7 Failure Prevention and Recovery

5/20/2018 7 Failure Prevention and Recovery

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Nigel Slack, Stuart Chambers & Robert Johnston, 2004 Operations Management, 4E: Chapter 19

19.1

Failure Prevention and Recovery

Chapter coverage:System failure

Failure detection and analysis

Improving process reliabilityRecovery


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19.2

Failure There is always a chance that things might go wrongwe

must accept this NOT ignore this.

Critical failure:

Lost of customer

High downtime High repair cost

Injury or lost of lives (company reputation)

Non - critical failurelesser effect Organizations must discriminate and give priority to

critical failurewhy things fail & how to measure the

impact of failure


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19.3

All failure can be traced back to some kind of humanfailure.

A machine failure might have been cause by

someones poor design or maintenance.

Delivery failure might have been someones error inmanaging the supply schedule.

Failures are rarely a random chance.

It can be controlled to a certain extentCan learn from failure and change accordingly

Opportunity to examine and plan for elimination

Failure as an Opportunity


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19.4

System Failure

Why things fail:1) Failure resulting from within the operation:

Design failure

Facilities failure People failure

2) Failure resulting from material or information input

Supplier failure3) Failure resulting from customer actions

Customer failure


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19.5

Why Things Fail

Design failure: Operations may look fine on paper but cannot cope withreal circumstances.

Type 1:Characteristic of demand was overlooked ormiscalculated.

Bearing factory designed to produce 100 bearingsper day but customers demand 125 bearings perday.

Type 2:The circumstances under which the operationhas to work are not as expected.

A factory building designed to house stationarymachinery fails when it was used to store avibrating machine.


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19.6

Why Things Fail

Facilities failure: All facilities (machines, equipment, buildings, fittings)are liable to breakdown.

Type 1:Partial breakdown

Worn out carpet in a hotel Machine can only half its normal rate

Type 2:Complete breakdown

Sudden stop of operation

It is the effect of the breakdown that is importantsomebreakdowns could paralyse the whole operation.

Some failures have a cumulative significant impact.


7/60 Nigel Slack, Stuart Chambers & Robert Johnston, 2004 Operations Management, 4E: Chapter 19

19.7

Why Things Fail

People failure: Type 1:Errors are mistakes in judgement

A managers decision to continue running the plantwith a partially failed heat exchanger resulted in amore expensive complete breakdown.

Type 2:Violation are acts which are contrary todefined operating procedures

A machine operator failure to lubricate thebearings of the motor resulted in the bearingsoverheating and failing



19.8

Why Things FailSupplier failure:

A supplier failed to

Deliver.

Deliver on time.

Deliver quality goods and servicescan lead to failure within an operation.

Customer failure:

Customer failure can result when customers misuse

products and services Example: Someone loading a 14kg washing

machine with 18kg of cloths will cause the machineto fail.



19.9

There are three main ways of measuring failure:

Failure rateshow often a failure occurs

Reliabilitythe chances of failure occurring

Availabilitythe amount of available usefuloperating time

Measuring Failure



19.10

Failure rate (FR):

Example: If an engine fails 4 times after operating for

300 hours, it has a failure rate of 0.013 (0.13%).

Example: If out of 250 products tested for operability 5

failed, the failure rate is 0.02 (0.2%)

testedproductsofnumbertotal

failuresofnumberFR

timeoperating

failuresofnumberFR

Measuring Failure



19.11

Failure over timethe bath-tub curve At different stages during the life of anything, the

probability of it failing will be different.

Most physical entity failure pattern will follow the

bath-tub curve.

Measuring Failure


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19.12

The bath tub curve comprises three stages:

The infant-mortality stagewhere early failures

occur caused by defective parts or improper use.

The normal life stagewhen the failure rate is low

and reasonably constant and caused by normalrandom factors.

The wear-out stagewhen the failure rate increases

as the part approaches the end of its working life and

failure is caused by the ageing and deterioration of

parts


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19.13

Bath-Tub Curve

Time

Failure

rate

Infant-

mortality

stageNormal-life

stage

Wear-out

stage

X y


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19.14

Reliability

Measures the probability of a system, product or serviceto perform as expected over time.

Values between 0 and 1 (0 to 100% reliability)

Used to relate parts of the system to the system.

If components in a system are all interdependent, afailure in any individual component will cause thewhole system to fail.

