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BITS PilaniPilani Campus

Concurrent Engineering

QMZG663

Lecture 8

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BITS PilaniPilani Campus

Design for

Reliability

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Contents

4/2/16 Concurrent Engineering BITS Pilani 3

• Reliabilit

• !"E#

• $esign %or Sa%et

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Why Engineering items fai!


• In&erentl incapable 'esign• ()erstresse'

• *ariation +Strengt& , -oa'.

• ear out +"o)ing members0 corrosion0 Insulation'eterioration.

• Sneas or bugs

• Errors in t&e assembl

• "an ot&ers

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Cassification of faiures

4/2/16 Concurrent Engineering BITS Pilani

• on Repairable , -ig&t bulb

, Transistor

, $io'e

, $omestic s5itc&es

suall a''resse' b "ean Time To !ailure +"TT!.

• Repairable , Sstems suc& as #utomobiles0 #ir con'itioners0 re%rigerators

suall a''resse' b "ean Time Bet5een !ailure +"TB!.

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"#$ecti%es of &eia#iity Engg


• To appl engineering no5le'ge an' specialist tec&ni7ues to pre)ent orto re'uce t&e lieli&oo' or %re7uenc o% %ailures8

• To i'enti% an' correct t&e causes o% %ailures t&at 'o occur0 'espite t&e

e%%orts to pre)ent t&em8

• To 'etermine 5as o% coping 5it& %ailures t&at 'o occur0 i% t&eir causes&a)e not been correcte'8

• To appl met&o's %or estimating t&e liel reliabilit o% ne5 'esigns0 an'

%or analsing reliabilit 'ata

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&eia#iity Economics


Traditional

Modern

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'efinitions

4/2/16 Concurrent Engineering BITS Pilani :

&eia#iity(

The ability of an item to perform a required function under

stated conditions for a stated period of time ( BS 4778)

)a*ar+ &ate(): (also called failure rate)

- Is the number of failures per unit time per the number of

items eposed for the same time

-ie 1; per 1

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Bathtu# cur%e

4/2/16 Concurrent Engineering BITS Pilani =

Applicable for electronic components

Initial failures (with in break-in) arehigh due to “infant mortality” due todesign, mfg , installation failures.

an be minimi!ed by "nal # at factoryto "lter good from bad

After initial break- in, failure will occurless and less and will reach constant$alue only random failures occur atconstant failure rate. %inally wear outwill accelerate the failure

Applicable for &echanicalcomponents

'hey do not ehibit constant

failure rate.

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'efinitions

• !ean time to failure (!TT"):

, T&e sum o% t&e sur)i)al time %or all o% t&e

components 'i)i'e' b t&e number o% %ailures8

T&is is use' %or parts t&at are not repaire' +lig&tbulb or transistor or spacecra%t.8 &en a part

%ails 0 t&e sstem %ails

• !ean time bet#een failures (!TB"):

, T&e mean time bet5een t5o successi)ecomponent %ailures8 "TB! is similar to "TT!0

but applie' %or components or sstems t&at can

be repaire'8

1

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E,am-es

"TT!

!or e>ample0 assume ou teste' 3 i'entical sstems starting %rom

time < until all o% t&em %aile'8 T&e %irst sstem %aile' at 1< &ours0 t&e

secon' %aile' at 12 &ours an' t&e t&ir' %aile' at 13 &ours8 T&e "TT!

is t&e a)erage o% t&e t&ree %ailure times0 5&ic& is 1186669 &ours

8 +1

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.%erage /aiure &ates for a 0ariety of

Com-onents an+ Systems

12Concurrent Engineering BITS Pilani4/2/16

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'efinitions

• I% R+t. is t&e reliabilit 5it& respect to time0t&en !+t. is t&e unreliabilit +probabilit o%

%ailure. in t&e same time8

• I% components are put on test0 t&e number

sur)i)ing to or at time t is 0 an' t&e number

t&at %aile' bet5een t@< an' t@t is 8

• $e%inition o% reliabilit

•


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Constant /aiure &ate

• !or t&e special case o% a constant %ailure rate

#lso items 5&ic& sur)i)e a%ter t&e t perio' s@ p +t . #

!or e>ample0 i% %ailure rate is 2 > 1

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'eri%ation

If there are s items sur$i$ing at time t, then in a smallperiod of ,

@

@

ln s ? ln C @ t

s @ C e>p + t . D I% p + t .

