15 reliability 1 - student

8/8/2019 15 Reliability 1 - Student

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Reliability

Assoc. Prof. Ir. Dr. Cheong Kuan YewSchool of Materials & Mineral Resources Engineering

Engineering CampusUniversiti Sains Malaysia



Topic Outcome:

At the end of this topic, students will be able to:

Describe the term “reliability”.

Discuss how to improve the reliability of a product.Apply statistical aspect in reliability.

Explain a life-history curve of a product.

Construct and discuss OC curve related to reliability.

Use life & reliability testing plan.

Differentiate between availability & maintainability



Topic Outline:

What is Reliability?Achieving Reliability.

Statistical Aspects of Reliability.Life-History Curve.Failure Analysis.Availability & Maintainability.



(1) What is Reliability?

A reliable product or service meaning that it works for longer period of time before failure occurs.

Reliability is:Quality over a long run, or

A probability of success, or,

A probability (numerical value) that a product or aservice will perform its intended function satisfactorily for a prescribed life under certainstated environmental conditions .

4 factors associated with reliability.



Can Reliability be measured ?

(numerical value)

Reliability, R, is usually measured on

a “0” to “1” scale,a percentage, or

parts per million (ppm).



Examples:

R = 1 or 100% no failure of the product or service.

R = 0 or 0% certain failure.

R = 0.5 or 50% the product or service isexpected to fail on half the occasions when it isused.



Common expressions of reliability also include:Failure rate per 1000 hours (2% failures per

1000 h).Failure rate per 1000 usage cycles.Mean Time Between Failure (MTBF) (MTBF =

3,500 h).Probability of an item or service failing (failure

= 2ppm).Reliability over a fixed period (99% reliability

after 5,000 h).

Failure

in time(FIT)



What is the intended function of the

product/service?

Product/service

designed for particular applications.expected to perform it task.



What is the intended life of the

product/service?How long is the product/service is expected to last.

Product lifeUsage

Time, or

Both.



What is the environment conditions

of the product/service usage?

Storage, transportation, and usage conditions.

Would it be more severe than actual recommendedcondition?



Relationship between Reliability

& Customer ExpectationKey Customer Variables Versus Product Categories/Applications

Environment.

E. R. Hnatek, “Practical Reliability of Electronic Equipment

and Products”, Marcel Dekker: NY, 2003, p3.

Calculator PC Pacemaker Auto Airline SatellitePrice Low Extremely

highDiscomfort &repercussion caused bymalfunction

Low

Designed-in Reliability Low

Customer expectations Low



Reliability vs Durability vs

RobustnessDurability: The ability to endure with ongoing preventive

maintenance/servicing

Functional: continue to function correctly with no unscheduled

breakdown.

Robustness: Ability to continuously functioning correctly under

stressed conditions (high temp, altitude, shock..)



(2) Achieving Reliability

Reliability must start from very beginning.

Fully understand customer’s needcustomer’s need , marketmarket

requirement, competitive analysis, comparisonrequirement, competitive analysis, comparisonwith previous productwith previous product , etc.

Then translate these to subsequence phases.

2 approaches: (1) top-down (2) bottom-up.

Top-downTop -down market demand & competitive analysis

Bottom-upBottom-up comparing current product to previousproduct in terms of complexity, technologycapability,and design/manufacturing process.



By combining the two approaches, there are6 main factors that a reliable

product/service can be achieved:(1) Emphasis (Penegasan).

(2) System.

(3) Design.

(4) Production.

(5) Transportation.

(6) Maintenance.



(1) Emphasis

Increase emphasis is needed because of:

Legal means,

More complicated product/service, or

Automation.



(2) System Reliability

Complex product/service

many different components

a system

chances of components not working is increased.

Therefore, the method of arranging the componentsaffects the entire system reliability.

Series, parallel, or combination.



(2) System Reliability(Components in Series arrangement)

Reliability of a system depends on individualcomponents.

Multiplicative theorem is applied.

System Reliability, R S =(R A)(R B)(R C)=0.7160.716

System reliability is always less than the lowestreliability value of the components.

