Ch.3 Design for Maintainability.

Part 2 : System Design & Development.

Ch.3 Design for Maintainability.

Edited by Dr. Seung Hyun Lee (Ph.D., CPL) IEMS Research Center, E-mail : [email protected]

Maintainability. [Langford, pp55 - 70]

Definition of Maintainability. Maintainability is the measure of the ability of a system to be restored to a specified level of operational readiness within defined intervals with

the

use

of

prescribed

personnel,

facility,

and

equipment

resources.

- 2 -


Two Elements of Maintainability. ․ Corrective Maintenance. The unscheduled actions, initiated as a result of failure, that are necessary to restore a system to its required level of performance.

․ Preventive Maintenance. The scheduled actions necessary to retain a system at a specified level of performance.

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The basic quantitative parameter for Maintainability ․ Corrective maintenance ( M ct ) . - Mean corrective maintenance time ( M ct) - Mean time between failure (MTBF) or Failure rate( λ )

․ Preventive maintenance ( M pt ) . - Mean preventive maintenance time ( M pt) - Mean time between preventive maintenance (MTBFpt) or Preventive maintenance frequency( f pt)

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The basic quantitative parameter for Maintainability ․ Logistics/administrative delay. - Mean logistics delay time ( M LD ) - Mean time between logistics delay (MTBL) or Logistics delay frequency ( f LD )

․ General maintenance, considering both corrective and preventive maintenance elements, is measured by - Mean active maintenance time ( M) - Mean time between maintenance (MTBM) - Maintenance downtime (MDT)

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Corrective Maintenance. [Langford, pp55 - 70]

Definition. Corrective maintenance relates to the repair and restoration of a system required as a result of a random, unplanned failure or a disabling

malfunction

of

the

system.

Corrective

maintenance

is

synonymous with repair.

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Corrective Maintenance. [Langford, pp55-70]

Corrective Maintenance Cycle. ․ Spare Parts and repair parts. Spares and repair parts covering items replaced as a result of corrective and preventive maintenance actions. Spares are major replacement items that are repairable, whereas repair parts are nonrepairable smaller components.

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Mean Corrective Maintenance Time. ․ Mean and Standard Deviation. n

․ M ct =

∑ (λ i ) (M ct ) i

i=1

n

or

∑ λi i= 1

∑ M ct

i

n

n

․ σ =

(M ct - M ct ) 2 ∑ i=1 i

n-1

where n = number of subsystems. λ i = failure rate of the i th system t i = time to repair the i th unit.

- 8 -


Maintainability. ․ Calculating Maintainability. ․ For a normal distribution, expected downtime (MTTR) is same as

M ct . The

probability of performing a maintenance action within an allowable time interval are M( t) = 1 - e

- t /MTTR

․ Example. What is the probability of completing an action within 5 hours if the

MTTR = 7 hours ?

Sol.)

M(t) = 1 - e

- t/MTTR

= 1-e

-

5 7

= 1 - 0.4895 = 0.5105 - 9 -

Preventive Maintenance. [Langford, pp55 - 70]

Mean Preventive Maintenance Time. ․ Preventive maintenance consist of the actions required to retain a system at a specified level of performance and may include such functions as periodic inspection, servicing, scheduled replacement of critical items, calibration, overhaul.

․ Mean preventive maintenance time. n

M pt =

∑ (f p t i ) ( M pt ) i

i=1

n

∑ f pt i

i=1

where f pt : the frequency of the individual ( i th ) preventive action. i

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Maintainability Measures. [Langford, pp55 - 70]

Mean Active Maintenance Time. ․ Mean Active Maintenance Time ( M ) is a function of both corrective maintenance and preventive maintenance. This parameter reflects the average maintenance task time, taking into consideration mean corrective maintenance time time

M ct and mean preventive maintenance

M pt .

M =

=

(

1 1 ×( M ct ) + ×( M pt ) MTBM ct MTBM pt 1 1 + MTBM ct MTBM pt

)

(

)

( λ) ×( M ct ) + ( f pt )×( M pt ) λ + f pt - 11 -


Mean Time Between Maintenance (MTBM). ․ Mean time between maintenance(MTBM) is the average interval between maintenance actions, taking into consideration both mean time between corrective maintenance

MTBM ct

and mean time

between preventive maintenance MTBM pt.

