concepts of reliability

7/28/2019 Concepts of Reliability

http://slidepdf.com/reader/full/concepts-of-reliability 1/25

1 Alan Pendry

Concepts

of

Reliability



2

Aims of this session

To examine the basic concepts of Reliability Engineering

To investigate Hazard Rates

To discuss the main characteristics of the Bathtub Curve

To discuss the Management of Reliability

Alan Pendry



3

What is Reliability?

Concerned with Failures during the lifeof a product

It is an aspect of engineeringuncertainty

It is concerned with the Probability thatan item will perform a required functionwithout failure under stated conditionsfor a stated period of time

Alan Pendry



4

Probabilistic ReliabilityWhen we consider Reliability as a Probability,any attempt to quantify it must involvestatistical methods

We can specify Reliability as: The mean number of failures in a given time

(Failure Rate)

The mean time between failures (MTBF) for itemsrepaired and returned to use

The mean time to failure (MTTF) for non-repaireditems

Alan Pendry



5

Probabilistic Reliability Variation is often a function of time or time-related factors – operating cycles, seasonality,maintenance periods, …

Historical Reliability data can not be used tomake credible forecasts of future behaviourwithout taking into account non-statisticalfactors – design changes, maintainer training,

unforeseeable problems, …

Alan Pendry



6

Repairable itemsReliability is the probability that failure willnot occur in the period of interest, whenmore than one failure can occur

Also expressed as Failure Rate or Rate of Occurrence of Failures (ROCOF), andMTBF under a constant rate of failure

Repairs take time, so we are alsoconcerned with Availability

Alan Pendry



7

Availability Availability is a function of Quality,Reliability and Maintenance orMaintainability (Mean Time To Repair

MTTR )It is affected by the failure rate andmaintenance time

Maintenance can be corrective orpreventive

Alan Pendry



8

Non-repairable itemsReliability is the probability of survival over theintended life, or for a period during its life, whenonly one failure can occur

The instantaneous probability of the first andonly failure is the hazard rate

Reliability characteristics include MTTF and theexpected time by which a certain percentagemight have failed

One failure leads to system failure, so systemfailure is a function of the time to the first failure

Alan Pendry



9

Hazard rate can vary with time

Constant rate for non-repairable itemscharacterises failures caused by excessiveloads at a constant average rate

Overstress failures due to accidental ortransient circuit overload and Maintenance-induced failures in mechanical equipmenttypically occur randomly and at a generallyconstant rate

Alan Pendry



10

Hazard rate can vary with time

Constant rate for repairable itemsindicates externally induced failures, asper non-repairable items, and is also

typical of complex systems subject torepair and overhaul, where different partsexhibit different patterns of failure with

time and parts have different ages sincerepair.

Alan Pendry



11

Hazard rate can vary with timeMaterial fatigue brought about by strengthdeterioration due to cyclic loading is a failuremode which does not occur for a finite time, and

then exhibits an Increasing probability of occurrence in non-repairable items

Increasing failure rates occur in repairable systems when wearout failure modes begin to

predominate

Alan Pendry



12

Hazard rate can vary with timeDecreasing rates for non-repairable items occurin items less likely to fail as their survival timeincreases

Repairable items show decreasing rates whenreliability is improved by progressive repair

“Burn In” of components improves reliability of surviving population

Combined effect generates the Bathtub Curvewhere death is analogous to failure

Alan Pendry



Bathtub Curve

Alan Pendry

Time

Infant

mortality

WearoutUseful

life

Externally

Induced

Failures

Total hazard rate bathtub

Wearout of

good items

Failures of weak items



14

Reliability as an Effectiveness Parameter

Reliability should be specified and paid for

Cost of non-availability can be very high

Reliability affects availability and is often related

to availability by the formula: Availability = MTBF

MTBF + MTTR

We are constrained by the statistical limits imposed by available data

Alan Pendry



15

Managing Reliability

Reliability can be improved by QualityControl where designs are simple andoccasional failures have no significant

operational or financial effectFormal reliability programmes are requiredwhere risks are high or systems are

complex

Alan Pendry



16

Reliability programmes

Reliability programmes should be basedon responsibility and authority given to anindividual based on defined objectives:

Warranty cost reduction/maximum figure

Demonstrable MTTF

A requirement that failure will not occur

Alan Pendry



17

Reliability programmes

A Reliability program must begin at theconcept phase where decisions related tothe following areas, amongst others, are

made: Risks involved in the specification

Development time scale

Resources applied to evaluation and test Available skills

Alan Pendry



18

Reliability programmes A Reliability program involves negotiation withmany departments over acceptable orpermissible trade-offs

As development proceeds to detailed design thereliability risks are controlled by a formal,documented approach to the review of thedesign and the imposition of design rules

relating to components, materials, processselection, tolerancing, . . .

Alan Pendry



19

Reliability programmesThe programme continues through initialproduction and testing by planning andexecuting tests to generate confidence in thedesign and by collecting, analysing and acting

upon test dataQC activities during production ensure that theproven design is repeated, with further testingto eliminate weaknesses

Data collection and analysis continuesthroughout production and use, giving theprogramme a cyclical nature

Alan Pendry



20

Reliability economics

Less than perfect reliability is the result of failures

All failures have causes

What is the cost of preventing orcorrecting the cause compared with doingnothing?

Total costs reduce as reliability improves

Alan Pendry



21

Reliability economics

Achieving reliable designs and productsrequires an integrated approach

Is is therefore difficult to separately

identify and cost the activities specificallydevoted to reliability

Alan Pendry



Reliability & lifecycle costs:Traditional view O’Connor, P15

Alan Pendry

Total costs

Q&R programme costs

Failure costs

100%



Reliability & lifecycle costs:Modern view O’Connor, P15

Alan Pendry

Total costs

Development/Production costs

Failure costs

100%



24

Reading around . . .

Characteristics of the Bathtub curve

Burn-in: What is it and how does itwork for electrical and mechanical

items?

Alan Pendry



25

Any questions?

Alan Pendry

concepts of reliability

Documents