trapped by mtbf

ARS, North America 2007ARS, North America 2007San Diego, California USASan Diego, California USA

Track 2, Session 9Track 2, Session 9

Trapped by MTBFTrapped by MTBFFred Schenkelberg

Ops A La Carte, LLC

Fred Schenkelberg, Ops A La Carte, LLC Slide Number: 2Session 9Track 2

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Introduction Introduction

How does your organization talk about Reliability?

How do your customers talk about Reliability?

How does your organization talk about Reliability?

How do your customers talk about Reliability?


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OutlineOutline

MTBF – calculationMTBF – a very poor four letter acronymHistory of Use It’s MisleadingA Better MetricActually, we’ve been talking about MTTF

MTBF – calculationMTBF – a very poor four letter acronymHistory of Use It’s MisleadingA Better MetricActually, we’ve been talking about MTTF


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MTBF CalculationMTBF Calculation

failures

hoursMTBF

#

#

1MTBF


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Mean (M)Mean (M)

The mean in MTBF

What does it mean to you? (no pun intended!)

Average?

The mean in MTBF

What does it mean to you? (no pun intended!)

Average?


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Start 1000 units, MTBF = 100 hrsStart 1000 units, MTBF = 100 hrs

0

200

400

600

800

1000

1200

1 8 15 22 29 36 43 50 57 64 71 78 85 92 99

368 still alive at 101 hours


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Note the exponential decayNote the exponential decay

0

200

400

600

800

1000

1200

1 21 41 61 81 101 121 141 161 181 201 221 241 261 281


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Other IssuesOther Issues

Time – just because it is hours…

Between – note the duration of the failure-free period!

Failure – use the customer definition

Time – just because it is hours…

Between – note the duration of the failure-free period!

Failure – use the customer definition


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History of UseHistory of Use

Remember Slide Rule and Mechanical Adding Machines

Victor Adding Machine

Remember Slide Rule and Mechanical Adding Machines

Victor Adding Machine


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History of UseHistory of Use

Early Parts Count based on adding failure rates of components (60’s and early 70’s)

Early Parts Count based on adding failure rates of components (60’s and early 70’s)

t

ttt

n

n

etR

eeetR)( 21

21

)(

)(


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Beta = 0.63Beta = 0.63

DEPTH.CUT

.0001

.0002

.0003

.0005

.001

.003

.005

.01

.02

.03

.05

.1

.2

.3

.5

10^-01 10^00 10^01 10^02 10^03 10^04

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Depth Cut Response data Weibull Probability Plot

Weibull Distribution ML FitExponential Distribution ML Fit95% Pointwise Confidence Intervals


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Beta = 1.97Beta = 1.97

Depth In

.00000001

.00000003

.0000001

.0000003

.000001

.000003

.00001

.00003

.0001

.0003

.001

.003

.01

.03

.1

.3

.7

1 10 100 1000 10000 100000

Fra

ctio

n F

aili

ng

test7.df data Weibull Probability Plot

Weibull Distribution ML FitExponential Distribution ML Fit95% Pointwise Confidence Intervals


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Use ReliabilityUse Reliability

R(t) is the probability that a random unit drawn from the population will still be operating by t hours

R(t) is the fraction of all units in the population that will survive by t hours

R(t) is the probability that a random unit drawn from the population will still be operating by t hours

R(t) is the fraction of all units in the population that will survive by t hours

Applied Reliability, 2nd Ed., pg 29


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The four (five) elementsThe four (five) elements

FunctionDurationProbabilityEnvironment

They all change over time

FunctionDurationProbabilityEnvironment

They all change over time


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Use better models/distributionsUse better models/distributions

Weibull Type I Gumbel Exponential Lognormal Etc.

Weibull Type I Gumbel Exponential Lognormal Etc.

)(

)(t

Weibull etR

)()(te

Gumbel etR

50ln

1)(Tt

tRlognormal

tlExponentia etR


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Other metricOther metric

What is the cost of a field failure?

Warranty $ per unit shippedReturns/field failure $ per unit shipped

What else could you use?

What is the cost of a field failure?

Warranty $ per unit shippedReturns/field failure $ per unit shipped

What else could you use?


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Actually…Actually…

MTBF is or should be used for repairable systems

MTTF is what I’ve been talking about MTTF is calculated the same way as MTBF

when we assume Negligible repair time Interarrival times as from an independent sample of

non-repairable parts Exponential distribution for lifetime of parts

See Chap 10, Applied Reliability for more info

MTBF is or should be used for repairable systems

MTTF is what I’ve been talking about MTTF is calculated the same way as MTBF

when we assume Negligible repair time Interarrival times as from an independent sample of

non-repairable parts Exponential distribution for lifetime of parts

See Chap 10, Applied Reliability for more info


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SummarySummary

MTBF


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Where to Get More InformationWhere to Get More Information

Tobias, Paul A. and Trindade, David C., Applied Reliability, 2nd Ed. Chapman & Hall, New York, 1995.

“The Limitations of Using the MTTF as a Reliability Specification” Reliability Edge, Qtr 2, 2000, Vol 1, Issue 1.

Ops A La Carte, LLC provides a full range of reliability engineering services including assessments, FMEA facilitation, HALT and ALT testing, data analysis and customized training.

Tobias, Paul A. and Trindade, David C., Applied Reliability, 2nd Ed. Chapman & Hall, New York, 1995.

“The Limitations of Using the MTTF as a Reliability Specification” Reliability Edge, Qtr 2, 2000, Vol 1, Issue 1.

Ops A La Carte, LLC provides a full range of reliability engineering services including assessments, FMEA facilitation, HALT and ALT testing, data analysis and customized training.


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Presenter’s Biographical SketchPresenter’s Biographical Sketch

Fred Schenkelberg, Consultant Independent Reliability Engineering and

Management Consultant for past 3 years. Previously at HP Corporate Reliability Engineering Program for 5 years.

MS Statistics Stanford, BS Physics USMA

[email protected](408) 710-8248

www.opsalacarte.com

Fred Schenkelberg, Consultant Independent Reliability Engineering and

Management Consultant for past 3 years. Previously at HP Corporate Reliability Engineering Program for 5 years.

MS Statistics Stanford, BS Physics USMA

[email protected](408) 710-8248

www.opsalacarte.com

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