07 Maintainability & Availability (1)

8/12/2019 07 Maintainability & Availability (1)

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MAINTAINABILITY

&AVAILABILITY


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TerminologyAvailability Measure of the degree to which an item is in an operable and committable

state at the start of a mission when the mission is called for at an unknown

(random) time.

Dependability Measure of the degree to which an item is operable and capable of performing

its required function at any(random) time during a specified mission profile,

given item availability at the start of the mission. See Availability.

Capability A measure of the ability of an item or system to achieve mission objectives

given the conditions during the mission

Inherent R & M Value Measure of reliability or maintainability that includes only the effects of an

item design and its application, and assumes an ideal operation and support

environment

Maintainability The measure of the ability of an item to be retained in or restored to specified

condition when maintenance is performed by personnel having specified skill

levels, using prescribed procedures and resources

Maintenance All actions necessary for retaining an item in or restoring it to a specified

condition

Maintenance,

Corrective

All actions performed as a result of failure, to restore an item to a specified

condition. Corrective maintenance can include any or all of the following

steps: Localization, Isolation, Disassembly, Interchange, Re-assembly,

Alignment and Checkout.


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Maintenance, Event One or more maintenance actions required to effect corrective and

preventative maintenance due to any type of failure or malfunction,

false alarm or scheduled maintenance

Maintenance,

preventive

All actions performed in an attempt to retain an item in specified

condition by providing systematic inspection, detection, and

prevention of incipient failures

Maintenance,

scheduled

Preventive maintenance performed at prescribed points in the items

life

Maintenance Time An element of downtime which excludes modification and delay time

Maintenance,

unscheduled

Corrective maintenance required by item conditions

Mean- Maintenance -

Time

The measure of item maintainability taking into account

maintenance policy. The sum of preventive and corrective

maintenance times, divided by the sum of scheduled and

unscheduled maintenance events , during a stated period of time.

Terminology


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Mean-Time-Between-

Failure (MTBF)

A basic measure of reliability for repairable items: The mean

number of life units during which all parts of the item perform

within their specified limits, during a particular measurement

interval under stated conditions.

Mean-Time - Between-

Maintenance(MTBM)

A measure of the reliability taking into account maintenance policy.

The total number of life units expended by a given time, divided by

the total number of maintenance events(scheduled and unscheduled)

due to that item.

Mean-Time-To-

Failure(MTTF)

A basic measure of reliability for non-repairable items: The total

number of life units of an item divided by the total number of

failures within that population, during a particular measurement

interval under stated conditions.

Mean-Time-To-Repair(MTTR) A basic measure of maintainability: The sum of correctivemaintenance times at any specific level of repair, divided by the total

number of item failures during a particular interval under stated

conditions.

Terminology


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Planning & Strategies

Maintainability & Availability Planning:Maintainability engineering is performed for the following

reasons:

To achieve ease of maintenance through design, reducing

maintenance time and cost, (increasing availability) To estimate maintenance and equipment downtime. This

will determine the need for redundancy, in order toincrease availability.

To estimate system availability by combiningmaintainability data with reliability data

To estimate labor, hours, time, and other resources forproper maintenance


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Designing and developing maintainable products and systems

provides guidelines for establishing a maintainability program:

Prepare program plan

Perform maintainability analysis

Prepare inputs to maintenance concept and plan

Establish maintainability design criteria

perform design trade-offs predict maintainability parameter values

incorporate maintainability requirements in subcontractor specs

Integrate other items

Participate in design reviews

Establish data collection, analysis and corrective action system

Demonstrate achievement of maintainability requirements

Prepare maintainability status reports

Maintainability Program


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An analysis of customer needs and requirements can determinethe maintainability and availability requirements for a systemor component.

The customer needs may include: Expected operating time per unit time

Downtime, maintainability time, or availability constraints

Operational/ambient environment (i.e. polar, tropic, severe)

Types of diagnostic and maintenance support equipment Maintenance skill levels

Maint & Availability Requirements


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A measure of the degree to which an item or system can be

expected to achieve a set of specific mission requirements, andwhich may be expressed as a function of availability,dependability, and capability.

SE= Availability x Dependability x Capability

SE= Mission Reliability x Operational Readiness x DesignAdequacy

System Effectiveness


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Time, active That time during an item is in an operational inventory

Time, administrative That element of delay time, not included in the supply delay time

Time, alert That element of up time during which an item is assumed to be in

specified operating condition and is awaiting a command to perform

its intended mission

Time, delay That element of down time during which no maintenance is being

accomplished on the item because of either supply or administrative

delay

Time, down(down time) That element of active time during which an item is not in condition

to perform its required function (Reduces availability and

dependability)

Time, inactive That time during which an item is in reserve

Time, mission That element of uptime required to perform a stated mission profile

Time, modification The time necessary to introduce any specific change(s) to an item

improve its characteristics or to add new ones

Time, not operating That element of uptime during which the item is not required to

operate

Time, reaction That element of uptime needed to initiate a mission, measured from

the time command is received


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Time, supply delay That element of delay time during which a needed replacement item

is being obtainedTime, up( uptime ) That element of active time during which an item is in condition to

perform its required functions

Uptime ratio A composite measure of operational availability and dependability

that includes the combined effects of item design, installation, quality,

environment ,operation, maintenance, repair and logistic support.

