Usacerl Tr-99 41 Rcm Guide 1999

8/13/2019 Usacerl Tr-99 41 Rcm Guide 1999

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US Army Corpsof EngineersConstruction EngineeringResearch Laboratories

Reliability Centered Maintenance

(RCM) Guide

Operating a More Effective Maintenance Program

Alan Chalifoux and Joyce Baird

This manual outlines a comprehensive method

of organizing an efficient maintenance program

by applying the concepts of Reliability Centered

Maintenance (RCM). RCM combines

professional intuition and a rigorous statistical

approach, and recognizes that different

maintenance strategies apply to different facility

equipment: run-to-failure, preventive, predictive,

and proactive maintenance. The RCM approach

applies these differing maintenance strategies in

an optimal mix, to ensure that facility equipment

is maintained sufficient to accomplish the facility

mission without wasting maintenance labor.

This guide is meant to help maintenance

supervisors, managers, and technicians

organize and operate an efficient and effective

maintenance program in an environment of

maintenance budget cutbacks.

Approved for public release; distribution is unlimited.

USACERL Technical Report 99/41

April 1999


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USACERL TR 99/41 1

Executive Summary

Maintena nce ma na gement is a complicat ed business. Fa cility ma intenance

budgets are continually scrutinized by fiscal managers in a constant effort to

trim dollars. Maintena nce mana gers are under constant pressure to squeeze

every last bit of productivity out of every ma intena nce dollar.

This ma nua l outlines a comprehensive method of organizing a n efficient ma inte-

nance program through applying the concepts of Reliability Centered Mainte-na nce (RCM). Combining professional intuition a nd a rigorous sta tistical ap-

proach, RCM recognizes that there are different maintenance strategies followed

for different facility equipment: run-to-failure, preventive maintenance, predic-

tive maint enan ce, a nd proactive maint ena nce. The RCM approach a pplies these

differing maintenance strategies in an opt imal mix, to ensure that facility

equipment is being main ta ined sufficient to accomplish the fa cility mission wit h-

out wast ing inordinate amounts of maintenance labor baby sit t ing facility

equipment.

This ma nua l presents th e RCM approach for ma intenance supervisors, ma na g-

ers, and t echnicians t o use as a guide in organizing a nd operat ing a t ight , cost-

effective, lean and mean maintenance program in light of and in spite of the

continual cutbacks in maintenance budgets.


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Foreword

This study was conducted for the Facilities Management Division (EMD) at

Ma diga n Army Medical C enter (MAMC) und er P roject 4A162720D048, In dus-

tr ia l Opera tions P ollution Cont rol Technology.; Work Un it Y67, Relia bility C en-

tered Ma intena nce. The technical monitor wa s Micha el Car ico, MAMC-FMD .

The work was performed by the Industrial Operations Division (UL-I) of the

Utilit ies and Industrial Operat ions Laboratory (UL), U.S. Army ConstructionEngineering Research Laboratories (CERL). The CERL principal investigators

w ere Alan C ha lifaux a nd J earldine I . Northrup. Special credit is given to the

National Aeronautics and Space Administration (NASA) for the use of its docu-

ment Reli abil it y Center ed M ain tenance Gui de for F acil i t i es and Coll ater al

Equ i pment , December 1996 in the prepa ra tion of this report. Walt er J . Mikucki

is Chief, CEC ER-UL -I; Dr. J ohn Ba ndy is La bora tory Operat ions Chief, CECE R-

U L; a nd G a ry W. S chanche w a s t he responsible Technical D irector, CE CE R-TD.

The C ER L t echnica l editor w a s William J . Wolfe, Technical In forma tion Tea m.

Dr. Micha el J . OConnor is Director of US ACE RL.


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Contents

Executive Summary ......................................................................................................... 1

Foreword............................................................................................................................ 2

1 Introduction.................................................................................................................9

Background.........................................................................................................................9

Objectives..........................................................................................................................12

Approach...........................................................................................................................13

Scope ................................................................................................................................13

Units of Weight and Measure............................................................................................13

2 RCM Definition and Philosophy .............................................................................14

Definition ...........................................................................................................................14

RCM Analysis....................................................................................................................14

RCM Principles .................................................................................................................15

The RCM Process.............................................................................................................17

RCM Program Benefits .....................................................................................................19

Impact of RCM on a Facilitys Life Cycle ...........................................................................22

3 RCM Program Components....................................................................................24

Reactive Maintenance.......................................................................................................24

Preventive Maintenance (PM) ...........................................................................................25

Preventive Maintenance Criteria..................................................................................................26

Determining PM Task and Monitoring Periodicity. .......... .......... .......... .......... .......... .......... .......... ..26

Condition Monitoring (CM) ................................................................................................28

Proactive Maintenance...................................................................................................... 30

Specifications for New/Rebuilt Equipment ........................................................................31

Balance........................................................................................................................................32

Alignment.....................................................................................................................................34

Alignment Effects .........................................................................................................................35

Failed-Part Analysis .....................................................................................................................37

Root-Cause Failure Analysis (RCFA)...........................................................................................37

Reliability Engineering and Reliability Calculations......................................................................38

Rebuild Certification/Verification ..................................................................................................42

Age Exploration............................................................................................................................42

Recurrence Control......................................................................................................................43


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Facility Condition Assessment .....................................................................................................44

4 Use of Condition Monitoring (CM) Technologies .................................................45

Introduction .......................................................................................................................45

Spot Readings versus Continual Real-Time Data Collection ...........................................45

5 Vibration Monitoring and Analysis.........................................................................47

Theory, Applications, and Techniques...............................................................................47

Basic Vibration Theory .................................................................................................................47

Information Obtained through Vibration Monitoring......................................................................51

Detection Interval/Amount of Data Collected ...............................................................................52

Overall Vibration...........................................................................................................................52

Spectrum Analysis and Waveform Analysis .......... .......... .......... .......... ........... .......... .......... ..........52

Torsional Vibration........................................................................................................................53

Multi-Channel Vibration Analysis..................................................................................................53

Shock Pulse Analysis...................................................................................................................53

Vibration Sensor Mounting (Permanent Installations) .......... .......... .......... .......... .......... ......... .......53

Laser Shaft Alignment..................................................................................................................54

Limitations .........................................................................................................................54

Logistics ............................................................................................................................54

Equipment Required ....................................................................................................................54

Operators.....................................................................................................................................55

