design for maintainability. maintainability2 definition maintainability, as a characteristic of...
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Design for Maintainability
Maintainability 2
Maintainability
DefinitionMaintainability, as a characteristic of design, can be defined on the basis of a combination of the following factors:
Maintenance times Maintenance frequency Maintenance cost
The above three factors are dependant on the fact that the system is operated and maintained in accordance with prescribed procedures and resources.
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Maintainability
Measures of MaintainabilityFrom a systems perspective
Corrective maintenance Preventive maintenance
From a software perspective Adaptive maintenance Perfective maintenance
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Corrective and preventive active maintenance times
Administrative and logistic delay times
Total maintenance downtime
Maintenance Elapsed -Time Factors
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Mean corrective maintenance time(Mct)
Mean corrective time is a composite value representing the arithmetic average of the individual maintenance cycle times(Mcti)
Mean time to repair(MTTR)Basic measure of maintainabilitySum of corrective maintenance times divided by the total number of failures within an itemAverage time it takes to fully repair a failed systemIncludes fault isolation,replacements of failed item(s) and check outIts also called the mean corrective maintenance time,Mct
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OR Disassembly Completed
AlignmentAnd
Adjustment
ReassemblyReassemblycompleted
ConditionVerification(Checkout)
RepairCompleted
Repair ofEquipment
Faulty Item IdentifiedDisassembly
(Access)
Active Maintenance CommencesLocalization
AndIsolation
Failure Confirmed(Notification)
Preparation forMaintenance
DetectionFailureOccurs
Removal ofFaulty Item
Installation ofSpare/Repair
Part
Corrective Maintenance Cycle
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10
2
4
6
8
12
14
5
7
10
12
9
4
3
029.5 39.5 49.5 59.5 69.5 79.5 89.5 99.5
Time(min)
Histogram of maintenance times
Frequency of Occurrence
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Arithmetic Mean
1
nMcti
iMctn
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Mean Preventive Maintenance Time ( )
Preventive Maintenance refers to the actions required to retain a system at a specified level of performance
Includes active maintenance timeDoes not include logistic delay and administrative delay time
i i
i
fpt MptMpt=
fpt
M pt
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Logistics Delay Time (LDT)
Refers to maintenance downtime, which is expended as a result of waiting for spare part to become availableWaiting for availability of an item of test equipment in order to perform maintenance, waiting for transportation and so onLDT does not include active maintenance timeIt does constitute a major element of total maintenance downtime (MDT)
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ADT & MDTAdministrative Delay Time (ADT)
That portion of downtime when maintenance time is delayed for administrative reasons.Examples:
Personnel Assignment IssuesLabor Strike
Maintenance Down Time (MDT)Total elapsed time required to repair or restore a system to full operational status or to retain a system in that conditionMDT includes:
Mean active maintenance time Logistics delay time (LDT)Administrative delay time (ADT)
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Uptime/Downtime FactorsTime
Uptime Downtime
Standby/Ready Time
SystemOperating Time
ActiveMaintenance Time
LogisticsDelay Time
AdministrativeDelay Time
CorrectiveMaintenance
PreventiveMaintenance
PreparationTime
InspectionTime
ServicingTime
CheckoutTime
Preparation forMaintenance
Localizationand FaultIsolation
Disassembly(Gain Access)
Repair of Itemin Place
Removal of FaultyItem and Replace
with Spare
Reassembly(Buildup)
Adjustment,Alignment, orCalibration
ConditionVerification(Checkout)
or
Preventive Maintenance Cycle
Corrective Maintenance
Fault Detection
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Maintenance Labor Hours
Elapsed time vs. labor hoursElapsed time reduction via more personnel Can be expensiveMaintainability is primarily concerned with ease and economy of maintainability
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Maintenance Labor HoursImportant factors in the labor-time element
Maintenance labor-hours per system operating hour (MLH/OH)Maintenance labor-hours per cycle of system operation (MLH/cycle)Maintenance labor-hours per month (MLH/month)Maintenance labor-hours per maintenance action (MLH/MA)
Note: Any of these factors can be specified in terms of mean values.