One component fails, the whole systemOne component fails, the whole system

not working.not working.

R A=0.955 R B=0.750 R C=0.999

Component A Component B Component C



(2) System Reliability(Components in Parallel arrangement)

One component fails, the whole system still able to work;until all parallel components do not function.

R s = 1 – (Probability of component I fail)(Probability of component J fail)

RS = 1 – (1-R I)(1-R J )=1-(1-0.750)(1-0.840)= 0.960

R I=0.750

R J=0.840

Component I

Component J



(2) System Reliability(Components in Parallel arrangement)

As the number of components in parallel increases, thereliability increases.

The reliability for a parallel arrangement of components isgreater than the reliability of the individual components.



(2) System Reliability(Components in Combination arrangement)

Series + parallel arrangements of components

RS = (R A)(R I,J )(R C) = (0.95)(0.96)(0.99) = 0.90

R A=0.955

R I=0.750

R C=0.999

Component A

Component I

Component C

R J=0.840Component J



Q & A

RS = (R A)(R I,J )(R C)(R K,L,M )

= (R A)[1-(1-R I)(1-R J )](RC)[1-(1-R K)(1-R L)(1-R M)

R A=0.955

R I=0.750

R C=0.999

Component A

Component I

Component C

R J=0.840Component J

R K =0.750Component K

R L=0.840Component L

R M=0.999

Component M



(3) Design

Fewer number of components, greater the reliability

Approximate calculation (series) R s =R n,

where n is the number of components and R is thereliability of the component,assuming the reliabilityis the same for all the components.



(3) Design

Other techniques:

Having a backup or redundant component (in parallelarrangement; cheaper redundant component)see next slide

Over-design.

Having a fail-safe type of device (safety concern)

Maintenance.Protection for certain environment.

Investment in reliability (RM) reliability

Up to certain level.



(3) Design

(redundancyredundancy )Redundancy (Kelebihan) is the duplication of partsdup lication of parts or

features in such a way that the duplicate can take over the function of another part in the event of failure.

Eg: In a two-engine aircraft, the second aircraft enginewill propel the aircraft if one engine fails.

The addition of redundant parts to a product can

improve the reliability of a system enormouslyimp rove the reliability of a system enormously .It is important with safetysafety related products and services.

There are 2 types of redundancy2 ty pes of redundancy :



(3) Design

(Active redundancy)Active redundancyActive redundancy occurs when all the redundant items

are in operation at the same timeoperation at the same time .

Examples:All 4 aircraft engines operating at the same time

when only 1 engine is enough at cruising altitude(one engine is not enough for take off).

Both hydraulic brake circuits in your car alwaysworking.Only one circuit is enough to stop the car innormal driving (but not enough for an emergencystop).



(4) Production

Basic quality techniquesBasic quality techniques will minimize the risk of product/service unreliability.

Emphasis should placed on those components which areleast reliable.



(5) Transportation

Product transports to customer.

The actual performance of the product by the customer is

the final evaluation.Good packaging techniques and shipment evaluation are

essential.



(6) Maintenance

Designers try to eliminate the need for customer maintenance.

Is it practical?Product should have ample warming when failure occurs.

(light or buzzer).

Maintenance should be simplesimp le and easyeasy to perform.



Discipline & Tasks Involved with

Product ReliabilityA study of 72 nondefense corporations revealed that the

product reliability techniques they preferred and felt tobe important were the following:Supplier control 76%Parts control 72%

Failure analysis and correction action 65%

Environment stress screening 55%

Test, analyze, fix 50%

Reliability qualification tests 32%

Design reviews 24%

Failure modes, effects, and critically analysis 20%

N.H. Criscimagna, “Benchmarking Commercial Reliability Practices”, IITRI, 1997.



Reliability Goals & Metrics

Typical reliability metrics for a high-reliability, high-availability,fault-tolerant product are shown below:

Metric Definition

Correctivemaintenance (CM)rate

What customers see CMs are maintenance activities done in a reactive mode andexclude proactive activity such as preventive maintenance.