Available Training .........................................................................................................................55

Cost .............................................................................................................................................55

6 Thermography ..........................................................................................................56

Theory and Applications ...................................................................................................57

Limitations .........................................................................................................................58

Logistics ............................................................................................................................58


Operators.....................................................................................................................................58

Training Available ........... .......... .......... .......... ........... .......... .......... ........... .......... .......... ........... .......58

Cost .............................................................................................................................................59

7 Passive (Airborne) Ultrasonics...............................................................................60

Theory, Applications, and Techniques...............................................................................60

Basic Theory of Ultrasonic Detection ...........................................................................................61

Leak Detection.............................................................................................................................62

Electrical Problems ......................................................................................................................65

Mechanical Inspection .................................................................................................................66

Ultrasonic Translators...................................................................................................................69

Limitations .........................................................................................................................69


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Logistics ............................................................................................................................69


Operators.....................................................................................................................................70

Training Available/Required..........................................................................................................70

Cost .............................................................................................................................................70

8 Lubricant and Wear Particle Analysis....................................................................71

Purpose.............................................................................................................................71

Machine Mechanical Wear Condition...........................................................................................71

Lubricant Condition......................................................................................................................71

Lubricant Contamination..............................................................................................................72

Standard Analytical Tests ..................................................................................................72

Visual and Odor...........................................................................................................................72

Viscosity.......................................................................................................................................73

Water ...........................................................................................................................................73

Percent Solids/Water....................................................................................................................73

Total Acid Number (TAN)..............................................................................................................73

Total Base Number (TBN)............................................................................................................74

Spectrometric Metals ...................................................................................................................74

Infrared Spectroscopy ..................................................................................................................74

Analytical Ferrography .................................................................................................................74

Special Tests .....................................................................................................................75

Glycol Antifreeze..........................................................................................................................75

Karl Fischer Water .......................................................................................................................75

Application.........................................................................................................................76

Motors, Generators, Pumps, Blowers, Fan...................................................................................77

Gearboxes ...................................................................................................................................77

Chillers.........................................................................................................................................78

Diesel Engines.............................................................................................................................78

Compressors ...............................................................................................................................78

Hydraulic Systems .......................................................................................................................78

Large Reservoirs..........................................................................................................................78

Lubrication Analysis.....................................................................................................................78

Sampling......................................................................................................................................79

9 Electrical Condition Monitoring..............................................................................80

Techniques ........................................................................................................................ 80

Megohmmeter Testing..................................................................................................................81

High Potential Testing (HiPot).......................................................................................................81

Surge Testing ...............................................................................................................................81

Conductor Complex Impedance...................................................................................................82

Time Domain Reflectometry ........................................................................................................82


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Motor Current Spectrum Analysis (MCSA) ..................................................................................82

Radio Frequency (RF) Monitoring................................................................................................82

Power Factor and Harmonic Distortion ........................................................................................82

Motor Current Readings...............................................................................................................83

Airborne (Passive) Ultrasonics.....................................................................................................83

Transformer Oil Analysis .......... ........... .......... .......... .......... ........... .......... .......... .......... ............ ......83

Applications.......................................................................................................................83

Equipment to be Monitored..........................................................................................................83

Conditions Monitored...................................................................................................................84

Detection Interval .........................................................................................................................84

Accuracy ......................................................................................................................................84

Limitations....................................................................................................................................84

Logistics ............................................................................................................................85

Equipment Required ....................................................................................................................85Operations ...................................................................................................................................85

Training Available ........... .......... .......... ........... .......... .......... .......... ........... .......... .......... ........... .......85

Cost .............................................................................................................................................85

10 Non-Destructive Testing ..........................................................................................86

Techniques ........................................................................................................................ 86

Radiography.................................................................................................................................86

Ultrasonic Testing (Imaging).........................................................................................................87

Magnetic Particle Testing .............................................................................................................88

Dye Penetrant ..............................................................................................................................89

Hydrostatic Testing.......................................................................................................................89

Eddy Current Testing....................................................................................................................89

Location and Intervals.......................................................................................................90

Intervals .......................................................................................................................................90

Locations .....................................................................................................................................92

Applications.......................................................................................................................92

Limitations .........................................................................................................................93

11 Conclusions.............................................................................................................. 95

References.......................................................................................................................96

Distribution


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List of Figures and Tables

Figures

1 Bearing life scatter. ....................................................................................................11

2 RCM logic tree...........................................................................................................17

3 Sample RCM system data sheet. ..............................................................................19

4 Sample failure mode sheet. .......................................................................................20

5 Failure mode sheet for bearings. ...............................................................................216 Failure mode sheet for stator. ....................................................................................21

7 Maintenance cost trends under an RCM program. ...................................................22

8 Stages of life cycle cost commitment. .......................................................................23

9 Decrease in life of cylindrical roller bearings as a function of misalignment. ............36

10 Sample vibration data. ...............................................................................................50

11 Two plots of vibration data juxtaposed in the same graph.........................................51

Tables

1 Recommended coupled alignment tolerances (General Motors, 1993)....................35

2 Recommended maximum inspection intervals (API 570). ........................................90


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1 Introduction

Background

Maintena nce often ta kes a low priority in the overa ll opera t ing str at egy of a fa-

cility. Maintenance programs are managed and funded by people, and human

na tur e seems to abide the old tenet, If it a int broke, dont fix it . In fa cilities

ma na gement the definition of broke is extreme. B roke typically mea ns tha t a

piece of equipment ha s cat a str ophica lly fa iled (e.g., resulting in a pollution fine),or (at the very least ) tha t i t h as fa iled to the point tha t i t ha s become an an noy-

ing distur ba nce in the norma l daily opera tion of a fa cility.

While few people will argue against the need for performing regular mainte-

na nce, few fisca l ma na gers will make the fina ncial commitment to funding ma in-

tenan ce programs at a level that w ill keep a facility w ell mainta ined. Fiscal

ma na gers usually assign ma intenance program s a very low priority. Compared

to other facility departments, maintenance departments have no real product

a nd - a s such - produce no real income. Ma ny fiscal ma na gers view money spent

on maint enan ce a s money thrown dow n a bla ck hole. In spite of an y life-cycle

proofs to the contrary, fiscal managers look to cut maintenance budgets first

when any other fiscal need arises. Not until they see the bathroom floor flooded

with sewage or swelter in an office working at 85 F for hours do they realize

tha t something is broken and m a y need repair.