•


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&eia#iity With . 0aria#e /aiure &ate

• "ec&anical %ailures0 an' some %ailures o% electronic components0 'onot e>&ibit a perio' o% constant %ailure rate8

• Since t&e %ailure rate is a %unction o% time0 t&e simple e>ponential

relation %or reliabilit no longer applie'8

• Instea'0 reliabilit is e>presse' b t&e eibull 'istribution

m, s&ape parameter , Scale parameter +C&aracteristic )alue.

t ran'om )ariable

•


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System &eia#iity

• T&e o)erall reliabilit o% t&e sstem 'epen's on &o5 t&e in'i)i'ualcomponents 5it& t&eir in'i)i'ual %ailure rates are arrange'8

• I% t&e components are arrange' so t&at t&e %ailure o% an component

causes t&e sstem to %ail0 it is sai' to be arrange' in series

•


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System &eia#iity

• # muc& better arrangement o% components is one in 5&ic& it is

necessar %or all components in t&e sstem to %ail in or'er %or t&e

sstem to %ail8 T&is is calle' parallel reliability:

•

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* +ystem ./

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System &eia#iity 12

• I% 5e are 'ealing 5it& a constant%ailurerate sstem

• Series

• Parallel

•

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Binomia +istri#ution

• T&e reliabilit o% an nouto%m sstem isgi)en b a binomial 'istribution

2

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So%e+ -ro#em( 1457 'ieter Schmi+t


ple engineering design can be described by a reliability block diagram as shobelow. In subsystem A, two components must operate for the subsystem to funssfully. +ubsystem has true parallel reliability. alculate the reliability of eachstem and the o$erall system reliability.

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Soution


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Maintenance an+ &e-air

• #n important categor o% reliabilit problems'eals 5it& maintenance an' repair o% sstems

• I% a %aile' component can be repaire' 5&ile a

re'un'ant component &as replace' it in ser)ice0

t&en t&e o)erall reliabilit o% t&e sstem is

impro)e'

• I% components subFect to 5ear can be replace'

be%ore t&e &a)e %aile'0 t&en t&e sstemreliabilit 5ill be impro)e'

• Pre)enti)e maintenance is aime' at minimiGing

sstem %ailure


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Maintaina#iity

• # re'un'ant sstem continues to operate 5&en a component &as%aile'0 but it ma become )ulnerable to s&ut'o5n unless t&e

component is repaire' an' place' bac in ser)ice8

&ere "TB!@ mean time bet5een %ailures

"TT!@mean time to %ail

"TTR@mean time to repair

• !aintainabiltiy is t&e probabilit t&at a component or sstem t&at

&as %aile' 5ill be restore' to ser)ice 5it&in a gi)en time8

t- repair rate

r- permissible time to carry out the repair

•


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Maintaina#iity

• !or a large computer installation0 t&e maintenance cre5 logboo s&o5s t&ato)er a perio' o% a mont& t&ere 5ere 1 unsc&e'ule' maintenance actions

or 'o5ntimes0 an' 12

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.%aia#iity

• #)ailabilit is t&e concept t&at combinesbot& reliabilit an' maintainabilitD it is t&e

proportion o% time t&e sstem is 5oring

Hon line to t&e total time0 5&en t&at is'etermine' o)er a long 5oring perio'8

•


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'esign for &eia#iity

• T&e 'esign strateg use' to ensure reliabilit can %all

bet5een t5o broa' e>tremes

• T&e fail-safe approach is to i'enti% t&e 5ea spot in t&e

sstem or component an' pro)i'e some 5a to monitor

t&at 5eaness +re'un'anc etc.