MTBM =

1 1 1 + MTBM ct MTBM pt

=

1 λ + f pt

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Maintenance Downtime (MDT). ․ Maintenance downtime represent the total system downtime attributable to maintenance actions and maintenance-related events. It takes into consideration the effects of corrective maintenance, preventive maintenance, and logistics delay periods.

M =

=

(

1 1 1 ×( M ct ) + ×( M pt ) + ×( M LD ) MTBM ct MTBM pt MTBL 1 1 1 + + MTBM ct MTBM pt MTBL

)

(

)

(

)

( λ) ×( M ct ) + ( f pt )×( M pt ) + f LD×( M LD ) λ + f pt + f LD

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Availability. [Langford, pp71 - 83]

Definition. ․ Maintainability and reliability determine the availability of systems and equipment. ․ Availability. A 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.

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Availability Measures. [Langford, pp71 - 83]

Inherent Availability ( A i ). ․ This is the ideal state for analyzing availability. The only consideration

are

the

MTBF(reliability)

and

the

MTTR(maintainability). This measure does not take into account the time

for

preventive

maintenance

and

assume

repair

begins

immediately upon failure of the system. The measure for A i .

Ai =

μ λ+μ

=

MTBF MTBF + M ct

where

λ = failure rate = 1 / MTBF. μ = repair rate = 1 / MTTR.

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Achieved Availability ( A a). ․ Achieved availability is somewhat more realistic in that it takes preventive

maintenance

into

account

as

well

as

corrective

maintenance. The assumption here is that, as in A i , there is no loss of time waiting for the maintenance action to begin. The measure for A a.

Aa =

MTBM MTBM + M

where MTBM is the mean time between maintenance action both preventive and corrective. M is mean active maintenance. - 16 -


Operational Availability ( A o). ․ This is what generally occurs in practice. Operational availability takes

into

account

that

the

maintenance

response

is

not

instantaneous, repair parts may not be in stock as well as other logistics issues. The measure for A o .

Ao =

MTBM MTBM + MDT

where MDT is mean down time.

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Design For Maintainability. [Blanchard, pp285 - 292]

Maintenance Task Analysis (MTA). ․ Evaluation of those maintenance functions that are to be allocated to the human. ․ Identification of maintenance functions/tasks in terms of task times and

sequences,

personnel

quantities

and

skill

levels,

and

supporting resources requirements. ․ Spares/repair parts, and associated inventories, tools and test equipment,

facilities,

transportation

and

handling

requirements,

technical data, training, and computer software.

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Maintenance Task Analysis (MTA).

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Reliability-Centered Maintenance (RCM). ․ Evaluation of the system/process, in terms of the life cycle, to determine

the

best

overall

program

for

preventive(scheduled)

maintenance. ․ Emphasis is on the establishment of a cost-effective preventive maintenance program based on reliability information derived from the FMECA.

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Level-of-Repair Analysis (LORA). ․ Evaluation of maintenance policies in terms of levels of repair. ․ Example. Should a component be repaired in the event of a failure or discarded, and, given the repair option ? Should the repair be accomplished at the intermediate level of maintenance, at the supplier's factory, or at some other level ?

․ Decision factors include economic, technical, social, environmental, and political considerations.

- 21 -


Level-of-Maintenance. Organizational Maintenance ․ At the operational site or wherever the prime equipment is located.

Intermediate Maintenance ․ Mobile or semi mobile units, Truck, van, portable shelter, or equivalent.

Depot, Supplier, or Manufacture Maintenance ․ Depot/Suppliers specialized repair activity, or manufacturer's plant.

․ Fixed field shop ․ System/equipment operating personnel. (low maintenance skills)

․ Personnel assigned to mobile, semi-mobile, or fixed units.

․ Depot facility personnel or manufacturer's production personnel.

․ Using organization's equipment.

․ Equipment owned.

․ Equipment owned.

