact assembly such as: 1. Limiting number of components to the fewest that are essential

to the working of the finished product 2. Using standard fasteners and integrating fasteners into the

product itself 3. Designing the product to have a base component on which

other components can be located 4. Designing the product to have components that facilitate

retrieval and assembly 5. Designing the product and its component parts to maximize

accessibility, both for manufacture and during repairs

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Bill of Materials (BOM)

A listing of all parts required to manufacture an object or system

Used in concert with both assembly and with the ordering and inventorying of parts

Also has a great use in studying the economics of a product

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Design for Affordability:How Much Does This Design Cost?

Design for Affordability (Engineering Economics)

Important in almost any projectMoney almost always has an effect on

design choices, and so should be understood in the proper context

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Time Value of MoneyMoney owed tomorrow is worth less than the

same amount actually paid todayMoney owed in the future is subject to opportunity

costs and risks1. Opportunity cost – Money could have been used for

something else (or at least put in a bank to earn interest)

2. Risk - Money paid in the future may be less useful, due to inflation or other factors, or may not be paid at all (bankruptcy)

3. To address these concerns, economists have developed the notion of discounting. Formula is given in the text on p278

4. We want to consider the full set of life cycle costs for a design, not merely the initial costs

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Time Value of Money:Affects Design Choices

Time value of money affects design choices in many ways

We are often comparing designs that are not identical in their time frames

We need a way to get them into comparable terms

Many engineers use Equivalent Uniform Annual Cost (EUAC) to do thisText has more details

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Estimating CostsA key skill in understanding the life cycle costs of

a designIn the simplest terms, costs include:

1. Labour costsInclude not only wages directly paid to workers who produce a

productAlso various indirect costs, such as unemployment insurance,

other insurance, pensions, etc.Most inexperienced designers tend to underestimate labour

costs by at least one half2. Material costs

Include costs of all parts in the Bill of Materials, and any inventory that is lost in the production process

whether component parts are ordered in large lots or not usually affects unit price through volume discounts

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Estimating Costs (continued)

3. Overhead costsAccount for costs shared by all the various products a

company makes, such as executive salaries, warehouse or factory costs, administrative services, etc.

In many companies, overhead can constitute as much as 50-100% of the direct labour expenditure

4. Profit for various stakeholdersMost companies want to make money on their products

When estimating costs, designers generally get higher levels of detail and accuracy in the later stages of designs. During conceptual design, rules of thumb that can be used

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Costing and Pricing

Costing and pricing are often confused by students and inexperienced engineers

With the exception of large public works, most products are priced on basis of what consumers will pay rather than costs

Firms tend to use cost to establish a floor to help them decide whether or not to remain in a market

For pricing, however, they will use market information

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Designing for Reliability:How Long Will This Design Last?

Design for reliability is important for most products

Few people knowingly select unreliable goods or services

In some cases, reliability can be the difference between life and death, as in the case of airplanes

Engineers often consider two related concepts, reliability and maintainability

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ReliabilityDefined as “the probability that an item will perform its

function under stated conditions of use and maintenance for a stated measure of the variate (time, distance, etc.).”

In practice, we think in terms of probability of failure 1. Types of failures: in-service failure, incidental failure, and

catastrophic failure. Designers are often concerned with the Mean Time Between Failures (MTBF)

2. To reduce the consequences of failure, engineers often introduce redundancy into systems, using parallel parts

3. Series systems are like chains - when the weakest link fails, the system goes down

4. Parallel systems are like multi-strand cables - all the parts in parallel must fail before the system will no longer work

5. Economics often drives the choice. Engineers determine the probability and costs of failures and design parallel systems when failure is costly (in dollar or life terms)

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Maintainability

Maintainability is “the probability that a failed component or system will be restored or repaired to a specific condition within a period of time when maintenance is performed within prescribed procedures.”

Closely related to the choices made in DFM and DFA.

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Design for Sustainability:What About the Environment?

Engineers have an ethical obligation to consider the environmental consequences of the things they design

Often done by first determining the types of impacts, and then estimating the overall size and scope of the impacts

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Environmental Issues and Design

One way to characterize environmental impacts of designed systems or artifacts is in terms of:Air qualityWater quality and consumptionEnergy demandsWaste streams

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Environmental Life Cycle Assessment

Environmental life cycle assessment has three essential steps:1) Inventory analysis — list all the inputs

and outputs as well as any intermediate products associated with the design

2) Impact analysis — determine all the effects on the environment and measure or estimate the magnitude of the effects

3) Improvement analysis — determine the needs and opportunities to address adverse effects found in the first two steps

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Design for Quality:Building a House

Quality is the concept which unites all the previous ideas

Quality can be defined as “fitness for use”Quality Function Deployment (QFD) is sometimes

referred to as the “house of quality”Refers to a graphical way of organizing all the major

relationships among stakeholder interests, desired design attributes, measures and metrics, targets, and current products or designs

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Abstract version – House of Quality

Fig. 11.4 p290

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House of Quality – Office Laptop

Fig. 11.5 p291

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Other Design for ‘X’ Considerations

DisassemblyTestingServiceabilitySafetySimplicityStandard PartsAccessibility (handicapped)…

engineering design ge121 design for ‘x’

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