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Maintenance Strategies

Introduction

Maintenance Strategies

On-Failure Maintenance

Fixed Time Maintenance

The Need for More Strategies

The ‘Bathtub’ & Other Curves

Predictive Maintenance

Design Out Maintenance

Selecting Maintenance Strategies

Key Points

IntroductionFrom day one, equipment and plant must be maintained if it is tofulfil its function for a useful life. This can be achieved using one ofthe four key maintenance strategies:

On-Failure MaintenanceFixed Time MaintenancePredictive MaintenanceDesign Out Maintenance

Each of these strategies has a place within an optimizedmaintenance plan (see Fig 1), but the distribution of the mix willdepend upon many factors, including;

The equipment to be maintainedThe operational context - both in terms of production and the prevailing environmental conditions, for example sub-seaThe maintenance resources availableHealth and Safety complianceGeneral practicalitiesCost

Each maintenance strategy has advantages when implementedcorrectly, but conversely, disadvantages when implementedincorrectly. For example, we would maintain the indicator bulbs onan auto using on-failure maintenance as this is both cost effectiveand practical. However, we would select an alternative to the on-failure strategy when considering the maintenance of the autobraking system.

Note: Many differing terms are used to describe the variousstrategies, planned maintenance, preventive maintenance, etc.and these will be discussed in the relevant sections.

On-Failure

X X

X

X

X

X X

Monday

Tuesday

Wed

Thursd

Friday

Fixed Time

Failure ModeParameter Trend

Condition Based

Design Out

Maintenance Plan

42

87

6

96

72

5

10

74

8

33

57

1

84

66

6

42

96

4

33

32

5

63

01

9

53

67

7

48

92

0

Fig 1: The Optimum Maintenance Plan

Maintenance StrategiesComplete the table below for each of the four maintenancestrategies. Enter a definition for the strategy, for example ‘On-Failure Maintenance - fix it when it breaks’, and list as manyadvantages for this strategy when implemented correctly. Inaddition list as many disadvantages as possible for when thisstrategy is implemented incorrectly. Also, provide one exampleof equipment relevant to your field that you would maintainusing this strategy.

Table 1: Maintenance Strategies

Strategy

Definition

Advantages(whenimplementedcorrectly)

Disadvantages(whenimplementedincorrectly)

Equipmentyou wouldmaintain

On-Failure Fixed Time Predictive Design Out

Fix it whenit breaks

Cheap

High sparesstock levels

Comments:Replacement, or repair of equipment after failure, is the default strategy - eventually failure will occur ifnothing else is done.

On-failure maintenance is sometimes called corrective maintenance, a term which is unspecific and can beapplied to other strategies and hence is better avoided. Unfortunately on-failure maintenance encourages the‘fire-fighting’ syndrome - respond fast and work furiously - with its attendant problems.

Definition:Maintain equipment when it has failed to fulfil itsfunction.

On-Failure Maintenance

Disadvantages:

there is no warning of failure - this can presenta significant safety and/or environmental riskuncontrolled plant outage will result - stoppages occur unexpectedlythere will be production losses or delays - these will inevitably result in increased costthere is a need to provide standby plant if higher levels of availability are required - this can lead to higher build costsit requires a large standby maintenance team -as you have no indication of what will fail, you need a standby team ready to repair any of your equipmentsecondary or consequential damage may occur- a bearing failure may lead to the failure of theshaft, followed by the pump, etc, which will lead to higher costs of spares and longer repair timesthere is a requirement for a large spares stock to cater for any kind of failure that may occur

Advantages:

it requires no pre-care of the plantit requires little management apart from organizing labor and storesif applied correctly, it can be very cost effective

X X

X

X

X

X X

42876

96725

10748

33571

84666

42964

33325

63019

53677

48920

Monday

Tuesday

Wed

Thursd

Friday

Comments:If we wish to improve on a ‘fire fighting’ situation dominated by on-failure maintenance, it is possible toimplement the fixed time maintenance strategy. That is, attempt to maintain the equipment before failure andthereby prevent failure.

Replacement, or repair at a fixed time interval before failure, is the strategy most commonly applied to reduceon-failure maintenance. The simple planning structure for manpower and spares makes this strategyattractive for the management of maintenance and gives the reasoning behind the very common name‘planned preventive maintenance’. Safety related checks are also often carried out on a fixed time basis.

Definition:Maintain equipment based upon fixed time - eithercalendar based, run hours based, or number ofequipment cycles based.