․ Detailed inspection and system checkout. ․ Major servicing. ․ Major equipment repair and modifications. ․ Complicated adjustments. ․ Limited calibration. ․ Overload from organization level of maintenance.

․ Complicated factory adjustment. ․ Complex equipment repairs and modification. ․ Overhaul and rebuild. ․ Detailed calibration. ․ Supply support. ․ Overload from intermediate level of maintenance.

․ ․ ․ ․ ․

Visual inspection. Operation checkout. Minor servicing. External adjustment. Removal and replacement of some component.

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Performance Check. 1. Availability is a function of : A. Reliability and Supportability.

B. Reliability and Maintainability.

C. Reliability and Failure Rate.

D. Reliability and Reliability Apportionment.

2.____________________ = MTBM/(MTBM + MDT) A. Operational availability.

B. Inherent availability.

C. Achieved availability.

D. Normal availability.

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Performance Check. 3. Primary input to maintainability analysis includes the following types of information : A. Operational support concepts and requirements; overall quantitative maintainability requirements; personnel subsystem limitations, characteristics, and description; projected facility, training program, and tool availability; cost constraints; studies and reports for the system; and standard tools and equipment. B. Operational support concepts and requirements; overall performance requirements; personnel subsystem limitations, characteristics, and description; projected facility, training program, skills, equipment and tool availability; cost constraints; studies and reports for the system; and standard tools and equipment. C. Operational support concepts and requirements; overall quantitative maintainability requirements; personnel subsystem limitations, characteristics, and description; projected facility, training program, skills, equipment, and tool availability; cost constraints; studies and reports for the system; and standard tools and equipment. D. Operational support concepts and requirements; overall quantitative maintainability requirements; personnel subsystem limitations, characteristics, and description; projected facility, training program, skills, equipment, and tool availability; environmental constraints; studies and reports for the system; and standard tools and equipment. - 24 -

Performance Check. 4. Preventive Maintenance is : A. The scheduled actions accomplished to retain a system at a specified level of performance by providing systematic inspection, detection, calibration, condition monitoring, and/or replacement of critical items to prevent impending failures. B. The scheduled actions accomplished to restore a system at a specified level of performance by providing systematic inspection, detection, servicing, calibration, condition monitoring, and/or replacement of critical items to prevent impending failures. C. The scheduled actions accomplished to retain a system at a specified level of performance by providing systematic inspection, detection, servicing, calibration, condition monitoring, and/or replacement of critical items to prevent impending failures. D. The scheduled actions accomplished to restore a system at a specified level of performance by providing systematic inspection, detection, calibration, condition monitoring, and/or replacement of critical items to prevent impending failures.

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Performance Check. 5. A system reflects MTBMct of 1000 hours and MTBMpt of 250 hours. The system MTBM is : A. 200 hours.

B. 333 hours.

C. 2000 hours.

D. 150 hours.

6. If the system MTBM is 1000 and the MDT is 50, the operational availability ( A o) is : A. 0.95

B. 1.05

C. 0.59

D. None of the above.

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Performance Check. 7. Based on the following subsystem data : Subsystem

Subsystem MTBF

Subsystem 1

1500

Subsystem 2

1200

Subsystem 3

1000

Subsystem 4

800

The system MTBF is : A. 250

B. 255

C. 1125

D. 267

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Performance Check. 8. If the system design

calls for MTBF of 1500 and M ct of 2.5, the projected Inherent

Availability ( A i ) of the system is : A. 0.998

B. 0.984

C. 0.894

D. 0.989

9. The Target A i of a system is 0.999 and the maximum acceptable M ct is 1.5 hours. The design MTBF must be a minimum of : A. 1499 hours.