Fiscal managers continually put maintenance budgets under the closest scrutiny

in an effort to reduce dollars spent on maintenance, while expecting facility per-

forma nce to rema in on a consta nt par. This forces maint enan ce supervi-

sor/ma na gers to tr im essentia l (but less obvious) w ork from t heir da ily a gendas .The most common area trimmed is preventive maintenance, i.e., those mainte-

nance activities performed on facility equipment before equipment fa ilure. The

importance of preventive maintenance is less obvious to those people not inti-

ma tely fa miliar w ith facility equipment and operat ion. The consequences and

cost of not performing PM only become obvious when it is too late.

P reventive maint enance requires tha t m a intenance personnel pay regular visits

to observe the condition of facility equipment. The most ba sic tasks on th ese PM

visits is to take a look at the equipment to see if there are any telltale signs of


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fa ilure or imminent failure. Also, depending on the type of equipment, the ma in-

tena nce mechanic may h a ve a checklist of tas ks he ha s to perform (e.g., dra ining

a lit tle oil and visua lly checking for foreign ma tt er or discolora tion). In la rge fa-

cilities such a s MAMC, t he basic P M ta sk of w a lking out to a piece of equipment

a nd giving it a quick look over requires a grea t deal of time. It a lso requires tha t

this time be invested by a trained mechanic; untrained personnel are likely to

miss telltale signs of failure. MAMC-FMD w a nted to determine if th ere w as a ny

mea ns of a utoma ting th is basic P M inspection a ctivity. B y doing so they (or any

other fa cility) could free up skilled labor for more other ta sks, t hereby sq ueezing

more out of every ma intena nce dollar.

P reventive ma intena nce is typically performed based on the calenda r. Ma inte-

na nce personnel schedule visits to a part icula r piece of equipment bas ed on cer-

ta in t ime intervals having elapsed. While certainly better tha n no PM at a ll,calendar-based P M ma y result in too much t im ebeing spent on a piece of equip-

ment. Ea ch visit to a properly functioning piece of equipment ta kes time aw a y

from other ma intena nce a ctivities. Numerous visits to a piece of equipment wit h

no news to report can be regarded as wa sted maintena nce dollars. Ca lenda r-

based P M, while much preferable to no PM, is not the optima l wa y to run a P M

program.

Typically, th e next st ep up (from calend a r-ba sed P M) is performing P M bas ed on

equipment run t ime. This method is thought t o provide a bett er means of get-

ting a maintenance mechanic out to a piece of equipment just as it is beginning

to show signs of w ear. Int uitively, performing P M based on equipment run time

ma kes sense. Eq uipment does not ha ve to be checked repeat edly if it ha s not

been used. G enerally speaking, it is th e actua l operat ion of the equipment th at

wea rs it down, so it ma kes sense to check the equipment a fter it ha s run a suffi-

cient a mount of time to incur some wea r. (How ever, run t ime is not a proper cri-

terion for performing P M on all equipment. There ar e certa in pieces of equip-

ment t ha t r equire visual inspection w hen they ha ve notbeen ru n).

Ca lenda r-based P M a ssumes tha t fa ilure proba bilit ies can be determined stat is-tically for individual machines and components and parts can be replaced or ad-

justments can be performed in time to preclude fa ilure. This is not a lwa ys tr ue.

A common practice has been to replace bearings after a certain number of oper-

at ing hours, the assumption being that bearing failure rate increases with t ime

in service. Figure 1 shows tha t this assumpt ion is not alw a ys true. This figure

shows the failure distribution of a group of 30 identical 6309 deep groove ball

bearings installed in bearing test ma chines that run th e bear ings to failure. The

wide variation in bearing life is evident; there is no strong correlation between

bearing operating hours and bearing life.


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Figure 1. Bearing life scatter.

An improvement on ca lendar-ba sed P M (a nd t he tra ditional n ext step up) is con-

dition monitoring (CM). Here t he ma intena nce supervisor/ma na ger defines some

critical parameters that reflect the condition of a piece of machinery (e.g., am-

perage draw, bea ring temperat ure, sha ft vibra tion). Accepta ble limits ar e de-

fined for ea ch of these para meters. Some sort of da ta a cquisition devices are a t-

tached to measure the crit ical parameters and the stream of real-t ime data are

compar ed to the limits. Once th e limits a re exceeded, an a lar m is issued so tha t

a P M visit can be scheduled.

The original focus of the CE RL r esearch project w a s t o int egrat e condition moni-toring equipment with the computerized maintenance management system

(CMMS ) at MAMC. The intent w a s to collect rea l-time da ta a nd feed it int o the

MAMC CMMS . How ever, once MAMC a nd US ACE RL personnel took a ha rd

look at w ha t da ta could/should be collected a nd t ra nsferred to t he MAMC

CMMC, issues a rose tha t w ere broader tha n the technical issues of dat a acquisi-

tion:

1. Ha rdw a re and softw ar e technology exists to collect, store, and a na lyze gigabyt es

of dat a . B ut does it make sense to replace all other ma intenance activities wit h

da ta collection? (No, not necessa rily.)

2. Is the pa th of da ta intensive ma intenance ma na gement alwa ys the best? (No,

dat a collection, stora ge, and a na lysis systems cost money to insta ll, opera te, a nd

ma inta in. They are not a lwa ys the most effective maintena nce technique.)

3. Is there a downside to taking the man away from the machine, replacing the

regula r huma n visits w ith electronic CM? (Yes, th e huma n technician ma y see

something w rong tha t is not monitored by the C M in place.)


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Given questions (and answers) such as the above, the process of replacing PM

w ith CM is a complica ted one. Any decision to replace calendar-ba sed P M wit h

CM genera tes severa l orga niza tional/policy-relat ed questions rega rding a facil-

itys ma intena nce progra m. Technical an a lysis of CM techniqu es quickly be-

comes oversha dowed; w orkable solutions t o the la rger issues seem out of rea ch.

MAMC and CERL found a workable solution in the concept of Reliability Cen-

tered Maintenance (RCM) as developed by the National Aeronautics and Space

Admin ist ra tion (NASA). The RCM approa ch is a dyna mic, ongoing effort, re-

quiring constant review today of the maintenance practices and policies put in

place yesterda y. It s bas ic a im is to increa se the reliability of machinery/syst ems

using a combination of four maintenance techniques: reactive maintenance, pre-

ventive maintenance, predictive maintenance, and proactive maintenance.

