Reliability Centered Maintenance Applied to Achieve...

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Dr Bill Wessels PE CRE Principal Research Scientist

bill.wessels@uah.edu

Reliability & Failure Analysis Laboratory http://rfal.uah.edu

Reliability Centered Maintenance Applied to Achieve Best Life Cycle Costs

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Dr Bill Wessels PE CRE Experience:

• 1975 – 1989 Tramp Engineer, mining mobile machinery & process equipment life-cycle sustainability

• 1989 – 2005 Contractor [Biomedical Devices, DoD and NASA] design-for- reliability, RCM

• 2005 – Research in design-for- reliability & life-cycle sustainability for mechanical and structural components

Credentials • BS USMA, West Point ’70 • PhD Reliability Systems Engineering, UAHuntsville ’96 • PE MechE, Pennsylvania, ‘82 • CRE, ASQ, ’96

Author – Practical Reliability engineering and Analysis for System Design and Life-Cycle Sustainment, CRC Press, 2010

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Introduction

Reliability-centered maintenance serves to preserve system functionality by performing scheduled maintenance actions that prevent unscheduled system downing events during scheduled operations

RCM achieves its objective by developing an understanding of critical part failure

Understanding part failure is manifested by the ability to measure the consumed life of a critical part

Scheduled maintenance is defined by application of a risk threshold to the consumed life

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RCM Roadmap

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Part Cost Objective Function· Labor Costs· Materials Costs· Overhead Costs

Logistics Downtime

Math Model

Spare Strategy

Math Model

PreventiveMaintenance Math Model

Resource Queuing

Math Model

PartReliability/Cost

Simulation

Assembly Reliability/Cost

Simulation

System Reliability/Cost

Simulation

PartFailure

Math Model

PartRepair

Math Model

Part ReliabilityLife-Cycle Affordability

Assembly ReliabilityLife-Cycle Affordability

System ReliabilityLife-Cycle Affordability

Condition Indicator

Time-to-Failure Reliability Failure

Math Model

CBM

TDM

SDMStress-Based

Reliability Failure Math Model

Single Fail MechSingle Fail Mode

f(t)

Y

E

S

Y

E

S

YES

NO

PartFailure Mode

Multiple Fail MechMultiple Fail Mode

f(s)

NO

Metric: Part Condition Indicator

Metric: Part Time in Service

Metric: Part Stress Profile

RCMDatabase

System

Subsystem

Critical Items List

Assembly

Part

ORFunctional

Failure

PartPart

RCMDatabase

Part Failure Consequences

Analysis

Perception of Part Failure

P-F Interval

Operator

Maintainer

Criticality A

nalysis

ORFailureMechanisms

FailureModes

PhysicalStress

ThermalStress

EnvironmetalStress

Change in Shape

(Strain)

Change in Geometry(Crack)

Change inMaterial Property

(Corrosion)

OR

Critical Items List

RCMDatabase

Part

FailureMechanisms

Sources

Transportation Storage Function

Operational Ambient

OR

OR

PartFailure Effects

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Part Reliability- Maintainability Cycle

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System Downtime

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Source of RCM Cost Benefit

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Part failsSystem down

Maintenance action initiated

Wor

k S

tart

s

Time

Time-to-Repair

post-Repair Logistics Downtime

pre-Repair Logistics Downtime

$Lost Opportunity

$Labor$Overhead

$Labor$Materials$Overhead

Wor

kRe

sum

es

$Labor$Overhead

$Benefit

Part failsSystem down

Maintenance action initiated

Wor

k S

tart

s

Wor

kRe

sum

es

$Labor$Overhead

$Labor$Materials$Overhead

Time

Time-to-Repair

post-Repair Logistics Downtime

pre-Repair Logistics Downtime

$Lost Opportunity

Total Downtime

$Labor$Overhead

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Top Level RCM Path Develop a rank ordered critical items list – CIL Perform reliability failure analysis to identify:

• Sources of failure mechanisms (Conditions of use) • Failure mechanisms action on the critical part (Cause of

part failure) • Failure modes (Damage to part) • Failure effects – part (Symptoms of the damage) • Failure effects – assembly (Consequences of part

failure)

Select appropriate RCM Path • CBM • TDM • SDM

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Condition-Based Maintenance CBM

Consumed life characterized by a condition that defines the failure mode/damage • Change in shape • Change in geometry • Change in material properties

Method for investigating the damage, NDI • Visual inspection of condition indicator • Manual measurement of condition indicator • Sensor measurement of condition indicator

Metrics that characterize the damage Risk Threshold

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Notional Linear CBM Concept

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Notional CBM Concept

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Decreasing Condition Indicator

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Diverging Condition Indicator

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Diverging Condition Indicator

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Converging Condition Indicator

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Converging Condition Indicator