Hence, reliability of the whole system, Rs,

Rs= R1R2R3Rn

Where: R1 = reliability of component 1R2= reliability of component 2

R3= reliability of component 3

Etc


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19.15

Worked Example

An automated pizza-making machine in a food manufacturers factoryhas five major components, with individual reliabilities (the probability

of the component not failing) as follows:

Dough mixer Reliability = 0.95

Dough roller and cutter Reliability = 0.99

Tomato paste applicator Reliability = 0.97

Cheese applicator Reliability = 0.90

Oven Reliability = 0.98

If one of these parts of the production system fails, the whole systemwill stop working. Thus the reliability of the whole system is:

Rs = 0.95 0.99 0.97 0.90 0.98

= 0.805


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19.16

Worked Example

Notes: The reliability of the whole system is 0.8 even though the

reliability of the individual components was higher.

If the system had more components, its reliability would be

lower. E.g. for a system with 10 components having reliability of

0.99 each, the reliability of the system is 0.9 BUT if the

system has 50 components having reliability of 0.99 each,

the reliability of the system reduces to 0.8.

Reliability chart given on page 687 of recommended text.


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19.17

Availability

Availability is the degree to which the operation isready to work.

An operation is not available if it has either failed or is

being repaired following a failure.

failuresofnumber

hoursoperatingMTBF

repairtotimemeanMTTRfailuresbetweentimemeanMTBF

Where

MTTRMTBF

MTBFAtyAvailabili


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19.18

The three tasks of failure prevention and recovery

Failure detection and

analysis

Finding out what is

going wrong and why

Improving system

reliabilityStopping things going

wrong

Recovery

Coping when things do

go wrong


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19.19

Failure detection and analysisMechanisms to detect failure:

1. In process checks

2. Machine diagnostic check

3. Point-of-departure interviews

4. Phone surveys

5. Focus groups

6. Complaint cards of feedback sheets7. Questionnaires


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19.20


1. In process checksemployees check that the process

is acceptable during the process.

Example: Is everything alright with your meal,

madam?

2. Machine diagnostic checka machine is put through

a prescribed sequence of activities to expose any

failures or potential failures.Example: A heat exchanger tested for leaks, cracks and

wear


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19.21


3. Point-of-departure interviewsat the end of a

service, staff may check that the service has been

satisfactory.

4. Focus groupgroups of customers are brought

together to some aspects of a product or service.

5. Phone survey, Complaint cards&Questionnaires

these can be used to ask for opinions about products orservices.


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19.22

Failure analysis:

1. Accident investigation Trained staff analyse the cause of the accident.

Make recommendations to minimize or eradicate ofthe failure happening again.

Specialized investigation technique suited to the typeof accident

2. Product liability

Ensures all products are traceable.

Traced back to the process, the components fromwhich they were produced and the supplier whosupplied them.

Goods can be recalled if necessary.


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19.23

3. Complaint analysis

Complaints and compliments are recorded and takenseriously.

Cheap and easily available source of informationabout errors.

Involves tracking number of complaints over time.4. Critical incident analysis

Requires customers to identify the elements ofproducts or services they found either satisfying or

not satisfying. Especially used in service operations.


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19.24

4. Failure mode and effect analysis (FMEA)

Used to identify failure before they happen soproactive measures can be taken.

For each possible cause of failure the following type

questions are asked:

What is the likelihood a failure will occur?

What would the consequence of the failure be?

How likely is such a failure to be detected

before it affects the customer?

Risk priority number (RPN) calculated based on

these questions.

Corrective action taken based on RPN.


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19.25

6. Fault-tree analysis

This is a logical procedure that starts with a failureor potential failure and works backwards to identifyall the possible causes and therefore the origins ofthat failure.

Made up of branches connected by AND nodes and

OR nodes.

Branches below AND node all need to occur for theevent above the node to occur.

Only one of the branches below an OR node needs to

occur for the event above the node to occur


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19.26 Fault-tree analysis for below-temperature

food being served to customers

Food served tocustomer is below

temperature

Cold plate

used

Plate taken

too early

from warmer

Plate warmer

malfunction

Oven

malfunction

Timing errorby chef

Ingredients

not

defrosted

Plate

is cold

Food

is cold

Key

AND node

OR node


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19.27

To be continued


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19.28

Improving Process Reliability

After the cause and effect of a failure is known, the next

course of action is to try to prevent the failures from

taking place. This can be done in a number of ways

Designing out fail pointsin the process

Building redundancyinto the process

Fail-safeingsome of the activities in the process

Maintenanceof the physical facilities in the process


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19.29

Designing out fail points

Identifying and then controlling process, product andservice characteristics to try to prevent failures.