This ma nual is meant to serve as a guide to MED COM ma intenance personnel

implementing a n RCM program . As such, is not a formula ic cookbook tha t can

be follow ed mindlessly. A good RCM progra m requires th a t ma intena nce super-

visor/ma nagers a nd sta ff be engaged in a nd consta ntly t hinking a bout t he value

of th eir present procedures. This ma nua l w ill hopefully provide some basic di-

rection t o the ME DC OM ma intena nce supervisors/in bringin g experience to bear

on the specific fa cility ent rust ed to their ca re.

Objectives

The Facilities Management Division (FMD) at Madigan Army Medical Center

(MAMC) realizes the importance of constant and regular maintenance, and has

pursued and practiced an aggressive maintenance program since its inception.

Its maintenance program was planned and implemented as the facility was be-

ing built , and the maintenance program has been managed and documented

from the beginning using commercial computerized maintenance management

syst em (CMMS ) softwa re. P reventive ma intena nce is a keyst one of the progra m

and accounts for about 60 percent of the maintenance dollars expended atMAMC . The objective of th is project wa s to help FMD d eterm ine if cert a in es-

sential, but rote, preventive maintenance efforts could be automated to free up

ma intena nce w orkers time.


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Approach

A literature search was done to uncover recent, relevant information in the area

of Reliability Centered Maintena nce. A ma nua l w a s prepar ed to serve a s a guide

to MED COM ma intenance personnel implementing a n R CM program.

Scope

Although this work was done specifically at the request of MAMC, it is impor-

ta nt t o note tha t RC M is a generally approa ch to equipment maintena nce appli-

cable at ma ny milita ry, industria l, or commercial facilities. How ever, RCM is not

a form ula ic cookbook meth odology tha t can be followed mindlessly. A good

RCM program requ ires tha t ma intena nce supervisor/ma na gers and st a ff be en-ga ged in and const a nt ly thinking about the value of their procedures. This man -

ual is meant to provide some basic direction to maintenance supervisors in

bringing their experience to bear on the specific facilities entrust ed to their ca re.

Units of Weight and Measure

U .S. sta nda rd units of measure ar e used throughout this report . A ta ble of con-

version fa ctors for Sta nda rd In tern at iona l (SI ) units is provided below.

SI conversion factors

1 in. = 2.54 cm

1 ft = 0.305 m

1 yd = 0.9144 m

1 sq in. = 6.452 cm2

1 sq ft = 0.093 m2

1 sq yd = 0.836 m2

1 gal = 3.78 L

1 lb = 0.453 kg1 oz = 28.35 kg

1 psi = 6.89 kPa

F = (C x 1.8) + 32


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2 RCM Definition and Philosophy

Definition

Reliability Centered Maintenance can be defined as an approach to mainte-

nance that combines reactive, preventive, predictive, and proactive maintenance

practices and strategies to maximize the life that a piece of equipment functions

in the required manner. RCM does this at minimal cost. I n effect, RCM str iv es

to cr eate the opti mal mi x of an i ntu it ive approach and a r i gor ous stati sti cal ap-pr oach to decid in g how t o mai ntai n facil i ty equi pment.

The key to developing an effective RCM program lies in effectively combining the

intuit ive a nd stat ist ical a pproaches. Int uit ion and sta t ist ics ea ch have strong

a nd w eak points . Int uition is an effective tool w hen a pplied judiciously; however,

if applied without serious reflection and review, it results in arbitrary, shoot-

from-the-hip solutions to problems. A rigorous sta tist ica l a pproach ha s its lim-

its , too. The first limit of th e st a tist ical a pproach is cost. D eveloping and /or

an alyzing a n a mount of dat a sufficient to provide a sta t ist ical ba sis is an expen-

sive ta sk. One may a lso fall into the ana lysis para lysis pit fa ll; the more one

delves into a problem the more da ta it seems is required to solve it . The second

limit of the sta tist ica l approa ch is a pplica bility. St a tist ics often do not tell the

w hole story. Da ta does not a lwa ys produce definite trends, since there ma y be

none.

RCM Analysis

RC M a na lysis ca refully considers the follow ing quest ions: Wha t does the syst em or equipment do?

Wha t fun ctiona l failures a re likely t o occur?

Wha t a re th e likely consequences of these functional fa ilures?

Wha t ca n be done to prevent these functional fa ilures?

To implement RC M, it is impera tive th at th e ma intena nce supervisor/ma na ger

and maintenance technician think about their facilities in terms of function.


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USACERL TR 99/41 15

That means thinking about facility equipment in terms of systems, subsystems,

components , and subcomponents. This terminology is used thr oughout this

manua l .

RCM Principles

The prima ry R CM principles ar e:

1. RCM is Concerned w it h M aint aini ng System Functionali ty. RCM seeks to pre-

serve system or equipment function, not just to maintain a piece of machinerys

operability for operabilitys sake. It should be noted tha t a common str at egy is to

ma inta in system function through equipment redundancy. Eq uipment redun-

dancy improves functional reliability but increases system life cycle cost (due to

th e increased first cost of inst a lling the redunda nt equipment ). The increa sed life

cycle cost of installing redundant equipment often eliminates redundancy as the

RC M method of providing syst em reliability.

2. RCM is System F ocused. It is more concerned with ma inta ining system function

th a n individua l component function. The question asked contin ua lly is: Ca n

th is system still provide its prima ry function if a component fails? (In t his exam-

ple, if the a nsw er is yes, then th e component is allowed t o run to failure.)

3. RCM is Reli abil i ty Centered. RCM treat s failure stat istics in an actuarial man-

ner. The relat ionship betw een operat ing a ge an d failures experienced is impor-

ta nt. RC M is not overly concerned with simple failure ra te; it seeks to know t he

conditional probability of failure at specific ages (the probability that failure will

occur in ea ch piece of equipment).

4. RCM Recognizes Design L im it ati ons. The objective of RCM is to ma inta in t he i n-

herent relia bility of syst em function. A ma intena nce program can only main ta in

the level of reliability inherent in the system design; no amount of maintenance

can overcome poor design. This makes it impera tive tha t ma intena nce knowl-

edge be fed back to designers to improve the next design. RCM recognizes tha tthere is a difference between perceived design life (what the designer thinks the

life of th e system is) a nd a ctua l design life. RCM explores this th rough the Age

Exploration (AE) process (see Section 3.4.6).

