Maintenance Strategies

Dr David BagleeSchool of Computing and Technology

Background

► Maintenance strategy is becoming a more important issue

► Today machines are integrated► Very little inventory stock held► Automation of processes increasing► Downsizing of workforce

Hidden Losses

Labour

BreakdownsPlant Trips

OperabilityReducedThroughput

Restart Losses

Poor QualityPoor Integrity

Low Morale

Visible Costs

Hidden Losses

Materials

Maintenance Strategies

► Reactive Maintenance (1)► Preventive Maintenance (2)► Conditional Maintenance► Predictive Maintenance (3)► Total Productive Maintenance (TPM)► Reliability Centred Maintenance (RCM)

Reliability DomainsR

ELI

AB

ILIT

Y

%

MAINTENANCE EXPENSE

Reactive Maintenance

Planned Maintenance

Predictive Maintenance

Reliability Centered Maintenance

Total Productive Maintenance

Reactive Maintenance

► Allow machinery to run to failure► Traditionally the exclusive maintenance

mode until fairly recently► Continues to be the predominate

method of maintenance► Many major industrial companies still

operate in this mode► Repair or corrective action is taken after

the fault has occurred

Advantages Disadvantages

► Low direct costs► Less staff required► Appropriate for certain

applications

► High costs due to unplanned downtime of equipment

► Increased labor costs, especially if overtime is needed

► Costs involved with repair or replacement of equipment

► Possible secondary equipment or process damage from equipment failure

► Inefficient use of staff resources

► Does not support the true definition of maintenance

Preventive Maintenance

► Actions that detect, prevent, or mitigate degradation of a component or system

► Aims to sustain or extend useful life by controlling degradation to an acceptable level.

► Actions performed periodically prior to functional failures to achieve desired safety or reliability levels.

Preventive Maintenance

► A step forward from reacting to breakdowns to preventing breakdowns: Pioneered by the U.S. Navy. Analogous to an automobile schedule of

maintenance. Time intervals based on manufacturer

recommendations and machinery history experience.

Preventive Maintenance

► Scheduled maintenance activities are based on specific time intervals

Calendar days Run time Parts produced

► Includes routine tasks such as: Changing oil Replacing filters Greasing bearings Instrument calibration

Preventive Maintenance

► Includes measurement of degradation prone areas for rapid changes in out-of-tolerance conditions. Wear rings Alignment Shaft end play

► Useful against age related modes of failure: Wear Fatigue Corrosion

Advantages Disadvantages

► Cost effective in many capital intensive or potentially high impact processes.

► Flexibility allows for the adjustment of maintenance periodicity.

► Increased component life-cycle.

► Energy savings.► Reduced equipment

or process failures.

► Catastrophic failures still likely to occur.

► Labor intensive.► Performance of

unneeded maintenance.

► Incidental damage to components through poor maintenance practices.

Conditional Maintenance

► Maintenance actions conducted as result of a specific condition, or as a result of specific circumstances or events. Examples of Conditional Inspections

Aircraft hard landing Unscheduled disassembly Power Interruption Over/Under Voltage Lightning strike Over stress Unscheduled or scheduled shut-down

Predictive Maintenance

► Measurements that detect the onset of a degradation mechanism thereby allowing causal stressors to be eliminated or controlled prior to any significant deterioration in the component physical state.

► Results indicate current and future functional capability.

► Involves use of specialised equipment to monitor the condition of machines (Vibration, Oil Analysis, Thermography, Acoustic Emission, etc)

Predictive Maintenance

► Schedule maintenance activities when mechanical or operational conditions warrant to repair or replace deteriorated equipment before obvious problems occur.

► Allows assessment of whether equipment will fail during some future period

► Hence indicates best time to perform repairs or maintenance

Advantages Disadvantages

► Increased component operational life/availability

► Allows for pre-emptive corrective actions

► Reduced equipment or process downtime

► Decreased costs for parts and labor

► Better product quality► Improved worker and

environmental safety► Improved worker morale► Energy savings

► Increased investment in diagnostic equipment

► Increased investment in training of staff

► Savings potentials not readily seen by management

Total Productive Maintenance► JIT based approach► Workers perform preventive maintenance on the

machines they operate► Skilled maintenance personnel train the operators

and develop ‘one-point lessons’.► Maintenance department moves from a ‘fire-

fighting’ mode to a prevention mode & re-engineering

► Restore deteriorated equipment through Improvement-Related Maintenance.

► Identify design weaknesses and improve equipment► Preventive (equipment manufacturers/operators

data)► Predictive (data analysis and periodic diagnostic

tests)

Reliability Centered Maintenance

► Process used to determine maintenance requirements of any physical asset in its operating context.

► Plan is based upon reliability criteria with priority given to the most critical components.

► Determine what types of failures are likely to occur.

► Focuses on preventing failures whose consequences are likely to be serious.

► Emphasizes the use of predictive maintenance practices.

► Utilizes previous aspects of reactive and preventive maintenance concepts, in concert with root cause analysis.

Strategy Development► Advantages

Towards more efficient maintenance programmes.

Eliminating unnecessary maintenance or overhauls.

Minimize frequency of overhauls.

Reduced chance of sudden equipment failure.

Focuses maintenance activities on critical components.

Increased component reliability.

Root causes of problems identified.

► Disadvantages Can have significant

startup cost Training Equipment etc.

Savings potential not readily seen by management.

Changes take time to implement

CostsD

ire

ct C

ost

Indirect Cost

TotalCost

Predictive

Preventive

Reactive

Trade-off between Repairs and PM

Ann

ual C

ost

Degree of Preventive Maintenance

Minimum TotalMaintenance Cost

PreventivePreventiveMaintenanceMaintenance

CostCost

BreakdownBreakdownand Repairand Repair

CostCost

Total Total MaintenanceMaintenance

CostsCosts

Service Organizations

► Maintenance issues are not limited to manufacturing

► Transportation firms (airlines, railways, haulage, dispatch companies, etc) must maintain vehicles in good operating condition

► Highways Department must maintain roads► Office personnel are reliant on computers,

printers, copiers and fax machines working properly

► As services become increasingly automated, service firms face more and more maintenance issues

Trends in Maintenance

► Production machinery is becoming more and more complex and maintenance personnel must keep pace

► Special training programs to maintain worker skill level

► Subcontracting service companies► Production workers maintain own

equipment► Computer assistance in maintenance