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The reliability factors MTBF and λ are the basis for determining the frequency of corrective maintenance.
Maintainability deals with the design characteristics necessary to minimize corrective and preventive maintenance
In this area reliability and maintainability factors should be compatible and mutually supportive.
Maintenance Frequency Factors
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Some Definitions
MTBF = Mean time between failureλ= Rate of breakdown = 1/MTBF
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Maintenance Frequency Factors(Continued)MTBM:Mean time between maintenance is the mean average time between all maintenance actions
where:
–MTBMμ: Unscheduled (corrective maintenance)
–MTBMs: Scheduled (preventive maintenance)
•MTBM is crucial to determine the achieved and operational availability.
MTBM=1
1MTBMs
+ 1MTBMμ
MTBMμ should be equivalent to the MTBF if all failures are considered
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Maintenance Frequency Factors(Continued)Example :
Consider a communication equipment with an MTBF (where all failures are considered) of 20 years and has a schedule of maintenance every 6 months, what is the MTBM ?
If all failures were measured to determine the MTBF is safe to assume that MTBF=MTBMμ
With a MTBMs= 0.5 years
MTBM=1
1MTBMs
+ 1MTBMμ
1
10.5 years
+ 120
years
= = 0.55 years
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MTBR:Mean time between replacement a factor of MTBM refers to all item replacement.
In corrective and schedule maintenance a replacement of a spare part is not always necessary.
A maintainability objective in system design is to maximize the MTBR
A low MTBR will mean a higher frequency of replacement which in turn will required higher levels of maintenance support.
Maintenance Frequency Factors(Continued)
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Maintenance Cost Factors
Important factor in the total life-cycle cost
Make design decisions in the early stages of System Development
Life-cycle must be considered when defining the system requirements
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Maintenance Cost Factors
Cost-related indices appropriate in system design
Cost per maintenance action ($/month)Maintenance cost per system operating hour ($/OH)Maintenance cost per month ($/month)Maintenance cost per mission or mission segment ($/mission)Ratio of maintenance cost to total life-cycle cost
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Related Maintenance Factors
Various logistics factors basedfrom the System Supply Responsiveness
Test and Support EffectivenessMaintenance facility availability and useTransportation modesMaintenance organizational effectiveness
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Related Maintenance Factors
Cost-effectiveness
Total system cost(Live cycle)
System effectiveness
CapabilityAvailability Dependability
Uptime(Operating/
standby)
Downtime(Non-use)
Active maintenance
time
Logistic support resource
consumption
Logistic delay time
Administrative delay time
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Maintainability Requirements in the System Life-Cycle
Conceptual Design========================•Feasibility study•Operational requirements•Maintenance Concept•Functional requirements•Technical performance measures•Advance system planning-----------------------------------------Maintenance concept, quantitative and qualitativemaintainability requirementsfor system (MTBM, MTBR, Mct,Mpt, MLH/OH, cost/MA, etc.),maintainability planning.
Preliminary System Design========================
•System functional analysis•Preliminary synthesis and allocation of design criteria•System optimization•System synthesis and definition-----------------------------------------
Allocation of maintainability requirements, maintainability analysis and trade-offs, maintenance engineering analysis, design support, maintainability predictions, formal design review and approval.
Detail Design & Development========================•System / product detail design•System prototype development•System prototype test and evaluation•System modification -----------------------------------------Maintainability analysis and trade-offs, maintenance engineering analysis, design support, maintainability predictions, maintainability demonstration, formal design review and approval.
Production / Construction==============================
•Fabrication, assembly, and test of system and its components•System construction---------------------------------------------------Maintainability test and evaluation; maintainability data collection, analysis, and corrective action
System Utilization & Life-Cycle Support
================================•Consumer use of system and its components•Life-cycle system support------------------------------------------------------
Maintainability data collection, analysis, and evaluation; system modification (as required)
System Retirement
and Phaseout
Feedback Loop
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Maintainability in the System Life Cycle
Maintainability is considered early in the process
During the conceptual design phase.Requirements developed from feasibility study
Qualitative RequirementsQuantitative Requirements
Maintainability testing is included as part of the overall system test and evaluation activity
After the physical models are developed and evaluation process is established.