5. RCM is Dr iven by Safety Fi r st, then Economics. Sa fety must be mainta ined at

an y cost; it a lwa ys comes first in a ny ma intenance task. Hence, the cost of main-

ta ining safe working conditions is not calculated a s a cost of RC M. Once sa fety

on the job is ensured, RCM a ssigns costs t o all oth er a ctivities.


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16 USACERL TR 99/41

6. RCM Defines Fai l ur e as an Unsatisfactory Condi ti on. (Fa ilure is not an option.)

Here fa ilure is defined a s a loss of accepta ble product/serv ice qu a lity level, or

failure is defined a s a function not being ma inta ined.

7. RCM Tasks Must Produce a Tangible Resul t. The ta sks performed m ust be show nto reduce th e number of failures, or at lea st t o reduce th e dama ge due to fa ilure.

8. RCM Recogni zes Four M ai nt enance Categor ies and Uses a Logic Tree to Screen

M aint enance Tasks. This ensures consistency in determining how to perform

ma intena nce on a ll ty pes of facility equipment. Ea ch piece of equipment is as-

signed to one of four categories:

a . Run-to-Failure - Under an RCM program, run-to-failure is a conscious deci-

sion reached after analysis of what facility function(s) would be affected by

syst em fa ilure versus the (life cycle) cost of preventing failur e.

b. Ca lendar-Ba sed Ma intena nce (P M) - This is the most bas ic approa ch. It

schedules ta sks based on the time since that ta sk wa s last performed. It is

the type of maintenance most often performed in Preventive Maintenance

programs.

c. Condition Monitoring (CM) - This maintenance is performed based on predic-

tive testing and inspection. Real-time data is ga thered and ana lyzed as a

w ay to determine when a piece of equipment requires maintena nce.

d. Proactive Maintenance - Efforts in this area of a maintenance program are

aimed at applying the lessons learned from past maintenance experience to

futur e situa tions. This includes writin g bett er specifica tions, precision re-

build, fa iled part a na lysis, a nd root-cause failure ana lysis. Figure 2 shows

the logic tree used to determine what kind of maintenance should be applied

to ea ch piece of fa cility eq uipment.

9. RCM is a n Ongoing Pr ocess. This is one of the most importa nt cha ra cterist ics of

RCM. No ma intena nce procedures esca pe review. Ma intena nce personnel

ga th er dat a fr om the successes/fa ilures achieved a nd feed th is dat a ba ck to im-prove future ma intena nce procedures an d design of new syst ems. This feedba ck

loop is a n essentia l part of the RC M process. This includes: chan ging old equip-

ment specifications that have been proven inadequate or incorrect, rebuilding

w orn/fa iled equipment to better r esist fa ilure, performing fa iled-part a na lysis,

and performing root-cause failure analysis.


17/97

USACERL TR 99/41 17

Figure 2. RCM logic tree.

The RCM Process

RCM grew out of the aircraft ind ustr y in the lat e 1960s a nd 1970s. Since ma ny

a ircraft equipment fa ilures ha ve disastr ous consequences, the basic RCM process

developed wa s very forma l an d rigorous. The basic steps in developing a forma l

RCM a nalysis ar e :

Is there an effective PM taskthat will minimize functional

Will failure of the facility or equip-

Is the itemexpendable?

Can redesign solve the problempermanently and cost effectively?

Is there a Condition Monitoring (CM) technology(e.g. vibration testing, thermography) that will givesufficient warnin alert/alarm of impendin

Is CM cost- andpriority-justified?

Is establishing redundancy cost-and priority- justified?

Accept Risk ofFailureRun-to-Failure

Install RedundantUnitsProactive Maint.

Define PM taskand schedulePreventive Maint.

Define CM taskand schedulePredictive Maint.

Redesign

NO YES

YES NO NO YES

NO YES

NO YES

NO YES

NO YES

Source: NASA Facilities RCM Guide pp 2-3.


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18 USACERL TR 99/41

1. Define the major systems and component s. The user defines th e sys tem s. Where

systems are extremely complex and this complexity makes analysis difficult, the

user may opt to define subsystems as a means of organizing the problem into

manageable pieces.

2. For each system, define all functi ons of that system.

3. For each of those functi ons, defi ne the possible functi onal fai lur esthat could oc-

cur (i.e., wha t could go wrong t ha t w ould prevent t he syst em function from occur-

ring).

4. For each functi onal fai l ur e, defi ne al l possible fai lur e modes (i.e., each equip-

ment failure could be the cause of the functiona l failure).

5. For each fail ur e mode, stat e whether i t would be due to improper operati on, im -proper maintenance, or both.

Figure 3 presents a sample RCM ana lysis sheet (RCM Syst em Da ta Sheet) tha t

would be generat ed in applying a rigorous RC M a na lysis to a chilled w at er sys-

tem supplying computer equipment. Figur e 3 a na lyzes the system a nd the func-

t ions it performs. It a lso lists the functiona l fa ilures tha t could occur. Figure 4

presents one of the sample Failure Mode Sheets that would be produced de-

scribing how one of the components of the chilled wa ter sy stem could fa il; it lists

the fa ilure modes of the component. Note tha t F igure 4 is a breakout of one of

the 12 distinct failure modes listed in Figure 3. In a forma l and complete RCM

analysis, 11 other Failure Mode Sheets such as Figure 4 would be produced.

Figures 5 a nd 6 present t he root cause fa ilure an alys es of two sub-components of

the component a na lyzed in Figure 4 (motor = component, w hile stat or = sub-

component of motor, a nd r otor = sub-component of motor in th is exa mple).

Figures 5 an d 6 represent t w o of nine sh eets deta iling how/w hy sub-components

of the motor would fail.

Review of the sample RCM Information Sheet, the sample component failure

mode sheet, and the two sample Root Cause Failure sheets illustrates how ex-tensive, time-consuming, a nd expensive a forma l RCM process can become. Due

to the extensive up-front effort involved in producing a formal RCM analysis, it

is recommended that MEDCOM facilities only pursue this level of detail for

th ose syst ems wh ere the consequences of fa ilure are cat as tr ophic.