• !re7uentl an absolute #orst-case approach is use'0 in it

t&e 5orst combination o% parameters is i'enti%ie' an' t&e

'esign is base' on t&e premise t&at all can go 5rong at

t&e same time

, T&is is a )er conser)ati)e approac&0 an' is o%ten lea's to

o)er'esign


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'esign for &eia#iity 12

• T5o maFor areas o% engineering acti)it

'etermine t&e reliabilit o% an engineering

sstem

18 Pro)ision %or reliabilit must be establis&e' 'uringt&e 'esign concept stage0 carrie' t&roug& t&e

'etaile' 'esign 'e)elopment0 an' maintaine'

'uring t&e man steps in manu%acture

28 (nce t&e sstem becomes operational0 it is

imperati)e t&at pro)ision be ma'e %or its

continue' maintenance 'uring its ser)ice


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Causes of 9nreia#iity

• $esign mistaes , #mong t&e common 'esign errors are %ailure to inclu'e all important

operator %actors0 incomplete in%ormation on loa's an' en)ironmental

con'itions0 erroneous calculations0 an' poor selection o% materials

• "anu%acturing 'e%ects

, Poor sur%ace %inis&0 'ecarburiGation crac in &eattreate' steel

• "aintenance

• E>cee'ing 'esign limits

, E>cee'ing limits o% temperature0 spee'0 etc8

• En)ironmental %actors

, SubFecting e7uipment to en)ironmental con'itions %or 5&ic& it 5as not

'esigne'

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Minimi*ing /aiure

• # )ariet o% met&o's are use' inengineering 'esign practice to impro)e

reliabilit

, "argin o% sa%et , $erating

, Re'un'anc

, $urabilit , $amage tolerance

, Ease o% Inspection

3

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/ME.

• !ailure mo'e an' e%%ects analsis +!"E#. is a team

base' met&o'olog %or i'enti%ing potential problems

5it& ne5 or e>isting 'esigns

• !"E# 5as %irst use' to i'enti% an' correct sa%et

&aGar's

• !"E# i'enti%ies t&e mo'e o% %ailure o% e)er

component in a sstem an' 'etermines t&e e%%ect on

t&e sstem o% eac& potential %ailure


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/actors in 'e%eo-ing /ME.

• T&ree %actors are consi'ere' in 'e)eloping a!"E#

, T&e se)erit o% a %ailure

, T&e probabilit o% occurrence o% t&e %ailure8

, T&e lieli&oo' o% 'etecting t&e %ailure in eit&er 'esign

or manu%acturing0 be%ore t&e pro'uct is use' b t&e

customer8

• Ris Priorit umber +RP.

• *alue o% RP can )ar %rom a ma>imum o%

1

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&ating for Se%erity of /aiure


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&ating of "ccurrence of /aiure


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&ating of 'etection of /aiure


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&esuts "f . /ME. .naysis


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.n e,am-e of /ME.


After

impro$ement

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Creating a /ME. Chart

18 T&e 'esign is re)ie5e' to 'etermine t&e interrelations o%assemblies an' t&e interrelations o% t&e components o%

eac& subassembl

28 o5 loo more broa'l0 an' as 5&at are t&e

conse7uences to t&e sstem o% eac& %ailure i'enti%ie' in

step1

38 !or eac& o% t&e %unctions0 list t&e potential %ailure mo'e

48 !or eac& o% t&e %ailure mo'es i'enti%ies0 'escribe t&econse7uences or e%%ect o% t&e %ailure

8 sing t&e se)erit o% %ailure table0 enter t&e numerical

)alue


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Creating a /ME. Chart 12

68 I'enti% t&e possible causes o% t&e %ailure mo'e

98 sing t&e occurrence o% %ailure table0 enter a )alue %or t&e

occurrence o% t&e cause o% eac& %ailure

:8 $etermine &o5 t&e potential %ailure 5ill be 'etecte'

=8 sing Table s&o5n earlier0 enter a rating t&at re%lects t&e

abilit to 'etect t&e cause o% t&e %ailure i'enti%ie' in step :