Part replacement(PR) rate

What factory &logisticsorganization see

A part replacement is any part replaced during a CM activity.

Failure rate What engineers see A returned part that fails a manufacturing or engineering test.Any parts that pass all tests are called no trouble found (NTF). NTFs are important because they indicate a problem with outtest capabilities, diagnostics, or support process.



Each of the stated reliability metrics takes one of threeforms:

CM/PR/failure rate goal based on market demand.Expected CM/PR/failure rate based on predictions

(Technology/Process Capability)

Actual CM/PR/failure rate based on measurement.

The relationship among the various metrics



ExpectedCM/PR/F

R

ActualCM/PR/F

R

GoalCM/PR/F

R

Actual > ExpectedPotential design or process problems

Expected > GoalConsider newtechnology or

design/mfg/maintenance

Actual > GoalPotential competitive

Disadvantage



ReliabilityModel

-baselineunderstanding

of product’sreliability

Reliability Prediction

ExpectedCM/PR/F

R

ActualCM/PR/F

R

GoalCM/PR/F

R



Reliability Prediction

Limitations of Reliability Prediction

Simple technique omit great deal of distinguishing

detail and the very prediction suffers inaccuracy.Detailed prediction techniques can become

bogged down in detail and become very costly. Theprediction will also lag far behind and may hinder

timely product development.



Considerable effort is required to generate sufficient dataon a part class/level to report statistically validreliability figures for the class/level.

Other variants that can affect the stated failure rate of agiven system are uses,operator procedures,maintenance and rework practices,measurement techniques or definitions of failure,operating environments, and excess handling differingfrom those addressed by modeling techniquemodeling technique .



(3) Statistical Aspects of Reliability

(Distribution Applicable to Reliability)

Types of probability distribution used in reliabilitystudies are:

continuous probability distributionExponentialNormalWeibull

Gammadiscrete probability distribution

GeometricNegative binomial



(Frequency Distribution Curves)Only Exponential, Normal, and Weibull distributions are

widely used.

Their frequency distributions, f(t), as a function of timeare given below.



(Reliability Curves)

Reliability curves for exponential,normal, and Weibulldistributions are given below:

Exponential Normal Weibull



(Reliability Curves)

( )β

θ

θ

t t

t t

t t

e R

Weibull

dt t f R

Normal

e R

l Exponentia

−

−

=

∫ −=

=

:

)(0.1

:

:

0

Reliability as a function of time.

R t = reliability at time t

t = test time or cycleθ = mean life or Mean TimeBetween Failure (MTBF)

β = Weibull slope(or shape parameter)

Area under Normal Curve



(Failure-Rate Curves)Failure-rate, , is important in describing the life-

history curve of a product.

Failure-rate probability of a failure during a statedperiod of time, cycle, or number of impacts.Failure rate can be estimated from test data by use of

the formulae:(1) time terminated without a replacement.



(Failure-Rate Curves)

est = estimated failure rate

r = number of test failurest = test time for a failed itemn = number of items testedT = terminated time

( )∑ −+=

∑=

T r nt r

cycleor timetest

failurestest of number

est

est

λ

λ )(



(Failure-Rate Curves)Q & A• Determine the failure rate for an item that has the test of

9 items terminated at the end of 22 hours. Four of theitems failed after 4, 12, 15, and 21 h, respectively. Fiveitems were still operating at the end of 22 h.

( )025.0

22)49()2115124(4 =−++++=

∑ −+=

est

est

T r nt

r

λ

λ




est = estimated failure rater = number of test failurest = test time for a failed item

∑=

t r

est λ

(2) Time terminated with replacement.



(Failure-Rate Curves)Q & A• Determine the failure rate for 50 items that are tested for

15 h. When failure occurs, the item is replaced withanother unit. At the end of 15 h, 6 of the items hadfailed.

008.0)15(50

6 ==∑

=

est

est

t

r

λ

λ



(Failure-Rate Curves)(3) Failure terminated

Q & ADetermine the failure rate for 6 items that are testedto failure. Test cycles are 1025, 1550, 2232, 3786,5608, and 7918.

