Transcript Design for Maintainability - San Jose State University

Design for Maintainability
Maintainability
Definition
Maintainability, 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 Maintainability
From a systems perspective
Corrective maintenance
Preventive maintenance
From a software perspective
Adaptive maintenance
Perfective maintenance
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Maintenance Elapsed -Time
Factors
Corrective and preventive active
maintenance times
Administrative and logistic delay times
Total maintenance downtime
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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 maintainability
Sum of corrective maintenance times divided by the total number
of failures within an item
Average time it takes to fully repair a failed system
Includes fault isolation,replacements of failed item(s) and check
out
Its also called the mean corrective maintenance time,Mct
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Failure
Occurs
Detection
Preparation for
Maintenance
Localization
And
Isolation
Disassembly
(Access)
Reassembly
Active Maintenance
Commences
Alignment
And
Adjustment
Faulty Item
Identified
Disassembly
Completed
OR
Removal of
Faulty Item
Failure Confirmed
(Notification)
Repair of
Equipment
Reassembly
completed
Condition
Verification
(Checkout)
Repair
Completed
Corrective Maintenance Cycle
Installation of
Spare/Repair
Part
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Frequency of
Occurrence
14
12
12
10
10
8
6
9
7
5
4
4
3
2
0
29.5 39.5
49.5
59.5
69.5
79.5 89.5 99.5
Time(min)
Histogram of maintenance times
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Arithmetic Mean
n
 Mct
i
i

1
Mct 
n
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Mean Preventive Maintenance
Time (M pt)
Preventive Maintenance refers to the actions
required to retain a system at a specified level of
performance
  fpt i  Mpt i 
Mpt=
 fpt i
Includes active maintenance time
Does not include logistic delay and
administrative delay time
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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 available
Waiting for availability of an item of test
equipment in order to perform maintenance,
waiting for transportation and so on
LDT does not include active maintenance time
It does constitute a major element of total
maintenance downtime (MDT)
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ADT & MDT
Administrative Delay Time (ADT)
That portion of downtime when maintenance time is
delayed for administrative reasons.
Examples:
Personnel Assignment Issues
Labor 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 condition
MDT includes:
Mean active maintenance time
Logistics delay time (LDT)
Administrative delay time (ADT)
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Uptime/Downtime Factors
Time
Uptime
Standby/
Ready Time
Downtime
System
Operating Time
Active
Maintenance Time
Preparation
Time
Fault Detection
Preventive Maintenance Cycle
Inspection
Servicing
Checkout
Time
Time
Time
Repair of Item
in Place
Corrective Maintenance
Localization
and Fault
Isolation
Administrative
Delay Time
Preventive
Maintenance
Corrective
Maintenance
Preparation for
Maintenance
Logistics
Delay Time
Disassembly
(Gain Access)
Reassembly
(Buildup)
or
Removal of Faulty
Item and Replace
with Spare
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Adjustment,
Alignment, or
Calibration
Condition
Verification
(Checkout)
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Maintenance Labor Hours
Elapsed time vs. labor hours
Elapsed time reduction via more personnel
Can be expensive
Maintainability is primarily concerned with
ease and economy of maintainability
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Maintenance Labor Hours
Important 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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Maintenance Frequency Factors
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.
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Some Definitions
MTBF = Mean time between failure
λ= 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
MTBM=
1
1
1
+
MTBMs
MTBMμ
where:
–MTBMμ: Unscheduled (corrective maintenance)
–MTBMs: Scheduled (preventive maintenance)
MTBMμ should be equivalent to the MTBF if all failures
are considered
•MTBM is crucial to determine the achieved and
operational availability.
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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
1
=
1
1
+
MTBMμ
MTBMs
= 0.55 years
1
1
+
0.5 years 20 years
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Maintenance Frequency Factors
(Continued)
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 low MTBR will mean a higher frequency of replacement
which in turn will required higher levels of maintenance
support.
A maintainability objective in system design is to
maximize the MTBR
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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 based
from the System
Supply Responsiveness
Test and Support Effectiveness
Maintenance facility availability and use
Transportation modes
Maintenance organizational effectiveness
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Related Maintenance Factors
Cost-effectiveness
Total system cost
(Live cycle)
System effectiveness
Availability
Capability
Dependability
Downtime
(Non-use)
Uptime
(Operating/standby)
Active
maintenance
time
Logistic delay time
Administrative
delay time
Logistic support
resource
consumption
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Maintainability Requirements in the System Life-Cycle
Conceptual Design
Preliminary System Design
========================
•Feasibility study
•Operational requirements
•Maintenance Concept
•Functional requirements
•Technical performance measures
•Advance system planning
----------------------------------------Maintenance concept,
quantitative and qualitative
maintainability requirements
for system (MTBM, MTBR, Mct,
Mpt, MLH/OH, cost/MA, etc.),
maintainability planning.
========================
•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.
Feedback Loop
System Utilization &
Life-Cycle Support
System
Retirement
and
Phaseout
================================
•Consumer use of system and its components
•Life-cycle system support
-----------------------------------------------------Maintainability data collection, analysis, and
evaluation; system modification (as required)
Maintainability
Production / Construction
==============================
•Fabrication, assembly, and test of system
and its components
•System construction
--------------------------------------------------Maintainability test and evaluation;
maintainability data collection, analysis,
and corrective action
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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 Requirements
Quantitative 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 Cycle
System 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 Method
Maintainability Analysis: Iterative process of system
synthesis, analysis and evaluation
Tools/Methods Used for Maintainability analysis
1, Reliability-Maintainability Trade-off Evaluation
2, Reliability-Centered Maintenance
3, Repair versus Discard Analysis
4, Maintenance Prediction
5, Maintenance Task Analysis (MTA)
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Maintainability Analysis Method
Reliability- 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 Method
Example:
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 Method
Reliability- 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 Prediction
Maintainability 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 cont
Standardization; 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
development
Create 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
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