tqm iem_reliability (1)

8/8/2019 TQM IEM_Reliability (1)

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Tapan Bagchi TQM IEM Reliability 1

ReliabilityReliabilityReliability


ReliabilityReliability

• Generally defined as the ability of a

product to perform as expected over

time

• Formally defined as the probability that

a product, piece of equipment, or

system performs its intended functionfor a stated period of time under

specified operating conditions




Maintainability

• The probability that a system or product

can be retained in, or one that has

failed can be restored to, operating

condition in a specified amount of time.


Types of Failures

• Functional failure – failure that occurs

at the start of product life due to

manufacturing or material detects

• Reliability failure – failure after some

period of use

These relate to the “bathtub curve”.




Types of Reliability

• Inherent reliability – predicted by

product design

• Achieved reliability – observed during

use; based on observed failure data


How do you measure Reliability?

• Failure rate (λ) – number of failures

per unit time

• Alternative measures

–Mean time to failure

– Mean time between failures (MTBF)




Failure Rate Curve

“Infant

mortality

period”


Cumulative Failure Rate Curve




Average Failure Rate = 0.02


Typical Forms of FailureTypical Forms of Failure

Early failuredue to design faults,

poor quality components,

manufacturing faults,

installation errors,

operator & maintenance errors

Useful life

has a low, constant failure rate

Wear-out failureparts approach the end of life

Time

F a i l u r e

R a t e

EarlyFailure Useful

Life

Wear-outFailure




Measuring ReliabilityMeasuring Reliability

Reliability R(t) :The probability of operating to anagreed level of performance

Unreliability F(t) :The probability of failing to operateto an agreed level of performance

( ) ( ) 1=+ t F t R


Reliability Function for Service Life

• Probability density function of failure time

is exponential: f(t) = λe-λt for t > 0

• Probability of failure from (0, T)

F(t) = 1 – e-λT

• Failure rate =λ

• Reliability function

R(T) = 1 – F(T) = e-λT




In general, Failure Times fit

Weibull DistributionIn probability theory and statistics, the Weibull distribution is a

continuous probability distribution with the probability density function

for and f ( x; k , λ) = 0 for x < 0, where k > 0 is the shape

parameter and λ > 0 is the scale parameter of the distribution.

The Weibull distribution is often used in the field of life data analysis due

to its flexibility—it can mimic the behavior of other statistical distributions

such as the normal and the exponential. If the failure rate decreases over

time, then k < 1. If the failure rate is constant over time, then k = 1. If the

failure rate increases over time, then k > 1.





The Weibull Distribution

expressions


1.0000.0000200010

2.0000.062123

0.9390.06212318779

0.6560.060120

0.8790.12224317578

0.2360.03365

0.8460.15430816927

0.2350.04182

0.8050.19539016106

0.2330.052103

0.7540.24749315075

0.2350.066131

0.6880.31262413764

0.2350.083166

0.6050.39579012103

0.2350.105210

0.5000.500100010002

0.2980.175350

0.3250.67513506501

0.3880.325650

0.0001.000200000

F(t) = 1 - R(t)

ReliabilityR(t)=

S(t)/20 00

r(t) = n(t)/ avg S

f(t)= n(t)/

2000 Surviving S(t)

Cumulative No.

of

Failures

ObservedNo. ofFailure

s n(t) Time t

Life Testing Data




r(t) and R(t) Calculations displayed

Failure Rate vs. Time

0.000

0.500

1.000

1.500

2.000

2.500

0 1 2 3 4 5 6 7 8 9 10Time

r(t) Faiure Rate

Reliability R(t)


Reliability ofReliability of NonNon--RepairableRepairable ItemsItems

Mean Time To Fail (MTTF)ratio of total up time to numberof failures.

Mean Failure Rate (λ )inverse to MTTF.

Mean Down Time (MDT)ratio of total down time tonumber of failures.

T1 T2 T3

Td1 Td2 Td3




Reliability ofReliability of RepairableRepairable ItemsItems

Mean Failure Rate (λ )inverse to MTBF.

Mean Time Between Failures(MTBF)ratio of total up time to numberof failures.

Total Up Time (T up)total time minus total down time

Tup1 Tup2 Tup3

Td1 Td2 Td3

T


Availability

• Operational availability

• Inherent availability

MDT MTBM

MTBF AO

+=

MTTR MTBF

MTBF

A +=

MTBM = mean time between

maintenance

MDT = mean down time

MTBF = mean time between

failures

MTTR = mean time to repair




Design for ReliabilityDesign for Reliability

Element selectionelements with well-established failure rate data

Environmentelements can withstand normal working environment

Minimum complexityfewer elements (series systems)

Redundancyseveral identical elements in parallel

Diversitya give function is carried out by two parallel systems


Series Systems

RS

= R1

R2

... Rn

1 2 n




Reliability of Series SystemReliability of Series System

Reliability Reliability of a series

system is the product of

individual element

reliabilities reliabilities .