19/97

USACERL TR 99/41 19

Building Chilled Water System

Function Functional Failures Failure Modes

Maintenance (M)

or Operation (O)

Provide chilled

water at speci-

fied flow rate

and tempera-

ture

Total loss of flow Motor Failure

Pump Failure

Catastrophic Leak

Blocked Line

Valve out of position

Both

Both

M

M

Both

Insufficient flow Pump cavitation

Drive problem

Blocked line


Instrumentation

O

M

M

Both

M

Chilled water tem-

perature too high

Chiller fatigue

Low refrigerant

Fouled heat exchanger

Instrumentation problem

Cooling tower problem


Both

M

M

M

M

Both

Figure 3. Sample RCM system data sheet.

RCM Program Benefits

1. Reliabil ity. The prima ry goal of RC M is to improve equipment relia bility. This

improvement comes through constant reappraisal of the existing maintenance

program and improved communication between maintenance supervi-

sors/ma na gers, ma intena nce mechanics, facility pla nners, building designers,

a nd equipment ma nufa cturers. This improved commun icat ion creates a feed-

back loop from the maintenance mechanic in the field all the way to the equip-

ment manufacturers.

2. Cost. Due to th e initia l investm ent required to obta in the technological tools,

tra ining, equipment condition ba selines, a new RCM program typica lly results in

a short-term increa se in main tena nce costs (see Figure 7). The increa se is rela-tively short-lived. The cost of rea ctive maint ena nce decrea ses as fa ilures ar e pre-

vented and preventive maintenance tasks are replaced by condition monitoring.

The net effect is a reduction of reactive maintenance and a reduction in total

ma intena nce costs. As a by-product, energy savin gs ar e often rea lized from the

use of the CM techniques tha t a re pa rt of any RC M program.


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20 USACERL TR 99/41

Electric Motor # 123456

Function: To provide sufficient power to pump 300 gpm chilled water

Component Functional Failure Failure Mode Source of Failure

Insulation Failure

Stator Motor will not turn

Open winding

Insulation contamination

Excessive current

Voltage spike

Phase imbalance

Excessive temperature

Motor will not turn Burnt rotor

Rotor

Wrong speed Excessive vibration

Insulation contamination

Excessive current


Imbalance

Bearings Motor will not turn Bearing seized

Fatigue

Improper lubrication

Misalignment

Imbalance

Electrical pitting

Contamination

Excessive Thrust


Motor will not turn Bearing seized

Motor Controller

Wrong speed VFD malfunction

Mainline contact failure

Control circuit failure

Loss of electrical power

Cabling failure

Overloads/fuse Motor will not turn Device burned out

Excessive current

Excessive torquePoor connection

Shaft/coupling Pump will not turn Shaft/coupling sheared

Fatigue

Misalignment

Excessive torque

Figure 4. Sample failure mode sheet.

3. Scheduling. The ability of a condition monitoring progra m to forecas t cert a in

maintenance activities provides time for planning, obtaining replacement parts,

making the necessary logistical arrangements (i.e., notifying occupants of equip-

ment dow ntime) before the ma intena nce is executed. CM reduces the unneces-sary maintenance performed by a calendar-based preventive maintenance pro-

gram, which tends to err consistently on the safe side in determining time

intervals betw een ma intenance ta sks.

4. Equi pment / Part s Replacement. A principal advant a ge of RCM is that it obta ins

th e ma ximum use from the equipment. With RCM, equipment replacement is

ba sed on equipment condition, not on the calenda r. This condition based a p-

proach to maint ena nce extends the life of the fa cility a nd its equ ipment.


21/97

USACERL TR 99/41 21

Root Cause of Failure Mode for Electric Motor Bearings

Failure Mode Mechanism Reason Root Cause

Seal FailureContamination

Cleanliness

Oil LeakInsufficient

Procedural

Excessive Procedural

Lubrication

Wrong Type Procedural

InherentMetallurgical

Excessive Temp.

Imbalance

Misalignment

Fit-up

Fatigue

Excessive Load

Application

Installation Procedural

Contamination See lubrication

Storage Procedural

Insulation

Bearing Seized

(This includes seals,

shields, lubrication

system, and lock nut

Surface Distress

ElectricalWelding

Figure 5. Failure mode sheet for bearings.

Root Cause Failure Mode for Electric Motors (Electrical)

Age InherentOxidation

Environment Chemical attack

Power quality

Phase imbalance

Short on/off cycle

Low voltage

Overheating Excessive current

Overloaded

Moisture

Improper lubeContamination Environment

Process related

Lack of winding

support

Phase imbalance

Imbalance

Misalignment

Stator Insulation resistance

reading zero ohms

Fatigue Excessive vibration

Resonance

Figure 6. Failure mode sheet for stator.


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22 USACERL TR 99/41

Figure 7. Maintenance cost trends under an RCM program.

5. Effi ciency/ Productivi ty. Sa fety is th e prima ry concern of RCM. The second most

import a nt concern is cost -effectiveness. Cost -effectiveness ta kes int o consider a -

tion the priority or mission criticality and then matches a level of cost appropri-

a te to tha t priority. The flexibility of the RCM a pproa ch to main tena nce ensures

th a t the proper ty pe of ma intena nce is performed wh en it is needed. Ma inte-

na nce tha t is n ot cost-effective is identified a nd n ot performed.

In summ a ry, the m ulti-faceted RC M a pproach promotes t he most efficient use of

resources. The equipment is maint a ined as required by its cha ra cteristics and

the consequences of its failures.

Impact of RCM on a Facilitys Life Cycle

RCM must be a consideration throughout the life cycle of a facility if it is toa chieve ma ximum effectiven ess. The four recognized ma jor phas es of a facility s

life cycle a re:

Planning

Design

Construction

Opera tions and Ma intenance.


23/97

USACERL TR 99/41 23

Figu re 8 show s t ha t pla nnin g (including conceptua l design ) fixes 2/3 (66.7 per-

cent) of a fa cility s life cycle cost . The subsequ ent design pha ses fix a bout a n-

other 30 percent of the life-cycle cost, leaving only about 4 percent fixable in the

lat er pha ses. Thus, the decision to institut e RCM a t a facility, including condi-

tion monitoring, w ill have a ma jor impa ct on the life-cycle cost of tha t RCM pro-

gra m. This decision is best ma de during th e plannin g phase. As RCM decisions

a re ma de later in th e life cycle, it becomes more difficult to achieve the maxi-

mum possible benefit from the RC M program .

