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[email protected] • ENGR-11_Lec-12_Chp10_Design_for_X.ppt1

Bruce Mayer, PE Engineering-11: Engineering Design

Bruce Mayer, PELicensed Electrical & Mechanical Engineer

[email protected]

Engineering 11

Design Design for for XX



OutLine OutLine Design for X Design for X

Trade-offs in Satisfaction Robust design Failure Modes & Effects Analysis Tolerance design

CustomerNeeds(CN)

FunctionalRequirements

(FR)

DesignParameters

(DP)

ProcessVariables

(PV)



Basic Design Engineering GoalsBasic Design Engineering Goals

Design Engineering Goals for Product:• “performs as expected”

• “works all the time” & “lasts long”

• “is easy to maintain”

and THAT• no damage occurs to product

• no damage or harm to environment

• no harm or injury to operator or user



Design for X (DfX)Design for X (DfX) During design, we often focus on the final product,

and not its manufacture. The Design For X (DfX) philosophy suggests that a

design be continually reviewed from the start to the end to find ways to improve production and other non-functional aspects

Advantages of DfX techniques include• shorter production times• fewer production steps• smaller parts inventory• more standardized parts• simpler designs that are more likely to be robust• they can help when expertise is not available, or as

a way to re-examine traditional designs• proven to be very successful over decades of application



Th

e “X” in

DfX

Th

e “X” in

DfX



Design for RobustnessDesign for Robustness Methods to reduce the sensitivity of product

performance to variations such as:• manufacturing (materials & processes)

• wear

• operating environment

Currently used methods• Taguchi Method

• Probabilistic optimal design (Monte Carlo)

Both Taguchi and Monte Carlo methods use statistics and probability theory



Failure Modes & Effects AnalysisFailure Modes & Effects Analysis

The FMEA Method seeks to systematically identify and correct potential product or process deficiencies before they occur

The Process• Identify EVERY Way in Which Product Can

FAIL; i.e., determine the Failure MODES

• Analyze the CONSEQUENCES of Every Failure; i.e., determine the EFFECTS



Failure Modes & Effects AnalysisFailure Modes & Effects Analysis

After Completion of the FMEA Work to REDUCE• The NUMBER of

Failure MODES

• The SEVERITY of the EFFECTS

Prioritize Risk Reduction using“Risk Priority No.”



FMEA Example FMEA Example Log Splitter Log Splitter FMEA considers Both DESIGN and

MANUFACTURING Deficiencies Example Hydraulic Log Splitter

• Hydraulic hose, on a home-use log splitter, begins to leak.

• The leak reduces the pressure to the piston/ram resulting in poor splitting.

• The leak drips oil on ground, creating a mess, costly too!

• Upon examination, a weak spot is found on hose due to poor manufacturing!



FMEA Main ConceptsFMEA Main Concepts Failure Mode: the “way” a part fails to perform

• e.g. failure mode: hose leaks

Effect: adverse consequence of failure mode• e.g. hose leak results in oil spills, refill costs

• Effects can be severe or hardly noticeable.

Cause: why it fails (or may fail)• e.g. poor hose manufacturing, improper pressure

• Causes occur with some likelihood or probability

Dectectability: the ability to discover the cause before the part is shipped from the factory.• e.g. conduct a pressure test to detect leaks?



FMEA Risk Metric FMEA Risk Metric RPN RPN Determine a rating for each mode of failure

…. using a “risk priority number” (RPN)

RPN = [Severity rating] x [Occurrence rating] x [Detection rating]

RPN = (S)•(O)•(D)

RPN will range from 1 to 1000• Large RPN “bad”

• Small RPN “good”



RPN CalculationRPN Calculation

Step 1: determine the failure modes• From:

– Engineering design specifications

– Function decomposition diagrams

– functions ---- matter, energy, signal

– HoQ

– free body diagrams

– force flow diagrams

– process flow diagrams

– configuration sketches / drawings



RPN CalculationRPN Calculation Step 2: determine potential effects of

each failure mode Step 3: determine a severity (S) rating for

each effect from the Severity rating table. Step 4: determine an occurrence (O)

rating for each cause from the Occurrence rating table.

