Reliabilty prediction and analysis

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RELIABILITY PREDICTION AND ANALYSIS

1. INTRODUCTION

Reliability prediction is the combination o the creation o a proper reliability model to!ether

"ith estimatin! #and $%stiyin!& the inp%t parameters or this model #li'e ail%re rates or a partic%lar

ail%re mode or e(ent and the mean time to repair the system or a partic%lar ail%re& and inally topro(ide a system #or part& le(el estimate or the o%tp%t reliability parameters #system a(ailability or a

partic%lar %nctional ail%re re)%ency&*

To perorm a proper )%antitati(e reliability prediction or systems may be diic%lt and may be

(ery e+pensi(e i done by testin!* On part le(el, res%lts can be obtained oten "ith hi!her conidence

as many samples mi!ht be %sed or the a(ailable testin! inancial b%d!et, ho"e(er %nort%nately these

tests mi!ht lac' (alidity on system le(el d%e to the ass%mptions that had to be made or part le(el

testin!* These a%thors ar!%e that it cannot be emphasi-ed eno%!h that testin! or reliability sho%ld be

done to create ail%res in the irst place, learn rom them and to impro(e the system . part* The !eneral

concl%sion is dra"n that an acc%rate and an absol%te prediction / by ield data comparison or testin!

/ o reliability is in most cases not possible* An e+ception mi!ht be ail%res d%e to "ear0o%t problems

li'e ati!%e ail%res*

In the desi!n and de(elopment o a ne" prod%ct, the desi!n and reliability en!ineers, may not

ha(e a(ailable ield data #reliability perormance data&, d%e to the simple act that the system has not

been ielded* In this case the reliability en!ineer m%st %se alternati(e methods to determine the

reliability o the proposed system* Early in the concept phase, "ith a minim%m depth o 'no"led!e o

the proposed system, a hi!h le(el %nderstandin! o the system co%ld be determined*

Reliability and 1aintainability !reatly inl%ence the lie0cycle cost o comple+ systems* The

more reliable and the more maintainable the prod%ct is, the lo"er its lie0cycle cost "ill be* A Portion

o the research in Reliability and 1aintainability #R21& addresses the importance o acc%rately

)%antiyin! the lie0cycle cost o a prod%ct and s%bse)%ently lo"erin! that cost* 3o"e(er, most R21

research in this area has not attempted to %tili-e R21 analysis as a desi!n tool* The accepted R21

analysis methods are %sed or the e(al%ation o e+istin! prod%cts based on test data retrie(ed rom theprod%ct*

Dept o 1echanical En!ineerin!, S4CE, 1ysore

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2. RELIABILITY PREDICTION

Reliability prediction describes the process %sed to estimate the constant ail%re rate d%rin! the

%se%l lie o a prod%ct* This ho"e(er is not possible beca%se predictions ass%me that,

The desi!n is perect, the stresses 'no"n, e(erythin! is "ithin ratin!s at all times, so that only

random ail%res occ%r

E(ery ail%re o e(ery part "ill ca%se the e)%ipment to ail*

The database is (alid

These ass%mptions are sometimes "ron!* The desi!n can be less than perect, not e(ery ail%re

o e(ery part "ill ca%se the e)%ipment to ail, and the database is li'ely to be at least 56 years o%t0o0

date* 3o"e(er, none o this matters m%ch, i the predictions are %sed to compare dierent topolo!ies

or approaches rather than to establish an absol%te i!%re or reliability* This is "hat predictions "ere

ori!inally desi!ned or*

Some prediction man%als allo" the s%bstit%tion o %se o (endor reliability data "here s%ch

data is 'no"n instead o the recommended database data* S%ch data is (ery dependant on the

en(ironment %nder "hich it "as meas%red and so, predictions based on s%ch data co%ld no lon!er be

depended on or comparison p%rposes*

The reliability analyses can be %sed to deine the )%antitati(e parameters or an item,

e)%ipment or a complete system, and may be e+pressed in n%mber o ail%res in a !i(en set period o

time, set n%mber o cycles or set n%mber o operations, s%ch as ro%nds ired rom a small caliber !%n,

etc*

A common e+pression %sed to deine an item7s reliability is its 1ean0Time0Bet"een08ail%re,

commonly 'no"n as its 1TB8* Once this i!%re is 'no"n it can be %sed to determine the reliability o

an item in terms o a probability o s%ccess or ail%re, o(er a !i(en operatin! period*