00027.0791856083786223215501025

6=

+++++=

∑=

est

est t r

λ

λ




Constant failure rate:

Exponential distribution andWeibull distribution ( =1)

1

2

2

:2

1

:

1

:

2

−

∞

−−

=

∫

−−

=

=

β

σ θ

θ θ β

λ

σ

θ λ

θ λ

t

Weibull

x

e

Normal

l Exponentia

t

t



(Failure-Rate Curves)Exponential Normal Weibull



(Failure-Rate Curves)In the previous equations, is the mean life or

Mean Times Between Failure (MTBF).

MTBF:How much time has elapsed between failures.It is used when speaking of repairable

systems .Another parameter that can be used to describe

reliability as a function of time is Mean Times toFailure (MTTF).

MTTF:It is used for non-repairable systems .



(Failure-Rate Curves)The amount of time that a system is actually

operating is of great concern.

Eg: without radar screen, air traffic controllers aresightless and therefore out of operation. To beconsider reliable, the radar must be functional for a significant amount of expected operating time.

Since many systems need preventive or Since many systems need preventive or corrective maintenance, a system’s reliabilitycorrective maintenance, a system’s reliabilitycan be judged in terms of the amount of timecan be judged in terms of the amount of timeit is available for use:it is available for use:




MTTF value can be replaced by MTBF.Mean time to repair = mean down time (MDT).

repair totimemeanMTTF

MTTF ty Availabili

+=



Q & AQ & A

• Determine the failure rate and MTBF for 6 items

that are tested to failure. Test cycles are 1025,1550, 2232, 3786, 5608, and 7918.

cycle

t r

est

est

37041

00027.0791856083786223215501025

6

==

=+++++=

∑=

λ θ

λ

λ Time -1



Q & AWindshield-wiper motors are readily available and easy

to install. Calculate the availability of the windshield

wipers on a bus driven eight hours a day, if the MTBFis 1250 hours. When the windshield-wiper motor mustbe replaced, the bus is out of service for a total of 24hours.

98.0241250

1250 =+

=ty Availabili

The bus is available 98 percent of the time.



Life-History Curve(Debugging Phase)

It is also called burn-in or infant-mortality or early failurephase.

A new machine or service, we often find it fails a fewtimes before it ‘settle down’ to a reliable state of performance.

Weibull distribution with < 1 is used to describe theoccurrence of failures in this phase.

Product is under warranty (usually).It is a significant quality costsig nificant quality cost.



Life-History Curve(Chance Failure Phase)

Random or constant failure phase.Failure rate is constant.

The product or service has ‘settle down’ and is reliable.Any failures that do occur are randomExponential and Weibull ( =1 ) distributions are used to

describe this phase.Reliability studies and Sampling Plans are concerned

with this phase.The lower the failure rate, the better the product.



Life-History Curve(Wear Out Phase)

The product is wearing out or the service supportsystems are beginning to fail.

Wear out failures tend to have a sharp rise in failure rate.Normal distribution is the best to describe this phase.Weibull distribution ( >1 ) can be used depending on the

type of wear-out distribution.



(8) Availability and

MaintainabilityTime related factors of availability, reliability, and

maintainability are interrelated.

Eg: when a water line breaks (reliability) it is nolonger available to provide water to customersand must be repaired or maintained.

Availability, A:

A time-related factor.Measures the ability of a product or service toperform its designated function.

Product available operation +standby.



Availability and Maintainability

MDT MTBF MTBF

DowntimeUptimeUptime

A+=+=

MTBF

MTTF MDT

Time

MTTF

If MTBF is defined as mean time before failure, then MTBF = MTTF.

MTTF



Availability and Maintainability

Maintainability:Maintainability:Preventive and corrective maintenance on a product

or service can be achieved.Mean time to repair,mean time to service,repair hoursper number of operating hours, preventivemaintenance cost, and down probability figure of merit for maintainability.

Keeping maintainability low more cost effectivemethod of keeping availability high thanconcentrating on reliability.

15 reliability 1 - student

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