Use of process maps to detect potential fail points inoperations.

Redundancy Building up redundancy to an operation means having

back-up systems in case of failure.

Increases the reliability of a component

Expensive solution Used for breakdowns with critical impact.


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19.30

Fail-safeing

Called poka-yoke in Japan.

Based on the principle that human mistakes are to some

extent inevitable.

The objective is to prevent them from becoming a

defect.

Poka-yokes are simple (preferably inexpensive) devices

of systems which are incorporated into a process to

prevent inadvertent operator mistakes resulting in adefect.


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19.31

Maintenance

Maintenance is the method used by organizations toavoid failure by taking care of their physical activities

Important to organizations whose physical activitiesplay a central role in creating their goods and service.

Benefits of maintenance: Enhanced safety

Increased reliability

Higher quality

Lower operating costs Longer life span

Higher end value


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19.32 Benefits of Maintenance

Enhanced safety: Well maintained facilities are lesslikely to behave in an unpredictable or non-standardway, or fail outright, all of which would pose a hazard tostaff.

Increased reliabilityThis leads to less time lost whilefacilities are repaired, less disruption to the normalactivities of the operation , and less variation in outputrates.

Higher qualityBadly maintained equipment is morelikely to perform below standard and cause qualityerrors.


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19.33 Benefits of Maintenance

Lower operating costsMany pieces of process

technology run more efficiently when regularly

serviced.

Longer life spanRegular care prolong the effective

life of facilities by reducing the problems in operation

whose cumulative effect causes deterioration.

Higher end valueWell maintained facilities are

generally easier to dispose of into the second-hand

market.


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19.34Approaches to maintenance

1. Run to breakdown (RTB)

Allowing the facilities to continue operating until

they fail.

Maintenance work is performed after failure has

taken place. The effect of the failure is not catastrophic or

frequente.g. does not paralyze the whole

operation.

Regular checks are sufficient.


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19.35Approaches to maintenance

2. Preventive maintenance (PM)

Attempts to eliminate or reduce the chances of

failure by servicing the facilities at pre-planned

intervals.

Used when the consequence of failure isconsiderably more serious.

Can be used to detect impending failures.

Remedial actions can be planned for, thus

improving overall efficiency. The useful life of certain components can be

increase beyond their recommended life span.


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19.36

3. Conditioned-based maintenance (CBM) Attempts to perform maintenance only when the

facilities require it.

May involve continuously monitoring parameters

(vibrations, temperature, displacement) of thefacility.

The results of the monitored parameter is used todecide whether to stop the facility to conduct

maintenance.

Approaches to maintenance


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19.37

4. Mixed maintenance strategies

Most operations adopt a mixture of theseapproaches because different elements of theirfacilities have different characteristics.


Use ???

Use ???

Use ???


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19.38

5. Run to breakdown versus preventive maintenance

The more frequent preventive maintenance iscarried out, the lesser chance it has of breakingdown.

The cost of preventive maintenance is often high.

Infrequent preventive maintenance will cost lessbut will result in higher chances of breakingdown.

The cost of an unplanned breakdown is oftenhigh.



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19.39 Cost of Preventive Maintenance

Costsof

PM

Amount of preventive maintenance


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19.40

Cost of Breakdown

Costsof

breakdo

wn



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19.41

Maintenance cost model 1: One model of the costs associated

with preventive maintenance shows an optimum level ofmaintenance effort.

Costs


Total cost

Cost of providingpreventive

maintenanceOptimum level of

preventivemaintenance

Cost ofbreakdowns


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19.42

Maintenance cost model 2: an optimum level of maintenance

effort.

Costs


Actual cost of providingpreventive maintenance

Model 1 cost of providingpreventive maintenance


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19.43


effort.

Costs


Actual cost ofbreakdowns

Model 1 cost of breakdowns


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19.44


effort.

Costs


Total cost

Cost of breakdowns

Cost of providing preventivemaintenance


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19.45

Notes:

In actuality the cost of PM does not increase as steeply asindicated in Model 1.

Model 1 assumes that all maintenance jobs must be

carried out by a specialist maintenance team but Model

2 recognizes that operators themselves can carry outsimple, in process maintenance. Etc

The cost of breakdown could be higher than indicated in

Model 1.

A breakdown may cost more than the cost of repairand the cost of the stoppage itselfa stoppage can

take away the stability in the operation.


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19.46

Run To Breakdown or Preventive

Maintenance?