Murphys La w being wha t it is, i t is ra re tha t a complete a nd w ell-plan ned RCM

program is instit uted at th e plannin g sta ge of a project. How ever, ma intena nce

personnel need not despair. Even th ough maint ena nce is a relat ively sma ll por-

tion of the overall life-cycle cost, a balanced RCM program is still capable of

achieving savings of 30 to 50 percent in a facilitys annual maintenance budget.While these opera tions a nd ma intena nce (O&M) sa vings ma y not be t he ma jority

of the fa cilitys life cycle cost, th ey a re still a significa nt portion of th e yea rly op-

erating costs of a facility, and would be well appreciated by any fiscal manager

looking to cut operating costs.

Figure 8. Stages of life cycle cost commitment.


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24 USACERL TR 99/41

3 RCM Program Components

An RCM program implements reactive maintenance, preventive maintenance,

condition monitoring, a nd proactive ma intena nce in a n optima l mix. It a lso

combines the intuitive approach with the statistical approach in determining

equipment condition.

Reactive Maintenance

Reactive ma intenance is referred to by ma ny different na mes: breakdown m ain-

ten a nce, repa ir, fix-w hen-fa il, a nd run-to-fa ilure (RTF) ma inten a nce. When ap-

plying this maintenance strategy, a piece of equipment receives maintenance

(e.g., repa ir or replacement) only w hen t he deteriora tion of th e equipments con-

dition ca uses a functiona l fa ilure. The stra tegy of rea ctive maint ena nce ass umes

tha t fa ilure is equa lly likely to occur in a ny par t, component, or system. Thus,

this assumption precludes identifying a specific group of repair parts as being

more necessary or desirable than others.

The major downside of reactive maintenance is unexpected and unscheduled

equipment dow ntime. If a piece of equipment fails and repair part s are not

a va ilable, delays ensue w hile the part s are ordered and delivered. If these part s

are urgently required, a premium for expedited delivery must be paid. If the

failed par t is no longer ma nufa ctured or stocked, more dra stic an d expensive ac-

tions are required to restore equipment function. Ca nniba lizat ion of like equip-

ment or ra pid prototyping technology ma y sa t isfy a temporar y need but a t s ub-

sta ntia l cost. Also, there is no ability to influence when fa ilures occur beca use no

(or minima l) a ction is ta ken to control or prevent th em. When th is is the sole

type of maintenance practiced, both labor and materials are used inefficiently.La bor resources ar e thrown a t w hat ever breakdow n is most pressing. In the

event th a t severa l breakdown s occur simulta neously, it is necessa ry t o pra ctice a

kind of ma intenance triage in a n a t tempt t o bring a ll the breakdow ns under con-

tr ol. Ma intena nce labor is used to sta bilize (but not necessa rily fix) th e most

urgent repair situation, then it is moved on to the next most urgent situation,

etc. Repla cement pa rt s must be consta nt ly stocked at h igh levels, since their use

cannot be ant icipat ed. This incurs high car rying char ges and is not an efficient

wa y to run a storeroom.


25/97

USACERL TR 99/41 25

A purely reactive main tena nce program ign ores the ma ny opportunit ies to influ-

ence equipmen t surviva bility. H owever, it can be effective if used selectively a nd

performed a s a conscious decision based on the results of an R CM a na lysis. This

RCM analysis compares the risk of failure with the cost of the maintenance re-

quired to mitigate that risk and cost of failure (again, refer to Figure 2, RCM

Logic Tree).

Examples of equipment that may be reactively maintained (run-to-failure) are:

non-critical electrical motors less than 7.5 HP, restroom exhaust fans, water

heaters serving bathrooms, lamps in areas where a few burned out lamps will

not pose any safety hazard or affect the use of the area (e.g., halls, cafeterias,

lounges).

Preventive Maintenance (PM)

PM consists of regularly scheduled inspection, adjustments, cleaning, lubrica-

tion, parts replacement, calibration, and repair of components and equipment.

P M is a lso referred to as time-driven or calenda r-bas ed maint ena nce. It is per-

formed w ithout rega rd to equipment condition or (possibly) degree of use.

PM schedules periodic inspection and maintenance at pre-defined intervals (in-

terva ls based on time, operat ing hours, or cycles) in a n a tt empt to reduce equip-

ment fa ilures for susceptible equipment. Depending on the interva ls set, P M ca n

result in a significant increase in inspections and routine maintenance; however,

it should also reduce the seriousness and frequency of unplanned machine fail-

ures for components w ith defined, age-relat ed wea r pat terns .

Tra dit iona l PM is keyed to failure rates a nd t imes betw een failures. I t assumes

that these variables can be determined stat ist ically, and that one can therefore

replace a pa rt t ha t is due for failure shortly before it fails. The ava ilability of

statistical failure information tends to lead to fixed schedules for the overhaul of

equipment or the replacement of par ts subject to wea r. P M is based on the as-sumption that the overhaul of machinery by disassembly and replacement of

parts restores the machine to a like-new condition with no harmful side effects.

In addition, this renewal task is based on the perception that new components

a re less likely to fail th a n old components of th e sam e design.

Failure rate or its reciprocal, Mean-Time-Between-Failure (MTBF), is often used

as a guide to esta blishing the interval at wh ich the ma intenance tasks should be

performed. The major wea kness in using these meas urements to esta blish task

periodicit ies is that failure rate data determines only the average failure rate.


26/97

26 USACERL TR 99/41

The reality is tha t fa ilures a re equally likely to occur a t ra ndom t imes a nd w ith a

frequency unrelat ed to th e avera ge failure rat e. Thus, selecting a specific time to

conduct periodic maintenance for a component with a random failure pattern is

difficult a t best.

As stated in Section 2.4, RCM grew out of the aircraft industry in the late 1960s

a nd ear ly1970s. This early R CM a pproach is documented in Reli abil i ty Center ed

Maintenance (Nowlan and Heap 1978), which demonstrated that a strong corre-

lat ion between age a nd failure did not exist and that the basic premise of time-

based ma intenance wa s false for t he ma jority of equipment.

In summary, PM can be costly and ineffective when it is the sole type of mainte-

na nce practiced.

Preventive Maintenance Criteria

Preventive maintenance criteria should reflect the age-reliability characteristics

of the equipment based on the equipment history. Eq uipment tha t has under-

gone P M should show a st rong failure rat e versus a ge correla tion. There should

be definite data (or at least definite conclusive in-house experience) to bear out

tha t a piece of equipment w ill fa il a t a certain a ge.