1

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'esign /or Safety

• Sa%et ma 5ell be t&e paramount issue in pro'uct 'esign

• ormall 5e tae sa%et %or grante'0 but t&e recall o% an

unsa%e pro'uct can be )er costl in terms o% pro'uct

liabilit suits0 replace' pro'uct0 or tarnis&e' reputation

• T&e pro'uct must be sa%e to

, "anu%acture

, se

, $ispose o% a%ter use

• # sa%e pro'uct is one t&at 'oes not cause inFur or

propert loss

4

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.s-ects To 'esign for Safety

18 "ae t&e pro'uct sa%e0 'esign all &aGar's out o% t&epro'uct

28 I% it is not possible to mae t&e pro'uct in&erentl

sa%e0 t&en 'esign in protecti)e 'e)ices lie guar's0

automatic cuto%% s5itc&es0 an' pressurerelie% )al)es0

to mitigate t&e &aGar'

38 I% step 2 cannot remo)e all &aGar's0 t&en 5arn t&e

user o% t&e pro'uct 5it& appropriate 5arnings lielabels0 %las&ing lig&ts0 an' lou' soun's

48 Pro)i'e training an' protecti)e clot&ing or 'e)ices to

t&e user or operator o% t&e e7uipment


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/ai:Safe 'esign

• # %ailsa%e 'esign sees to ensure t&at a %ailure 5ill eit&er nota%%ect t&e pro'uct or c&ange it to a state in 5&ic& no inFur or

'amage 5ill occur

• T&ere are t&ree )ariants o% %ailsa%e 'esign

, !ailpassi)e 'esign

• &en a %ailure occurs0 t&e sstem is re'uce' to its lo5estenerg state0 an'

t&e pro'uct 5ill not operate until correcti)e action is taen

, !ailacti)e 'esign

• &en %ailure occurs0 t&e sstem remains energiGe' an' in a sa%e operatingmo'e

, !ailoperational 'esign

• T&e 'esign is suc& t&at t&e 'e)ice continues to pro)i'e its critical %unction

e)en t&oug& a part &as %aile'


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Gui+eines for 'esign for Safety

18 RecogniGe an' i'enti% t&e actual or potential &aGar's0an' t&en 'esign t&e pro'uct so t&e 5ill not a%%ect its

%unctioning

28 T&oroug&l test prototpes o% t&e pro'uct to re)eal an

&aGar's o)erlooe' in t&e initial 'esign38 $esign t&e pro'uct so it is easier to use sa%el t&an to

use unsa%el

48 I% %iel' e>perience turns up a sa%et problem0 'etermine

t&e root cause an' re'esign to eliminate t&e &aGar'$% &eali'e that humans #ill do foolish thins and allo# for

it in your desin


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Gui+eines for 'esign for Safety 12

68 T&ere is a close correspon'ence bet5een goo'ergonomic 'esign an' a sa%e 'esign

98 "inimiGe t&e use o% %lammable materials0 inclu'ing

pacaging materials

:8 Paint an' ot&er sur%ace %inis&ing materials s&oul' be

c&osen to compl 5it& EP# an' (S# regulations %or

to>icit to t&e user an' %or sa%et 5&en t&e are burne'0

reccle'0 or 'iscar'e'

=8 T&in about t&e nee' %or repair0 ser)ice0 or maintenance

1

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Warning La#es

• it& rapi'l escalating costs o% pro'uct liabilit0manu%acturers &a)e respon'e' b plastering t&eir

pro'ucts 5it& 5arning labels

• arnings s&oul' supplement t&e sa%et relate' 'esign

%eatures b in'icating &o5 to a)oi' inFur or 'amage%rom t&e &aGar's t&at coul' not be %easibl 'esigne' out

o% t&e pro'uct 5it&out seriousl compromising its

per%ormance

• #ttention is ac&ie)e' b printing , $anger

, arning

, Caution


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T&an ou

Documents

QM ZG663-L8