Step 5: determine a detection (D) rating for each cause from the Detection rating table



Severity Rating CriteriaSeverity Rating Criteria

Severity (S) Rating

Type of effects

Description

10 Catastrophic Causes injury to people, property and or the environment

9 Extremely Harmful Causes damage to product, property or environment

8 Very Harmful Causes damage to product

7 Harmful Major degradation of function

6 Moderate Causes partial malfunction of product

5 Significant Performance loss causes customer complaints

4 Annoying Loss of function is annoying, cannot be overcome

3 Minor Some loss of performance, but can be overcome

2 Insignificant Very little function degradation

1 None No noticeable effects in function or harm to others



Occurrence Rating CriteriaOccurrence Rating Criteria



Detection Rating CriteriaDetection Rating Criteria



RPN Calculation & ReductionRPN Calculation & Reduction

Step 6: calculate the risk priority number for each effect

Step 7: prioritize or rank the failure modes for action

Step 8: take action to eliminate the failure mode or reduce its severity

Step 9: recalculate the risk priority number as failure modes are reduced or eliminated



RPN Calculation SummaryRPN Calculation Summary

RPN Calculations are Usually Tabulated or put in a SpreadSheet

Severity (S) Occurrence (O) Detection (D)

Failure mode Effects S

Rating Causes O

Rating controls

tests D

Rating RPN Recommended

Action



RPN Example RPN Example Hose Failure Hose Failure

Log-Splitter RPN & Remediation



Design for Design for SafetySafety Issues Issues

Injury Hazards Conditional Circumstances Legal Responsibilities Guidelines for Safe Products/Systems Safety Hierarchy Safe Design Principles



Define Safe Product/SystemDefine Safe Product/System

No injury to user, (products liability)

No injury to consumer/society

No injury to production worker

No damage to personal property

No damage to real property or the environment



HazardsHazards Hazard ≡ a source of danger which has

the potential to injure people or damage property or the environment

Partial Hazard List• Entrapment – pinch, crush• Contact – heat, sharp edges, electric• Impact – hammer, robot arm• Ejection – grinder sparks, saw dust• Entanglement – hair, clothing• Noise & Vibration – hearing loss, HAVS



Conditional CircumstancesConditional Circumstances

hazard is inherent during normal use hazard originates from a component

failure hazard caused by user misuse hazard exists during normal maintenance hazard created by improper maintenance hazard stems from lack of maintenance



Product Legal-LiabilityProduct Legal-Liability Plaintiff’s attorney will try to prove that the company

or its employees failed to:• perform “appropriate analyses.”

• comply with published standards.

• make use of state-of-the-art technology, due to ignorance.

• include reasonable safety features or devices.

• take into account how the user might misuse the product.

• consider hidden dangers that might surprise the user.

• consider variations in materials, mfg processes, or effects of wear.

• carry out appropriate testing, or interpret results correctly.



Guidelines for Safe ProductsGuidelines for Safe Products

1. Perform appropriate analyses

2. Comply with published standards

3. Use state-of-the-art technology

4. Include reasonable safety features or devices

5. Take into account how the user might misuse the product

6. Consider hidden dangers that might surprise the user



Guidelines for Safe ProductsGuidelines for Safe Products

7. Consider variations in materials or manufacturing processes, or effects of wear

8. Carry out appropriate testing and interpret results correctly

9. Provide adequate warnings

10. Implement superior quality control

11.Document everything



Safety Hierarchy MethodSafety Hierarchy Method1. Eliminate the hazard - ProActive approach,

“design-out” the hazard (eliminate any moving parts, hot or sharp surfaces)

2. Protect against the hazard with passive approach, (machine guards, seat belts)

3. Warn against the hazard - weak remedy (warning labels, alarms)

4. Provide Training - Provide and require operating training.

5. Provide Personal Protection - LEAST effective, (safety glasses, gloves, shoes)



Safe-Design PrinciplesSafe-Design Principles Safe-Life Good

• entire predicted useful life without malfunction.• designers to identify all operating conditions, misuses

and abuses• design appropriate maintenance and repair schedules.

Fail-Safe Better • upon failure of a component, product/system

SHUTS DOWN safely, • critical functions are sometimes still performed

– e.g. boiler feed-water valve failing in the open position

Redundant design Best (but EXPENSIVE) • additional product components or systems are designed

to take over the principle function of the failed component or system. – e.g., multi-engine airplanes, emergency brakes, BackUp

pumps in nuclear PowerPlants



All Done for TodayAll Done for Today

EnvironmentalTolerance

Zone

The ETZ is the Limits of SURVIVAL• Well Beyond the Comfort Zone



Bruce Mayer, PERegistered Electrical & Mechanical Engineer

[email protected]

Engineering 11

AppendiAppendixx



Method 6-3-5 (Brain-Writing)Method 6-3-5 (Brain-Writing) The traditional brainstorming relies on verbal

communications.• Idea generation may be dominated by a small

number of aggressive members. Guidelines for 6-3-5 method Team members are arranged around a

circular table to provide continuity. Six (6) members are ideal.

Each member sketches three (3) ideas for the product configuration or functions. Sketches should be the focus of this activity. The top five product functionswith respect to the customer needs are considered.

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