The quotient of uptime divided by uptime plus downtime

Useful life The number of life units from manufacture to when the item has an

unrepairable failure or unacceptable failure rate

Wear out The process which results in an increase of the failure rate or

probability of failure with increasing number of life units


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Maintenance strategies should be chosen to ensure a high

level of availability while controlling cost. Preventivemaintenance does not improve the inherent reliability of thesystem. Preventive maintenance will maintain the reliabilitylevel of useful life, keeping the failure rate low. It will also delaythe onset of were, thus increasing the length of useful life.

A single unit, such as a pump, could be considered to be asystem. Or the pump could be considered to be a component of alarger system. In either case, system reliability is at the highestlevel if all units are in their useful life phase. Preventivemaintenance strategies such as reliability-centered maintenance

and predictive maintenance are proactive and require thereplace or components of the system that are nearing the end oftheir useful life or entering wear-out. Replacement is madebefore failure occurs. Condition-based maintenance is reactiveand is a corrective maintenance strategy. Replacement is made

after failure occurs.

Maintenance Strategies


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Predictive maintenance assumes that the operator can detect theimminent failure of a unit. This detection can be by observation,

analysis, or using test equipment. The analysis of an oil sample or themeasurement of increased vibration might indicate were and anincreasing failure probability. If the increasing failure probability cannot be detected by the operator, a reliability-centered maintenancestrategy could be adopted. This strategy use the predicted failure

distributions to determine the optimum replacement time for unitsabout to enter the wear-out phase. Replacement of units that areentering the wear-out portion of the bathtub curve will maintain thesystem reliability at the useful life level.

It is possible that any maintenance action can negatively impactreliability by inducing failure modes due to the maintenance action.These maintenance induced failures are similar to the early-life failuresdue to the manufacturing or installation of a system. An effort shouldbe made during design to ensure that standard preventive maintenanceaction can be preformed quickly and a low risk of introducing problemsdue to the maintenance actions.

Predictive & Reliability Centered Maintenance


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Maintainability

The measure of the ability of an item to be retained or restored to a specified

condition when maintenance is performed by personnel having specified skilllevels, using prescribed procedures and resources, at each prescribed level of

maintenance and repair.

Maintainability is most commonly measured by mean time to repair(MTTR)

Probability of Repair within the Allowable Downtime:To calculate the probability of performing a maintenance action within an

allowable time interval use:

t =Allowable downtime

MTTR=Expected downtime(MTTR)


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Example 8.3: What is the probability of completing a maintenance action

within 5 hours if the MTTR=7 hours?

Solution:=1-0.4895=.5105

There is approximately a 51% probability of completion.

The average length of failures during a mission length of time T is T. The

number of failures which cannot be repaired within the allowable time is

given by:

Where: = failures per hour


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AvailabilityA measure of the degree to which an item is in the operable and

committable state at the start of a mission, when the mission is called

for at an unknown (random) time.

The three common measures of availability are:

Inherent Availability(A1)

Achieved Availability(AA )

Operational Availability(Ao)

The measure of Inherent(potential) Availability(A1):


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The measure of Achieved (Final) Availability (AA ) :

Where : MTBMA is the mean time between maintenance actions both

preventive and corrective.

MMT is the Mean Maintenance Action Time, and MMT is further

decomposed into the effects of preventive and corrective maintenance andis given as:


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The measure of Operational(actual) Availability (A o) :

Where: MDT is the mean down time.

The instantaneous availability,(probability) that an item will be

available at time t is:

When t is large, the expression reduces to:

The system steady state unavailability=


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Availability for constant failure rates

Series Configuration

Parallel Configuration


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Preventive Maintenance Analysis

Objective of preventive maintenance is to ensure that the probability offailure of equipment is minimized

Preventive maintenance decisions will depend on factors such as

distribution of failure times, cost of repairs, cost of down time

Time to failure distribution pattern in a system have major influence on

the replacement strategy

Following example shows optimization of replacement intervals to

minimize total cost


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Given a decreasing hazard rate , it is best to not replace the part.

Given a constant hazard rate, scheduled replacement does not

reduce failure rates.

Given a increasing hazard rate, scheduled replacement reduces

failure rates.

Given a nearly failure free, but increasing hazard rate, scheduledreplacement will make for nearly zero failures.