The ollo"in! acti(ities are co(ered or the )%antitati(e reliability analysis9

Reliability Analysis #methods&

Reliability Probability

Reliability Bloc' Dia!rams #1odelin!&

Reliability Analysis #1ethods&

There are methods a(ailable or determinin! the reliability o an item #this co%ld be a piece part

to a complete system& and are9

Reliability Prediction9This is the process %sed to determine the 1TB8 o an item* This is achie(ed by

perormin! a prediction analysis*


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!. RELIABILITY ANALY"I" #ET$OD"

Reliability predictions are cond%cted d%rin! the concept and deinition phase, the desi!n and

de(elopment phase and the operation and maintenance phase, at (ario%s system le(els and de!rees o

detail, in order to e(al%ate, determine and impro(e the dependability meas%res o an item* S%ccess%lreliability prediction !enerally re)%ires de(elopin! a reliability model o the system considerin! its

str%ct%re* The le(el o detail o the model "ill depend on the le(el o desi!n detail a(ailable at the

time* Se(eral prediction methods are a(ailable dependin! on the problem

The dierent reliability analysis methods are as ollo"s9

System 1odelin!

Predict 1TB8

Predict 1TB1A

Predict 1TB=R

!.1 "y%te& #odelli'

Some ass%mptions m%st be made %p ront to simpliy the analysis method* 8irst, all

components are ass%med to be replaceable on the maintenance line and that maintenance personnel

ha(e all the necessary tools and trainin! to perorm the remo(al* Second, only one component ail%re

is considered to ha(e occ%rred at any point in time, no m%ltiple component ail%res are considered*

Third, no passi(e ail%res or ail%res "itho%t indication are considered, and o%rth, no indicator

ail%res are considered* The system modelin! phase "ill con(ert the 8EA o the system into %sable

inormation or the analysis*

The 8EA method contains n%mero%s amo%nts o inormation that is not %tili-ed by this

analysis method* >hat m%st be deri(ed rom the 8EA is the type o ail%res that each component can

ha(e and "hat ail%re indications are present "hen that type o ail%re has occ%rred*

!.2 Predicti' #ea Ti&e Bet(ee )ail*re% +#TB),The ob$ecti(e o this analysis method is to s%pply estimated 1TB8 data or the system

components "itho%t ha(in! historical or test data* Traditionally, component ail%re rates are

determined either rom the %nderlyin! physics or rom historical data* In the concept%al desi!n phase

o a prod%ct, historical or test data is typically not a(ailable and the desi!n may not be o eno%!h

detail to perorm an analytical ail%re analysis* To prod%ce a ro%!h estimate o ail%re rates or

components in a system, "e propose e+trapolation rom the 'no"n ail%re rate data or one


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component* By %tili-in! en!ineerin! $%d!ment and common ail%re patterns, estimated 1TB8 data

can be !enerated (ery early in the desi!n o a prod%ct*

1ean time bet"een ail%res #1TB8& is a basic meas%re o reliability or repairable items*

1TB8 can be described as the time passed beore a component, assembly, or system ails, %nder the

condition o a constant ail%re rate* Another "ay o statin! 1TB8 is the e+pected (al%e o time

bet"een t"o consec%ti(e ail%res, or repairable systems* It is a commonly %sed (ariable in reliability

and maintainability analyses* 1TB8 can be calc%lated as the in(erse o the ail%re rate, @, or constant

ail%re rate systems* 8or e+ample, or a component "ith a ail%re rate o : ail%res per million ho%rs,

the 1TB8 "o%ld be the in(erse o that ail%re rate @*

!.! Predicti' #ea Ti&e Bet(ee #aiteace Actio%

8or each %ni)%e ail%re indication, there is a dia!nostic process that m%st be perormed by the

maintenance personnel* The de(eloped method attempts to sim%late that dia!nostic process and yields

a maintenance rate or all the possible components or that ail%re indication* The method that is

presented m%st be perormed or each dierent ail%re indication to ind the o(erall maintenance rates,

and con(ersely the 1TB1A7s, or each component*The main modiications to this method are the incl%sion o the probabilities that a component

ailed in a partic%lar mode to ca%se a partic%lar ail%re indication* This data "as de(eloped in the

system modelin! phase* Also, the end res%lt o the method %tili-ed here is a prediction o 1TB1A

data, "hile >on! attempted to sol(e directly or cost*

The irst step inds the probability that a partic%lar ail%re indication "ill occ%r* The ne+t step

de(elops an optim%m chec'in! order to dia!nose system components and the chec'in! order is