System reliability isSystem reliability is

lower than the lowestlower than the lowest

element reliabilityelement reliability

R R11

I

R R 2 2

R R n n

O

t

t t t

nsystem

n

n

e

eee

R R R R

)(

21

21

21

λ λ λ

λ λ λ

+⋅⋅⋅++−

−−−

=

⋅⋅⋅⋅=

⋅⋅⋅⋅=


Parallel Systems

RS = 1 - (1 - R1) (1 - R2)... (1 - Rn)

1

2

n




Reliability of Parallel SystemReliability of Parallel System

Reliability Reliability of a parallel

system is determined by

the product of individual

element unreliabilities unreliabilities .

System reliability isSystem reliability is

greater than thegreater than the

greatest elementgreatest element

reliabilityreliability

)1()1()1(1

1

21

21

t t t

nsystem

neee

F F F R

λ λ λ −−−−⋅⋅⋅−⋅−−=

⋅⋅⋅⋅−=

R R11

I R R 2 2

R R n n

O


Series-Parallel Systems

• Convert to equivalent series system

AA BB

CC

CC

DD

RRAA RRBB RRCCRRDD

RRCC

AA BB CC’’ DD

RRAA RRBB RRDD

RRCC’’ = 1= 1 – – (1(1--RRCC)(1)(1--RRCC))




Reliability Management

• Define customer performance requirements

• Determine important economic factors and

relationship with reliability requirements

• Define the environment and conditions of

product use

• Select components, designs, and vendors

that meet reliability and cost criteria

• Determine reliability requirements for machines and equipment

• Analyze field reliability for improvement


Configuration Management

1. Establish approved baseline

configurations (designs)

2. Maintain control over all changes in the

baseline programs (change control)

3. Provide traceability of baselines and

changes (configuration accounting)




Design Issues

• Access of parts for repair

• Modular construction and

standardization

• Diagnostic repair procedures and

expert systems


MaintainabilityMaintainability

• Maintainability is the totality of design

factors that allows maintenance to be

accomplished easily

• Preventive maintenance reduces the

risk of failure

• Corrective maintenance is the

response to failures




Reliability Engineering

• Standardization

• Redundancy

• Physics of failure

• Reliability testing

• Burn-in

• Failure mode and effects analysis

(FMEA)• Fault tree analysis (FTA)


FTA

http://www.weibull.com/basics/fault-tree/index.htm




BulbFails

Noelectricity

Power Plant

Fails

Power Line

Fails

GlassBroken

FilamentBroken

Connector

Corroded

VacuumLeak

Tree Breaks

Line

Wind Breaks

Line

Impurities Vibrations

Fault Tree Analysis (FTA)

Example:


Bicycle fails when I rush to class

Draw the FTA:

Hint: Draw an FTA diagramfor the total system first.




Faults/Pathways Magnified N -fold for a

Simple Manufacturing Process!


FMEA

http://www.npd-solutions.com/fmea.html




High-Level Combinations of

Severity and Probability

Increasing Severity of Harm/Consequence

I n c r e a s i n g P r o b a b i l i t y o f

O c c u r r e n c e

Low

Risk

Medium

Risk

High Risk


FMEA – Why?

• Why FMEA’s?

• Definition, Purpose,

Types, Benefits

• Team Approach

F M E A F M E A

IntroductionIntroduction




FMEA – Definition

• Identify potential failure modes

• Prioritize actions

• Document the process.

FMEA is a Structured group of activitieswhich...


FMEA – Purpose

F a i l u r e s

TimeCrisis (Production start)

FMEA




• Rate severity of failure modes

• Identify actions to reduce occurence

• Test adequacy of controls

FMEA – Purpose

FMEA’s are intended to ...


Potential failure Modes

No function

Partial function

Failure Mode Type Example

Not operational

Not all of function operating




Severity (Weightfactor)

What is the severity ofeach effect identified?


Rating criteria for Severity(Weightfactor)Criteria: Severity of effect ClassEffect

Accepted failure DNone

Safety failureS

Unacceptable

risk

Non-conformingwith safety

Correction is nescessary

A

Relative bigrisk

Correction is recommendedB

risico Correctie is nuttigCMinimum risk Correction is usefull C




Potential Cause of Failure

It is a weakness in the design with a failure modeas effect.

(see next slide)


Manufacturing misbuilds

Due to design Deficiencies

+

-

+

-




Manufacturing misbuilds

Robust Design done after FMEA

+

-+

-


Use FishboneUse Fishbone Diagram:Diagram:

Searching for Causes of failure

“Textunreadable”

Ink of poor quality

Text in wrong location




Cause of failure – “Why”-ladder

Design requirement

WHY?level 1

““TextText unreadableunreadable””

WHY?level 2

WHY?level 3

Ink doesn’t stick

Surface roughness

not ok.


Sentencing Technique: Is it an

effect or a cause?

FailureMode

Could result in Effect

Due to

Cause




Sentencing Technique -

Example

“TextUnreadable”

Could result in

Dissatisfiedcustomer

Due to Surfaceroughness

(designreq.)


Product-FMEA – Occurrence

What is the probability that

the failure will occur?




Possible Failure Rates RankingProbability of failure

> 1 of 20

Very high

Moderate > 1 of 400

> 1 of 15000

Low < 1 of 15000

4

5

3

2

1

Rating criteriaRating criteriaof occurrenceof occurrence

≥ 1 of 3


Product-FMEA – Actions/Solutions

What are the possible actions to:

- eliminate the failure

- reduce effect

- reduce occurrence




Do the Bicycle exercise again—by

FMEA

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