3. Concepts of Reliability

8/8/2019 3. Concepts of Reliability

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RELIABILTY



RELIABILTY

DEFINITION

RELIABILITY IS THE PROBABILITY OF AN DEVICE

PERFOMING ITS INTENDED FUNCTION OVER A GIVEN

PERIOD OF TIME UNDER A GIVEN OPERATING CONDITION.

RELIABILITY IS THE PROBABILITY THAT A SYSTEM WILL

OPERATE WITHOUT FAILURE FOR A GIVEN PERIOD OFTIME UNDER A GIVEN OPERATING CONDITION.



DETERMINANTS OF RELIABILITY

PROBABILITY

IT IS PROBABLE THAT A GROUP OF ITEMS

IN A SYSTEM WILL HAVE AVERAGE LIFE WHICH CAN BE

PREDICTED IF WE KNOW THE FREQUENCY DISTRIBUTION

OF LIFE SPAN OF ALL ITEMS.

INTENDED FUNCTION :

PERFORMANCE CHARACTERISTICS.

OPERATING CONDITIONS

CONDITIONS OF STRESS, TEMPERATURE,

HUMIDITY, SHOCK, PRESSURE, VIBRATION, CORROSIVE

ENVIRONMENTS, ACOUSTICS,VOLTAGE ETC.



TIME

FUNCTIONING OF THE ITEM SHOULD BE IDENTIFIED FOR

A STATED PERIOD OF TIME.THE STATED PERIOD CANSECONDS, MINUTES, HOURS OR EVEN YEARS.

OPERATIONAL READINESS

PROBABILITY THAT AT ANY POINT OF TIME IT EITHER

OPERATES SATISFACTORILY OR READY FOR OPERATION.

AVAILABILITY

PROBABILITY THAT AN EQPT OPERATE SATISFACTORILY

AT ANY POINT OF TIME OUT OF THE TOTAL TIME

AVAILABLE. [ OPR TIME, REPAIR TIME, ADMIN TIME etc].



DEVICE: STANDS FOR EQPT , SYSTEMS, SUBSYSTEMS OR

COMPONENTS.

MEASURES: RELIABILITY MEASURED IN TERMS OF MEAN

TIME BETWEEN FAILURE (MTBF).



WHY RELIABILITY STUDY

SOME FACTS OF WW II

FOR EVERY VACCUM TUBE IN USE THERE WAS ONE SPAREAND SEVEN IN TRANSIT.

AN ARMY STUDY REVEALS THAT BETWEEN TWO THIRDAND THREE FOURTH OF EQPT UNDER REPAIR OR U/S AT

ANY MOMENT.

AIR FORCE STUDY REVEALS THAT REPAIR ANDMAINTAINENCE COST WAS TEN TIMES THE ORIGINAL COST.

IN 1949 24 MAINTENANCE MANHOURS WERE REQUIRED

PER FLIGHT HOUR IN NAVY. THIS ROSE TO 80MAINTENANCE MANHOURS IN 1965 DUE TO INCREASE INELECTRONICS EQPT FROM ABOUT 120 IN 1949 TO ABOUT3300 IN 1965.



AREAS OF RELIABILITY

Design

Establishment of performance and specification, replaceability, economy,

anticipated level of user, reliability planning, systems design, operating,

environmental and usage conditions, human engg and safety factors,

derating, tolerances, fool proofing, fail safe, considerations, redundancy,life tests and life predictions, accelerated tests, deterioration tests, test

production.

Purchase

Quality and reliability requirements, incentives, reliable procurement

Planning.



Manufacture

Process planning, investigation of process, effectiveness, worker

education and training, maintenance of mfg stds, material testing,intermediate inspection, warranty inspection, ageing, debugging.

Storage and Transportation

Packing, Storage environment conditions, transportation and handling,reliability check.

Management

Training of servicing personnel, service network, preventive maint,

replacement policies, instruction manuals.



Usage

Use and procedures, operator training, Environmental control,method of detecting failure and disposition, preventive maint, periodic

inspection and maint, maint contract, procurement and storage of

spares, replacement of standards.

User inspection

Plant inspection, acceptance sampling plans, process checks,

reliability demos.



FEATURES OF RELIABILITY

IT IS A DESIGN /PRODUCTION FEATURE.

IS NOT PERFORMANCE,BUT REPEATABILITYOFPERFORMANCE.

IS NOT QUALITY AS QUALITY RELATES TODEGREE OF CONFORMANCE TO SPECIFICATIONS

AND NOT TO AVAILABILITY/EFFECTIVENESS INOPERATION.



RELIABILITY SPECIFICATIONS

1. DEFINATION OF THE SYSTEM.

2. INFO CONCERNING SYSTEM, PRODUCTION &

MODIFICATION STATUS.

3. DESCRIPTION OF OPERATING & MAINT. CONDITION.

4. DEFINATION OF MALFUNCTION

5. OTHER CONSIDERATIONS LIKE STORAGE

HANDELING,QUALITY CONTROL WORKMANSHIP.