B. 150 hours.

C. 14985 hours.

D. None of the above.

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Performance Check. 10. A system is allocated a system level M ct of 4.5 hours and has been estimated to have the following subsystem Reliability attributes : Subsystem Subsystem Failure Rate Subsystem 1 0.002 Subsystem 2 0.004 Subsystem 3 0.006 Subsystem 4 0.008 The Maintainability allocation of M ct hours A. Subsystem 1 : 2.0 Subsystem Subsystem 2 : 5.0 Subsystem B. Subsystem 1 : 4.0 Subsystem Subsystem 2 : 8.0 Subsystem C. Subsystem 1 : 8.0 Subsystem Subsystem 2 : 7.0 Subsystem D. Subsystem 1 : 16.0 Subsystem Subsystem 2 : 12.0 Subsystem

to each subsystem should be as follows : 3 4 3 4 3 4 3 4

: : : : : : : :

7.0 8.0 12.0 16.0 5.0 2.0 8.0 4.0 - 29 -

Performance Check. 11. If the system MTBM is 800 hours and the M is 5 hours the Achieved Availability ( A A) is : A. 0.994

B. 0.949

C. 0.941

D. 0.999

12 - 14. A System has the following specifications: ․ M ct (Hours) : 3.0

․ M pt (Hours) : 2.0

․ M LD (Hours) : 25.0 ․ MTBMpt (Hours) : 1,500

․ MTBF (Hours) : 2,000 ․ MTBL (Hours) : 1,000 (Logistics Delay Interval)

12. The System Failure Rate( λ ), Preventive Maintenance Frequency( f pt) and Logistics Delay Frequency( f LD ) are : respectively : A. λ : 0.001

f pt : 0.007

f LD : 0.005

B. λ : 0.0005

f pt : 0.00067

f LD : 0.0001

C. λ : 0.0005

f pt : 0.00067

f LD : 0.001

D. λ : 0.0005

f pt : 0.00067

f LD : 0.0001 - 30 -

Performance Check. 13. The Mean Active Maintenance Time ( M) (hours) is : A. 7.35 B. 3.97 C. 2.5 D. 2.427

14. The Maintenance Down Time (MDT) (hours) is : A. 10.28

B. 12.83

C. 13.28

D. 18.295

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Performance Check. 15 - 16. A system has a target Reliability of 0.95 ; the projected MTBF is 10,000 hours and the planned operating cycle is 500 hours. 15. If the reliability of 0.95 must be maintained, but the MTBF is reduced to 7,500 hours, the impact on the operating cycle would be : A. Reduction from 500 hours to 435 hours. B. Reduction from 500 hours to 385 hours. C. No effect. D. Reduction from 500 hours to 405 hours.

16. If the reliability of 0.95 must be maintained, but it is planned to increase the operating cycle from 500 hours to 750 hours the maintainability parameter would have to be redesigned by : A. Increasing MTBF from 10,000 hours to 14,622 hours. B. Reducing Failure Rate to 6839 x 10-6. C. Increasing MTBF from 10,000 hours to 16,422 hours. D. Doing nothing; no redesign would be required. - 32 -

Performance Check. 17. The results of a maintainability demonstration document the following : M ct : 25 minutes

Sample size : 30 task observations Sample standard deviation (s) : 3.2 Confidence factor : 95% (z = 1.65) The calculated upper limit is : A. 25.964

B. 34.064

C. 26.176

D. 25.166

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Performance Check. 18. The following is a true statement : A. Inherent availability is sometimes greater than Achieved Availability. B. Operational availability is sometimes greater than A i C. Operational availability is always less than A i

and always less than A A.

and always less than A A.

D. None of the above are correct statements.

19. A system has the following subsystem characteristics : Failures x 10-6

Subsystem Subsystem Subsystem Subsystem Subsystem

1 2 3 4

Mean Corrective Maintenance Time ( M ct)

250 375 125 500

5.0 4.0 8.0 2.0

The calculated system level M ct would be A. 4.75

B. 3.8

C. 7.5

D. 3.5 - 34 -

Performance Check. 20. The results of a maintainability analysis should includes : Ⅰ. The depth and frequency of maintenance requirements at each level. Ⅱ. The facilities required. Ⅲ. The support equipment and tools required. Ⅳ. The skill levels and tools required.

A. I and III only

B. I, III and IV only

C. II, III and IV only

D. I, II, III and IV

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Performance Check.

solutions. 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

B

A

C

C

A

A

D

A

A

C

A

C

D

B

B

16

17

18

19

20

A

A

C

B

D - 36 -