Fixed Time Maintenance

Disadvantages:

the maintenance activity and associated costs will increaseit can only be applied effectively on fixed time intervals where the deterioration is age related(see overleaf)the actual maintenance activity can sometimescause failures itselfmay be replacing a component that still has useful life

Advantages:

it reduces failures (compared to the on-failure strategy)it uses the workforce cost-effectively with planned work schedules it allows work to be planned well in advance

The ‘Bathtub’ & Other CurvesThe curve shown (Fig 2), the typical‘Bathtub’ curve, illustrates theclassic profile of failure rate againsttime for equipment. As can be seenthere is a run-in phase where thereare a large number of failures, oftenknown as high infant mortality. Thenumber of failures then reducesduring the normal level during therunning phase of the life cycle. Thenas the equipment ages and startsto wear-out the number of failuresstart to rise again, this is known asthe wear-out phase. Fig. 2: The Bathtub Curve

Operating Time (component age or life)

Failu

re R

ate

Run inPhase Wear Out

PhaseUseful Life Phase

DecreasingFailure Rate

IncreasingFailure RateConstant Failure Rate

The failures of equipment that illustrate wear-out can also be shown on a graph as a failure distribution againsttime (the total operating time before failure):

1 2 3 4 5

2

4

Num

ber

of F

ailu

res

Years of operating life

Ideal time to replace = 2.4 years

Fig. 3: Failure Distribution for 10 Motors

Fig 3 illustrates a possible failuredistribution for 10 motors. As can beseen, all the failures occur between 2.5and 3.5 years of operation, and so theideal time to replace these motors wouldbe after 2.4 years of operation. This wouldeliminate the failures, whilst keepingreplacement costs to a minimum.

The Need For More StrategiesAs mentioned previously, on-failure maintenance, whilst applicablein some instances, is not appropriate for most industrial plantwhere, for example, high levels of safety and availability withreduced cost are required.

Fixed-time maintenance is often an improvement upon the‘breakdown’ strategy, but it is only applicable to age relateddeterioration.

For example, if historical analysis of a plain bearing illustrates thatit will fail due to general wear once every two years, then it wouldbe sensible to replace the bearing before wear-out occurs, perhapsevery 23 months. However, this is not always the case.

For example, if you replace the motors

after 5 months then the cost of replacement is too high,after 2.4 years then the cost of replacement and failure is too high,after 3.6 years then the cost of failures is too high,

and so there is no ideal time for replacement. In other words, fixed time maintenance would not beappropriate.

Unfortunately, studies have shown that plantequipment fails in many ways that are notrelated to wear-out, and in fact the bathtubcurve is only applicable to some ways inwhich the equipment fails. Fig 5 illustratessome other failure rate profiles that aretypical for plant equipment. As can be seen,there is no clear wear-out phase for theseprofiles, and as such, no ideal time tomaintain the equipment using fixed timemaintenance. This in turn means thatalternative strategies are required ifmaintenance is to reduce both the numberof failures, and the overall costs ofoperating the plant.

1 2 3 4 5

2

4

Num

ber

of F

ailu

res

Years of operating life

After 2.4 years

Replacement& failures toohigh After 3.6 years

Failures toohigh

After 5 months

Replacementcosts toohigh

When to Replace ?

Fig. 4: Alternative Failure Distribution for 10 Motors

Fig. 5: Alternative Failure Rate Curves

Operating Time (component age or life)

Failur

e Rate

Failur

e Rate

Failur

e Rate

Failur

e Rate

However, if the failure distribution of these motors was as shown in Fig 4, then there is no ‘ideal’ time toreplace the motors.

Failure ModeParameter Trend

Comments:Maintenance on inspection or monitoring is an attractive concept, in that the maintenance action will only beperformed when knowledge of the equipment indicates that failure to fulfil its function is imminent. Thoughthe maintenance work will often be corrective, the ability to plan the work also makes it preventive in nature.

Definition:Maintain equipment based upon its known condition.

Predictive Maintenance

Disadvantages:

Different organizations will present differing reasonsfor not implementing Predictive Maintenance, but theonly disadvantage to the strategy is that:

it is costly if implemented incorrectly

That is, if condition monitoring is undertaken withoutan understanding of the equipment, and not as a partof an overall maintenance plan, then it is likely that itwill fail, result in expenditure, and lose credibilitywithin the organization. To succeed, PredictiveMaintenance must be implemented correctly withinthe improvement context and against a completeunderstanding of the equipment, its failure modes,and root causes.