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Maintainability in the System Life CycleSystem Requirements Definition of system performance factors, the
mission profile, and system use requirements. Definition of the operational life cycle. Definition of the basic system maintenance. Definition of the environment in which system
is expected to operate and be maintained.
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Component Selection and Application
Select standardized components and materials.
Select items with built-in self-test features and diagnostics.
Select items that do not require highly specialized skills.
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Component Selection and Application…
Ensure appropriate accessibility to subsystems for maintenance.
Incorporate modularized functional-package approach.
Avoid selection of short-life components.
Incorporate proper identification of components.
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Maintainability Analysis MethodMaintainability Analysis: Iterative process of
system synthesis, analysis and evaluation
Tools/Methods Used for Maintainability analysis1, Reliability-Maintainability Trade-off Evaluation2, Reliability-Centered Maintenance3, Repair versus Discard Analysis4, Maintenance Prediction5, Maintenance Task Analysis (MTA)
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Maintainability Analysis MethodReliability- maintainability trade off
Evaluation:
Reliability = The probability that a system will perform in a satisfactory manner for a given period when used under specified operating conditions.E.g.; Equipment A MTBF = 100 hrs
Equipment B , MTBF = 125 hrs >> Reliability of equipment B is higher.
Maintainability = It is the ability of a product to be maintained
E.g.; Equipment A Mct = 2 hrs, Equipment B Mct = 3 hrs >> maintainability of the equipment A is higher
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Maintainability Analysis MethodExample:Requirement: To replace an existing equipment item
with a new item for the purpose of improving operational effectiveness.
Alternatives: 3 different design configurations are available
Evaluation: Reliability-Maintainability-Cost>>decision
Result: Configuration A satisfies reliability maintainability with least cost.
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Maintainability Analysis MethodReliability- Centered Maintenance(RCM)
•Cost-effective preventive maintenance program
•Best initiated during the early process
•RCM analysis leads to specification for preventive maintenance or recommendation for redesign
Repair Versus Discard Analysis:
It is economically feasible to repair certain items and/or to discard certain when failure occur.
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Maintainability PredictionMaintainability Predictions are done periodically at different stages in the design process.
Predictions of the MTBM, Mct, Mpt, MLH/OH, and so on, are made and compared against the requirements identified earlier in the Maintainability Allocation Process.
Areas of noncompliance are evaluated for possible design improvement.
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Prediction of Mct
The Prediction covers subsystems, units, assemblies, subassemblies and parts.
Maintainability Characteristics such as localization, isolation, accessibility, repair, and checkout are evaluated.
Times applicable to each part are combined to provide factors for the next-higher level (subassemblies, assemblies, etc.).
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Design Review and Evaluation
The characteristics of the system and its elements are evaluated in terms of the initially specified maintainability requirements for the system.A check list may be developed to facilitate the review process. Ex:
Have servicing and lubrication requirements been held to a minimum (if not eliminated)?Have adjustment, alignment, and calibration requirements been minimized (if not eliminated)?Have all system maintainability requirements been met?
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Maintainability Demonstration It is conducted as part of the system test
and evaluation effort
It is usually accomplished during the latter part of detail design.
It should be conducted in an environment that simulates, as closely as practical, the operational and maintenance environment planned for the item.
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Checklist for review contStandardization; Minimize spares?
Modular packaging achieved?
Are modules interchangeable?
Diagnostic testing provisions?
Quick disassembly possible?
Accessibility to parts requiring replacement?
Is alignment, adjustment and calibration in field minimized?
Are the parts labeled?Servicing and lubrication minimized?Handling of heavy items designed?
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Design Review & Evaluation
Done at various stages of design and developmentCreate a formal checklist for the reviews
Maintainability requirements adequately defined and specified?
Compatible with other requirements ? Realistic ?
Compatible with maintenance concept?
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Maintainability Demonstration
It is usually accomplished during the latter part of detail design.
It should be conducted in an environment that simulates, as closely as practical, the operational and maintenance environment planned for the item