%tili-ed alon! "ith ail%re data to determine the probabilities that a partic%lar component "ill be the

ca%se o a ail%re indication* The inal step is to determine the 1aintenance Rate or each component

per indication, "hich is a %nction o the probability o the indication occ%rrin! m%ltiplied by the

probabilities that the pre(io%s chec'ed components in the dia!nostic process ha(e not ailed*

!.- PREDICTIN #EAN TI#E BET/EEN UN"C$EDULED RE#O0AL

The phase to predict the 1TB=R7s is a three step process* The irst step in this phase i!%res

the $%stiied 1TB=R or each component, "hich is e)%i(alent to its 1TB8* The second step in this

phase inds the %n$%stiied 1TB=R or each component* The %n$%stiied 1TB=R is o%nd or each


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component in each dierent ail%re mode based on the 1TB87s o other components and the Line and

Shop Labor 3o%rs per Remo(al #LL3PR and SL3PR&* All the %n$%stiied 1TB=R7s or each

component are combined to !et a total %n$%stiied 1TB=R* The third step in this phase %ses the

$%stiied and %n$%stiied components o the 1TB=R to !i(e a total 1TB=R or each component in

the system*


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-. PROBLE#" "OLUTION"


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. A""U#PTION" AND LI#ITATION"

8ail%re rate predictions are based on the ollo"in! ass%mptions9

The prediction model %ses a simple reliability series system o all components, in other

"ords, a ail%re o any component is ass%med to lead to a system ail%re*

Component ail%re rates needed or the prediction are ass%med to be constant or the time

period considered* This is 'no"n to be realistic or electronic components ater b%rn0in

Component ail%res are independent*

No distinction is made bet"een complete ail%res and drit ail%res

Components are a%ltless and are %sed "ithin their speciications*

Desi!n and man%act%rin! process o the item %nder consideration are a%ltless*

Process "ea'nesses ha(e been eliminated, or i not, screened by b%rn0in*

Limitations o ail%re rate predictions are9

Pro(ide only inormation "hether reliability !oals can be reached*

Res%lts are dependent on the tr%st"orthiness o ail%re rate data*

The ass%mption o constant component ail%re rates may not al"ays be tr%e* In s%ch cases this

method can lead to pessimistic res%lts*

8ail%re rate data may not e+ist or ne" component types*

In !eneral red%ndancies cannot be modeled*

Other stresses as considered may predominate and inl%ence the reliability*

Improper desi!n and process "ea'nesses can ca%se ma$or de(iations*


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3. APPLICATION"

8ail%re rate predictions are %se%l or se(eral important acti(ities in the desi!n phase o

electronic e)%ipment in addition to many other important proced%res to ens%re reliability*

E+amples o these acti(ities are9

To assess "hether reliability !oals can be reached,

To identiy potential desi!n "ea'nesses,

To compare alternati(e desi!ns,

To e(al%ate desi!ns and to analy-e lie0cycle costs,

To pro(ide data or system reliability and a(ailability analysis,

To plan lo!istic s%pport strate!ies,

To establish ob$ecti(es or reliability tests*


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4. CONCLU"ION

This report has briely loo'ed at reliability en!ineerin!, its terms and orm%lae, and ho" to

predict reliability and demonstrate it "ith tests and ield data*

>e ha(e seen that reliability predictions are cond%cted d%rin! the concept and deinition

phase, the desi!n and de(elopment phase and the operation and maintenance phase, in order to

e(al%ate, determine and impro(e the dependability meas%res o an item*

8ail%re rate predictions are %se%l or se(eral important acti(ities in the desi!n and operation o

electronic e)%ipment* These incl%de assessment o "hether reliability !oals can be reached,

identiication o potential desi!n "ea'nesses, e(al%ation o alternati(e desi!ns and lie0cycle costs, the

pro(ision o data or system reliability and a(ailability analysis, lo!istic s%pport strate!y plannin! and

to establish ob$ecti(es or reliability tests*


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Reliabilty prediction and analysis

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