Advantages:

The advantages of utilizing the PredictiveMaintenance strategy are many, some of which arenot so tangible - such as increased motivation of theworkforce through increased competency. The mainadvantage to Predictive Maintenance is thatimpending equipment failure can be detected, whichin turn provides the following benefits:

equipment can be shut-down before severe damage occursalternatively, equipment can be run to failure ifdesiredproduction can be modified to extend the unit life, for example, decrease the load on equipment so that it can continue to run until the next planned plant shutdownthe required maintenance work can be planned, with labor organized, spares assembled, etc.

All of the above will increase safety, plant output andavailability, and will lead to improvements in the finalproduct quality.

In addition, as a result of the additional informationcollected during the inspection or monitoring process:

the real cause of failure can be analyzed, for example, if the vibration indicates an impending bearing failure, reasons for that failure mode can be investigated - this would be more difficult to identify following a catastrophic failure resulting in a pile of brokencomponentsfollowing identification of the real root cause for failure, it is possible to plan in design modifications if required

Comments:Redesigning a unit is also part of the maintenance function. The other strategies discussed involve a degreeof repetition and they are concerned with responding to or anticipating failures in an effective manner. Incontrast, redesigning to avoid failure is, or should be, a one-off activity. There is, therefore, considerable scopefor plant improvement by the application of design out maintenance.

Definition:Design, or redesign, equipment to eliminate the rootcause of failure and resulting failure modes so as toeliminate or minimize the need for maintenance.

Design Out Maintenance

Disadvantages:

large design changes can be costlyplant outages may be required to effect design changesunless root causes of failure are fully understood, changes may be ineffectual, or potentially detrimentalthere are sometimes unknown ‘knock-on’ effects of design changes that can cause further difficulties later on

Advantages:

it is a one-off maintenance activityit reduces failuresit will lead to increased productionit will remove or reduce the need for maintenance

Selecting Maintenance StrategiesFor each of the cases shown in the table below, mark the mostappropriate maintenance strategy or strategies that should beimplemented.

Maintenance Strategy

Equipment / Why it fails

Airplane wing / falls off due to poor fixtures

Air handling unit / belts worn out - general wear

Car indicator bulb / ceases to work due to element failure

Pump / seal leaks

Plain bearing / general wear

Computer / failed mother board

Pump bearing / misaligned during installation

Pump impeller / wear due to cavitation

Pump impeller / general wear

Fan rolling element bearing / fan out of balance

Vibrating feeder / incorrect operator settings

Director's office air handling unit / not known

On

-Fai

lure

Fix

ed T

ime

Pre

dic

tive

Des

ign

Ou

t

Table 2: Selecting Maintenance Strategies

Key Points

All four maintenance strategies are required to make an effective Maintenance Plan

All four maintenance strategies have advantages when implemented correctly

All four maintenance strategies have disadvantages when implemented incorrectly

The appropriate strategy can only be chosen when the reasons for equipment failures are fully understood

Notes

Notes

Task 1 - Maintenance StrategiesSolutions to this task can be found in the main body of text and there are several ‘correct’ answers, howeveran example solution is shown below.

Table 1: Maintenance Strategies

Strategy

Definition

Advantages(whenimplementedcorrectly)

Disadvantages(whenimplementedincorrectly)

Equipmentyou wouldmaintain

On-Failure Fixed Time Predictive Design Out

Fix it whenit breaks

Cheap

etc

High sparesstock levels

etc

Conductmaintenanceat regularintervals

Can beplanned for

etc

Unnecessaryreplacementof parts

etc

Maintainbased uponknowncondition

Spot apotentialfailure

etc

Costly ifimplementedincorrectly

etc

Redesign t0eliminateroot causeof failure

Lessmaintenancerequired

etc

Could beexpensive

etc

Headlights Oil change Automobiletires

Dysonvacuums

Maintenance Strategy

Equipment / Why it fails

Airplane wing / falls off due to poor fixtures

Air handling unit / belts worn out - general wear

Car indicator bulb / ceases to work due to element failure

Pump / seal leaks

Plain bearing / general wear

Computer / failed mother board

Pump bearing / misaligned during installation

Pump impeller / wear due to cavitation

Pump impeller / general wear

Fan rolling element bearing / fan out of balance

Vibrating feeder / incorrect operator settings

Director's office air handling unit / not known

On

-Fai

lure

Fix

ed T

ime

Pre

dic

tive

Des

ign

Ou

t

Table 2: Selecting Maintenance Strategies

Task 2 - Selecting Maintenance StrategiesFor some situations it is possible to address the way equipment fails through different maintenance strategies.Operational requirements will dictate the most appropriate strategy to select.