Based on the arguments above, theshift is more towards the use of

Preventive Maintenance.


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19.47

6. Failure distributions

The shape of the failure probability distribution of afacility can determine if it benefits from preventive

maintenance.

Machine A

Machine B

Proba

bilityoffailure

Timex y


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19.48

Notes:

Machine AThe probability that it will break down before timexis

relatively low.

It has high probability of breaking down between

timesxandy. If preventive maintenance was carried out just before

pointx, the chances of breakdown can be reduced.


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19.49

Notes:

Machine B It has a relatively high probability of breaking down at

any time.

Its failure probability increases gradually as it passes

through timex.Carrying out preventive maintenance at point x or any

other cannot dramatically reduce the probability of

failure.


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19.50

Total Productive Maintenance (TPM) Approach

Total productive maintenance (TPM) is defined as:

the productive maintenance carried out by all

employees through small group activities

Where productive maintenance is:

maintenance management which recognizes

the importance of rel iabi l i ty, maintenance and

economic efficiency in plant design


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19.51

The five goals of TPM:

1. Improve equipment effectiveness: Examine how the facilities contribute to the

effectiveness of the operation by examining all the

losses which occur.

2. Achieve autonomous maintenance:

Allow people who operate the equipment to take

responsibility for some maintenance task.

Maintenance staff to take responsibility for theimprovement of maintenance performance.


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19.52

There are three levels at which maintenance staff

can take responsibility for process reliability: Repair levelstaff carry out instructions but do not

predict the future, they simply react to problems.

Prevention levelstaff can predict the future by

foreseeing problems, and take corrective action.

Improvement levelstaff can predict the future by

foreseeing problems, they not only take corrective

action but also propose improvements to prevent

recurrence.


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19.53

Example:

Suppose the screws on a machine become loose. Each weekit jams up and is passed to maintenance to be fixed.

A repair level maintenance engineer will simply

repair it and hand it back to production.

A prevention level maintenance engineer will spot

the weekly pattern to the problem and tighten the

screws in advance of their loosening.

An improvement-level maintenance engineer willrecognize that there is a design problem and modify

the machine so that the problem cannot recur.


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19.54

The five goals of TPM (cont):

3. Plan maintenance: To have a fully worked out approach to all

maintenance activities. Includes

the level of preventive maintenance required

for each piece of equipment. the standard for condition-based maintenance

the respective responsibilities of operating staffand maintenance staff. See Slide 19.55

4. Train all staff in relevant maintenance skills: TPM emphasises on appropriate and continuous

training to ensure staff have the skills to carry outtheir roles.


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19.55

The roles and responsibilities of operating staff and

maintenance staff in TPM

Maintenance staff Operating staff

Roles To develop:

Preventive actions

Breakdown services

To take on:

Ownership of

facilities Care of facilities

Responsibilities Train operators

Device maintenance

practice Problem-solving

Assess operating

practice

Correct operation

Routine preventive

maintenance Routine condition-

based maintenance

Problem detection


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19.56

The five goals of TPM (cont):

5. Achieve early equipment management: This goal is directed at avoiding maintenance

altogether by maintenance prevention (MP).

MP involves considering root causes of failure and

maintainability of equipment during the designstage, manufacture, installation and itscommissioning.


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19.57

Reliability Centred Maintenance (RCM) Approach

1. TPM tends to recommend preventive maintenance evenwhen it is not appropriate.

2. Uses the pattern of failure for each type of failure modeto dictate the approach of maintenance.

3. The approach of RCM is sometimes summarized as Ifwe cannot stop it from happening, we had better stop itfrom mattering efforts need to be directed at reducingthe impact of the failure.


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19.58

Example:

Take the process illustrated in Slide 19.59. This is a simpleshredding process which prepares the vegetables prior to

freezing. The most significant part of the process which

requires the most maintenance attention is the cutter sub-

assembly. However, there are several modes of failure.1) They require changing because they have worn out

through usage

2) They have been damaged by small stones entering the

process

3) They have shaken loose because they were not fitter

correctly.

O t i h l


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19.59 One part in one process can have several

different failure modes, each of which

requires a different approach

Failures

Failures

Failures

Time

Time

Time

Cutter shake loose

failure pattern

Cutter damage

failure pattern

Cutter wear outfailure pattern

Solution

Preventive maintenancebefore end of useful life

Solution

Preventive damage, fixstone screen

Solution

Ensure correct fittingthrough training

Cutters

Shredding

process


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19.60

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