Decreasing hazard rate,

scheduled maintenance willreturn the part to the top of the

curve.


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Constant hazard rate ,

replacement of a part will

result in the same probabilityof failure as before.

Increasing hazard rate,

scheduled replacement of a partwill reduce the probability of

failures.

Increasing hazard rate with nearfailure free life, scheduled

maintenance will ensure near

failure free probability.


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Corrective Maintenance Analysis

Corrective maintenance should aim at quick repair, accurate diagnosis so

that the same fault is not repeated.

Measures of overall effectiveness would include MTBF and MTTR.

MTBF should be high while MTTR should be low.

Maximising the MTBF requires good diagnostic skills and tools, trained

technical persons, adequate tools that are located easily and quickly

Machines and equipment should be accessible for repairs. Layout of the

equipment should therefore be planned for accessibility during breakdown.

Data collection system should facilitate separate measurement and analysis

of preparation time , fault isolation time, active maintenance and delays.


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As per MIL-HDBK-472, corrective maintenance can be analyzed as seven

steps:

1. Localization: Determining system fault without using test equipment

2. Isolation: Verification of system fault using test equipment3. Disassembly

4. Interchange or replace defective parts

5. Reassembly

6. Alignment

7. Checkout

Reduction of repair time implies that time for each step should be reduced

Time to repair could be severely impacted when required spare parts arenot available

T bili


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Testability

.A test strategy should be developed for performing

measurements on a specific item or program. Test programsusually occur at the end of development and production segments

and are used to determine if the design requirements have been

achieved. Test strategy includes all testing done on a product.

Testability is an important design feature.

Testability affects reliability, since production tests which are not

detected by testing can lead to failures in service, and circuits (or

complex systems) which are difficult to diagnose are more likely

to be inadequately or incorrectly repaired. Complex systems,

either mechanical or electronic, can benefit from good testability

principles.

The maintainability of a system is influenced by the ability to

detect a system fault and to isolate the component that has failed.


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TestabilityThe testing requirements for a system should provide for a

method to detect a system fault and isolate the failed component: In some cases, such as for a complex system, the requirement

could be met with a built-in test system.

by providing easily accessible test points for critical

measurements using external test equipment

The testability of a system must be developed as part of the

system design.


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Built in test (BIT)

The testing of complex electronic systems such as

laboratory instruments, printed circuit boards, etc. are

made easier with built-in test(BIT) facilities or ports.

BITs can increase system availability and userconfidence, but can also provide false conditions if

BIT components malfunction.


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Preventive Maintenance


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PMs can be performed at constant intervals or upon

failure of the part or system. The optimum point would be

at the point where the total of maintenance and failure

costs are minimal. Other models for PM strategy are:

1. Constant interval replacement policy

2. Preventive replacement at predetermined age

3. Optimum replacement, under minimal repair

4. System shocks, time independent

5. System shocks, time dependent

6. Group maintenance7. Inspection policy

8. Periodic inspection and maintenance

9. Continuous monitoring

Preventive Maintenance

S P t A l i


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Spare Part Analysis

The spare part inventories needed for equipment andmachinery should not approach the raw material

levels required for production needs. Spare are

necessary due to factors such as:

Long spare lead times (weeks, months, or years)

Avoidance of expensive production downtime

Penalties for late deliveries (as a supplier)Unreliability of suppliers(as a producer)


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The MTTR calculation assumes that spare parts are readily

available. This means that spares must be in inventory when

they are needed. If there is waiting time to obtain a spare, theunit downtime will increase and availability will suffer.

If the unit has a constant failure rate, the probability of

requiring no more than r replacement units can be found

using the cumulative Poisson distribution.The probability of having r or fewer failures in operating time t

if the unit is operating with a constant failure rate of is


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If preventive maintenance required spare parts, the number of spares

necessary is a function of the operating time and the maintenance

cycle. The number of spares required for preventive maintenance is

the product of the maintenance cycle and the total operating time. The

total number of spares that must be kept in inventory is a function of

the delivery time, the cost of maintenance the inventory, and the

availability requirement.

A unit that operates an average of 2000 hours per year has a constant

failure rate

The maintenance strategy is to replace the unit every 5000 operatinghours. In a five- year period:

a. How many spares will be necessary for preventive maintenance?

b. With a.90 probability, how many spares will be necessary for

corrective maintenance?


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b. P (r )= 0.9

From the cumulative passion tables in appendix N:

P(r=2)=.92t=1.0

With a probability of .92 the number of spares needed for

corrective maintenance is no more then two.

Solution:

The operating time in a four-year period is t=5 x 2000 =10,000

hours

The preventive maintenance cycle is 1/5000 hours

The number of spares required for preventive maintenance is

n=(1/50000)x(10,000)=2

07 Maintainability & Availability (1)

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