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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
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