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Design for Maintainability

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Page 1: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

Design for Maintainability

Page 2: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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.

Page 3: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

Maintainability 3

Maintainability

Measures of MaintainabilityFrom a systems perspective

Corrective maintenance Preventive maintenance

From a software perspective Adaptive maintenance Perfective maintenance

Page 4: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

Maintainability 4

Corrective and preventive active maintenance times

Administrative and logistic delay times

Total maintenance downtime

Maintenance Elapsed -Time Factors

Page 5: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 6: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 7: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

Maintainability 7

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

Page 8: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

Maintainability 8

Arithmetic Mean

1

nMcti

iMctn

Page 9: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 10: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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)

Page 11: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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)

Page 12: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 13: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 14: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 15: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 16: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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Some Definitions

MTBF = Mean time between failureλ= Rate of breakdown = 1/MTBF

Page 17: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 18: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 19: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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)

Page 20: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 21: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 22: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 23: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 24: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 25: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 26: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 27: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 28: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 29: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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)

Page 30: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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

Page 31: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 32: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 34: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.).

Page 35: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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.

Page 37: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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?

Page 38: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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?

Page 39: Design for Maintainability. Maintainability2 Definition Maintainability, as a characteristic of design, can be defined on the basis of a combination of

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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