CAUSES OF UNRELIABILITY :

POOR DESIGN AND OR DEVELOPMENT

SUB STANDARD WORKMANSHIP.

INADEQUATE TESTING.

MISMATCH IN ENVIRONMENT CONDITIONS SPECIFIED

AND OPERATED IN.

INCREASED COMPLEXITY OF PRODUCT/ENGG DESIGN.

LACK OF LIASION BETWEEN DESIGN AND/OR DEVELOPMENT AGENCIES AND USERS.

ATTEMPTING FALSE COMPETITIVE GAIN BYSTRETCHING PERFORMANCE / REDUCTION IN WEIGHTETC.

HUMAN ERROR.



IMPROVING RELIABILITY

SIMPLIFY DESIGN & THEREBY REDUCETHE NO

OF COMPONENTS IN ASYSTEM.

FIND MORE RELIABLE COMPONENTS ,MATERIALM & TECHNOLOGY.

ADOPT COMPONENT DERATING TECHNIQUE TOREDUCE FAILURE RATES WHICH ARE BELOWAVERAGE.

USE REDUNANCIES TO INCREASE RELIBILITY.

ESTABLISH REALISTIC ECONOMIC LIFE &OPERATING CONDITIONS.



AVIATION RELIABILITY INDICIES

No of PrematureWithdrawals.

Pilot¶s & Engineers Report on Technical Defects.

No of Defect Reports Raised.

Average Time for Rectification of Defects.

Occurrence of Repetitive Snags.

Maintenance Man Hours per Flying Hour.

Consumption Data of Spares.

Maintenance Cost per Flying Hour.



RELIABILITY & QUALITY

Quality means Conformations to Specificationswhile Reliability is Particularly Concerned with

Time.

Ability to Perform for a Stated Time period is

Central to Reliability while Ability of the Itemto Perform a Required Function Under StatedCondition is Common for Both.



RELIABILITY FUNCTION



DERIVATION OF RELIABILITY FUNCTION.

LET N BE THE TOTAL NO OF COMPONENTS TO BE TESTED

AFTER TIME t, FAILED COMPONENTS = mCOMPONENT SURVIVED = n.

N= m+n, RATE OF FAILURE (P) = m / ( m+n) = m/N.

Let Probability of Survival be R(t) & Prob of Failure be F(t).

Then we have R(t) + F(t) =1 or

PROBABILITY OF SURVIVAL, R(t) = 1- F(t) =1-m/N

Differentiating w.r.t time we have dR(t)/dt = -1/N* dm/dt.

or dm/dt = -N* dR(t)/dt

or 1/n*dm/dt = -N/n* dR(t)/dt which is the component failure rate.

P( t)= -1/ R(t) *dR(t)/dt

´P( t)* dt = -´ dR(t)/dt or - ´ P( t)* dt = In R(t) Therefore

R(t) = exp[- ´ P( t)* dt ] is the general formula for Reliability Functio

or R(t)= exp[ - P( t)] = exp [ -t/m ] = exp[ -t/ MTBF]



PROPERTIES OF RELIABILITY

The Probability of Survival or Reliability R(t) at time

t, has the following properties

0 <R(t) < 1

R(0) = 1 & R( ) = 0

R(t) in general is a decreasing function of

time.



WEIBULL ANALYSISA Graphical technique based on past Data for

Identifying Failure Distributions

Risk Predictions

Forecast Analysis for R&M Studies



WEIBULL DISTRIBUTION

F(t) = 1- R(t) = 1- exp-{( t- tc) F} tu0 , F>0.

Where F(t) is the Failure Rate Function or Fraction Failure

time t.

tc the starting Point or Origin.

the Scale Parameter or characteristic life . F the Shape or Slope Parameter.

& exp the exponential.

By taking a double log we can deduce the above functionin form a st. line function of Y = Bx + A and thus solve

Weibull Curve.

The slope of this function i.e F is very useful in analyzing

the failure Mechanisms.



Failure Distribution Characteristics

F ( t )

t

= 2.5

= 3.44 = 0.5

= 5.0

= 1.0



ANALYSIS OFWEIBULL CURVES

F= SHAPE PARAMETER.

F<1: FAILURE RATE DECREASING FUNCTION OF TIME.

Effect is that reliability increases as unit ages. Failure Rate Higher at

Initial Periods or Infant Mortality Failure Mode for Quality Control

or Assembly Problems.

F=1: FAILURE RATE IS CONSTANT.

This Reflects Original Design Deficiencies, Insufficient Redundancies,

Unexpected Failures or even Product Misuse.

F>1: FAILURE RATE INCREASINGW

ITH TIME.RepresentsWear Out Modes. Between 1.8 & 3.0 more scatter in

failure Data & failure Predictions cover long Time Spans

Reflecting uncertainty.

F =4: WEIBULL DISTRIBUTION BECOMES NORMAL DISTN.

3. Concepts of Reliability

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