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Reliability – Lesson 1

The Basics

Overview: Application of Reliability in different

industries Importance of Reliability – Cost Impact Bathtub curve Predictability vs. Failure Mode Avoidance P-Diagram Strategies for Improvement

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“A man who lacks reliability is utterly useless.”  

Confucius (551-479)

“Engineering is the science of economy, of conserving the energy, kinetic and potential, provided and stored up by nature for the use of man. It is the business of engineering to utilize this energy to the best advantage, so there may be the least possible waste.”

Willard A. Smith (1908)3

Wikipedia - the ability of a system or component to perform its required functions under stated conditions for a specified period of time.

ASQ – the probability that an item can perform its intended function for a specified interval under stated conditions

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Products: Design for Reliability Software

Equipment: Reliability Centered Maintenance

Medical – Survival rate

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Reliability field

  Nuclear Aerospace Medical Consumer Products

Reliability Criteria        

# units in field 00 000 00000000 00000000

Quality of field records:        

Failed units excellent excellent Reasonable>good fair

Unfailed units excellent excellent none none

Units lost to follow-up no no often yes

Noise space simple moderate complicated complicated

Competing risks no no yes yes

Key Reliability redundancy, redundancy, intervention robustness,

Strategy intervention intervention   some intervention

Key reliability measure probability probability cure rate and side effects distance from failure modes

Memo: Design improvement yes yes no yes

Safety Customer Satisfaction Warranty Costs Reputation Repeat Business Cost Analysis Customer Requirements Competitive Advantage

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Set goals & requirements (customer driven)

Perform an assessment Testing Data collection Failure reporting, analysis and

corrective action system (FRACAS)

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Mean Time to Failure (MTTF) Mean Time Between Failures (MTBF) Hard Failures – product function ceases Soft Failures – degraded to unacceptable

level Failure rate Hardware Breakdown Structure FMEA – Failure Modes, Effects & Analysis SPC – Statistical Process Control DOE – Design of Experiments

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“It’s unwise to pay too much, but it’s foolish to spend too little”— John Rustin

Present Value vs. Future Value of money Acquisition Costs Sustaining Costs Cost Breakeven Include Cradle-to-Grave costs

converted to NPV models

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http://www.barringer1.com/pdf/LifeCycleCostSummary.pdf

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Product B has a lower acquisition cost but needs repaired more often and at greater expense

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Prevention activities - intended to ensure that processes work without fail, such as failure mode and effects analysis, training and preventive maintenance

Appraisal activities - how well processes such as product and process approvals are actually working via activities such as inspection and testing

Failure costs - related to failures that do occur and are further classified as internal (failure identified before it reached the customer) or external (detected after reaching the customer). The time and expense of reprocessing failed products or services and investigating failures could fall into either failure category, depending on when the event occurs. Warranty costs are decidedly external failures.

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Number of

failures

Time

Infant Mortality

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Number of

failures

Time

Infant Mortality

Useful Life

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Time

Infant Mortality

Useful Life

WearoutNumber

of failures

We do not have access to all products in the field, particularly those that have not failed

We cannot specify a period of time to measure reliability

We cannot specify a set of operating conditions

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Reliability is the probability that an item can perform its intended function for a specified interval under stated conditions, However:

Reliable products are robust and mistake free

Failure modes of our products are primarily that something breaks or performance degrades

Avoiding failure modes will decrease the probability of failure

Reliability prediction is not easy for many industries

We need to reduce variation and the sensitivity to noise (ie. Six Sigma and P-diagram)

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P-DiagramNoise Factors123

Input Signal Ideal Function

System

Error StateControl Factors 1

1 22 3345

Ideal Function – primary intended function of the design

Input Signal – energy which is put into the system to make it work

Error State – undesirable output of the system

Control Factors – features that can be controlled by design or process

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Product variations Changes over time/usage Customer duty cycle External environment System interactions

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Sources of disturbing influences that can disrupt ideal function, causing error states which lead to quality problems

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  P-Diagram            

  Self contained jet pack   Noise Factors        

      1. Fuel volatility        

      2. Wind        

      3. Temperature

       

  Input Signal

 

      Ideal Function  

 

Throttle position

 

System

   

Lifts person off ground

 

          Error State    

      Control Factors   1. No ignition

   

      1. Material properties   2. Flame too large    

      2. Design

  

   

      3. Manufacturing process        

      4. Assembly process        

      

       

               

Bring P-Diagram List error states (potential failure

modes) Ties together P-Diagram and Testing Living and changing document Discovery, legal document Ultimate failure mode avoidance

document

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Captures the effects of noise factors over useful life

Generates a measurable event Generates a failure of degradation of the

ideal function Generates results which are directly

correlated with real-world performance Can be accelerated for reliability

improvement experiments (HALT, HASS)

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HALT testing

Change the design concept Make basic current design assumptions

insensitive to the noises (beef up design, use Design of Experiments, redundancy)

Reduce or remove the noise factor(s) Insert a compensation device Send the error state/noise elsewhere else

where it will do less harm (disguise effect)

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Failure Reporting, Analysis and Corrective Action System

Prioritizes field failures Root Cause Investigation Teams Needs input from Team members Drives problems to closure Documentation in central location Closed Loop System

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Application of Reliability in different industries

Importance of Reliability – Cost Impact Bathtub Curve Predictability vs. Failure Mode

Avoidance P-Diagram Strategies for Improvement

Reliasoft - http://www.reliasoft.com/ RIAC -

http://src.alionscience.com/src/training.do Hobbs Engineering -

http://www.hobbsengr.com/ ASQ Reliability Div. -

http://asq.org/reliability/ SRE - http://www.sre.org/ You’ll find many more on the web….

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Dr. Tim Davis -“Science, engineering, and statistics”. Applied Stochastic Models in Business and Industry, 2006, Vol. 22, Issue 5-6, pp. 401-430

Dr. Vasiliy Krivstov Dr. Paul Barringer Reliasoft