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Fatigue Condition Indicator

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Fatigue Condition Indicator

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Step Condition Indicator

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Step Condition Indicator

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Notional Attribute CBM Concept

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Time Directed Maintenance, TDM

Consumed life characterized by time in service, but…

Single failure mechanism with single failure mode

Failure mode correlated to operating time

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Time-to-Failure Data

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Time-to-Failure Histogram

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Fit Data to 3-Parameter Weibull Median Ranks Regression

First: Fit the 2-Parameter Weibull

Then: Select a value for the location parameter, g

• Choose the mid-time between 0 and Tmin

• Calculate Xg = Xi – g

• Calculate r2 for Xg & Y

If: r2 for Xg < r2 for the 2-Parameter Weibull, use the 2-Parameter Weibull

If: r2 for Xg > r2 for the 2-Parameter Weibull,

fit the 3-Parameter Weibull

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Fit Data to 2-Parameter Weibull Median Ranks Regression

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2-P Weibull Median Ranks Regression

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2-P Weibull Median Ranks Regression

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TTF Location Parameter, g

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TTF Location Parameter, g

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3-P Weibull Failure Math Model - TTF

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3-P Weibull Failure Math Model

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Failure Math Model pdf – f(t)

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Risk Threshold & Hazard Function

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Risk Threshold Logic

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Part Alternatives Hazard Functions & Risk Threshold

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Parts Alternatives Risk-based Scheduled Maintenance Interval, SMI

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Exponential v Weibull Failure Math Models

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Exponential v Weibull Hazard Function w/Risk Threshold

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Time-to-Repair Data

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Time-to-Repair Histogram

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3-P Weibull Failure Math Model

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Repair Math Model

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LDT Math Model - Baseline

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pre-Repair LDT Math Model

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Baseline

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post-Repair LDT Math Model

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Baseline

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pre-Repair LDT Math Model

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post-Repair LDT Math Model

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Deterministic Reliability, Maintainability & Availability

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Reliability Simulation Inputs

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Baseline Maintenance Labor Inputs

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PM Maintenance Labor Inputs

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Maintenance Costs Inputs

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Part Life-Cycle Simulation Factors

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Baseline Reliability Simulation Report

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Simulation Report for Part 1 Baseline

Results from 259 runs of sim time 40000-hrs

Part 1 Minimum Mean Maximum St.Dev 95% CL SEM 95% CL

Availability 0.98984 0.99051 0.99137 0.00030 0.99002 0.00002 0.99048

MTBF 628.51 683.68 734.35 19.74 651.09 1.23 681.65

MDT 6.32 6.54 6.80 0.08 6.68 0.005 6.55

MMT 1.67 1.76 1.88 0.03 1.81 0.002 1.76

LDT 4.79 4.86 4.79

System Failures 54 58.00 63 1.66 0.10 58.17

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PM Reliability Simulation Report

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Simulation Report for Part 1 - PM

Results from 259 runs of sim time 40000-hrs

Part 1 Minimum Mean Maximum St.Dev 95% CL SEM 95% CL

Availability 0.99305 0.99328 0.99342 0.00006 0.99318 0.000004 0.99328

MTBF 467.32 467.43 467.49 0.03 467.38 0.00 467.42

MDT 3.10 3.16 3.27 0.03 3.73 0.00 3.16

MMT 0.75 1.71 2.38 0.04 1.78 0.00 1.71

LDT 0.83 1.40 2.19 1.95 1.45

System Failures 85 85 85

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Reliability Simulation Summary

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Labor, Materials, Overhead & Lost Opportunity Costs Table

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Engineering Economics Basics

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,

0

, ,

1 1

1

n

i t

t

t

i t i t t

NPV PV

iPV A

i i

1

1 1

n

eqv n

i iA NPV

i

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Economic Analysis Expensed Costs &

Lost Opportunity Costs

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Part Uniform Recurring Equivalent Cost

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DetMean Det95%CL BSimMean BSim95%CL PMSimMean PMSim95%CL

A(CICC+Event) -$335 -$490 -$332 -$348 -$425 -$432

A(CExp+LostOp) -$2,118 -$3,816 -$2,090 -$2,239 -$1,452 -$1,747

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Part Operational Availability

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Baseline RCM

Part AO 0.99051 0.99328

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Stress-Directed Maintenance, SDM

Consumed life characterized by applied stress profile

Measures failure mechanisms acting on part

Not well developed for use by practioner

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SDM - The Present – Static Loading

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sop sdesign g

f (s) 1

f e

s

s s

s

95%LCLStrength

h(x)

x

1

( )x

h x

g

r

xr

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SDM - The Future - Wearout

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Summary

RCM yields definition of scheduled maintenance interval for critical parts

RCM measures part consumed life constrined by a risk threshold

RCM metrics: • CBM – Condition indicator

• TDM – Time in service

• SDM – Stress load profile

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