However, whether a piece of equipment should receive PM is not necessarily a

function of tha t equipments mission criticality. In selecting equipment th at

should receive P M, th e ma intena nce supervisor/ma na ger should use the process

shown in Figure 2. The selection process guides maintenance supervi-

sor/ma na ger in selecting t he ma intena nce stra tegy a ppropriat e to each piece of

equipment.

Determining PM Task and Monitoring Periodicity

This section offers suggestions for selecting equipment monitoring periodicities,

i.e., determining the time intervals between PM visits.

Although many ways have been proposed for determining the correct frequency

of preventive maintenance tasks, none are valid unless the in-service age-

reliability (i.e., failure rate versus age) characteristics of the system or are

known. This information is not normally available and must always be collected

for new systems and equipment. Condition monitoring techniques (e.g., taking

real-time data to determine the health of a piece of machinery) can be used to

help determine equipment condition vs. a ge.


27/97

USACERL TR 99/41 27

Careful analysis of similar kinds of hardware in industry has shown that , over-

all , more than 90 percent of the hardware analyzed showed no adverse age-

reliability relationship. This does not mean that individual parts do not wear;

they do. It means that the ages at fa ilure are distributed in such a way that

there is no value in imposing a preventive maintenance task. In fact, in a some

cases, imposing an arbitrary preventive task increases the average failure rate

because some PM tasks are actually detrimental to machines (e.g., machine dis-

a ssembly, overgrea sing). Of course, one w ould hope that d etriment a l tasks a re

never intentionally assigned, but overzealous PM technicians have sometimes

done this.

The Mean Time Between Failures (MTBF) is often used as the initial basis for

determining PM interval. This approach is incorrect in that it does not provide

any information about the effect of i ncr easi ng age on r eli abil it y. It provides onlythe a vera ge ag e (for a group of components ) a t w hich failure occurs, not t he most

likely age (for a specific component). In many cases a Weibull distribution, as

used by the bearing industry to specify bearing life, will provide more accurate

informat ion on the distribut ion of failures.

If good information on the effect of age on reliability is lacking, the best thing

tha t ca n be done is t o monitor the equipment condition (condition m onitoring).

This is explained in the next section.

The goals of a P M visit to a piece of equipm ent a re: (1) to determ ine equipment

condition, and (2) to develop a trend to forecast future equipment condition. The

follow ing t echniq ues a re recommended for sett ing init ial periodicity:

1. Anti cipati ng Fail ur e fr om Experi ence. For some equipment, failure history an d

personal experience provides an intuitive feel for when to expect equipment fail-

ure. In these cases, failure is time relat ed. Set monitoring so tha t t here are at

least t hree monitoring P M visits before the ant icipat ed onset of failur es. These

th ree visits will give the ma intena nce technician enough of a look a t t he piece of

equipment to become fa miliar w ith it. In most ca ses it is prudent t o shorten the

monitoring interval as the wear-out age is approached:

2. Fail ur e Di str ibuti on Statistics. In using sta tist ics to determine the basis for se-

lecting periodicities, the distribution and probability of failure should be known.

Weibull distributions can provide informa tion on the probability of an equipment

exceeding some life. For example, bearings are normally specified by their B 10

life; i.e., the number of revolutions that will be exceeded by 90 percent of the

bearings. Depending on the criticality of the equipment, an initial periodicity is

recommended that allows a minimum of three monitoring samples prior to the


28/97

28 USACERL TR 99/41

B 10 life or, in less severe cases, prior t o the MTB F point. In more critical cases, a

B 2 life ca n be ca lcula ted a nd t he monitoring interva l ca n be adjust ed accordingly.

3. Lack of I nformati on or Conservati ve Appr oach. The most common practice in

the industry is to monitor the equipment biweekly or monthly due to lack of in-formation and poor monitoring techniques. This often results in excessive moni-

toring. In th ese cases, significant increases in the monitoring interval ma y be

made without adverse impacting equipment reliability.

When indications of impending failure become apparent through trending or

other predictive analysis methods, the monitoring interval should be reduced

and additional analysis should be done to gain more detailed information on the

condition of th e equipment.

Condition Monitoring (CM)

Condition monitoring, also known as predictive maintenance, uses primarily

nonintrusive testing techniques, visual inspection, and performance data to as-

sess machinery condit ion. It replaces arbitrarily t imed maintenance tasks with

maintenance scheduled only when warranted by equipment condition. Continu-

ing analysis of equipment condition monitoring data allows planning and sched-

uling of ma intena nce or repairs in a dva nce of cata str ophic a nd functiona l failure.

The CM da ta collected is used in one of the follow ing w a ys t o determine the con-

dition of th e equipment a nd t o identify t he precursors of failure:

Trend A nal ysis. Reviewing da ta to see if a ma chine is on a n obvious a nd im-

mediate down w ard slide tow ard fa ilure.

Patt er n Recogni ti on. Looking at the data and realizing the causal relat ion-

ship betw een certa in events a nd ma chine failure. For exa mple, noticing tha t

aft er machine xis used in a certa in production run , component axfails due to

stresses unique to that run.

Tests agai nst L im i ts and Ran ges. Sett ing ala rm limits (based on professional

intuit ion) and seeing if th ey a re exceeded.

Stat i stical Pr ocess Anal ysi s. I f published failure dat a on a certa in ma-

chine/component exist s, compar ing fa ilure da ta collected on site w ith t he

published da ta to verify/disprove tha t you can use th a t published dat a .


29/97

USACERL TR 99/41 29

For tr ending purposes, a minimum of three monitoring points before failure ma y

rea sonably be expected a re recommended. Three dat a points a llow one to deter-

mine w hether eq uipment condition depreciat es linearly.

CM does not lend itself to all types of equipment or possible failure modes and

therefore should n ot be th e sol e type of m ai nt enance pr acti ced. Chapters 5-10

give informa tion on specific CM t echnologies and inst rument a tion. For example,

to obtain the total picture of a chilled watered system, a CM effort would have to

collect the following data (one can see how extensive - and costly - this approach

could become):

1. Fl ow Rates. Ch iller wa ter flow w ould be mea sured using precision, nonintrusive

flow detectors.

2. Temperature. Differential tempera ture would be measured to determine heat

tr a nsfer coefficients a nd t o indicat e possible tube fouling.

3. Pressure. Differentia l pressures across the pump w ould be mea sured to deter-

mine pump performance, and differential pressures across the chiller evaporator

and condenser sections should be measured to determine the condition of the

chiller t ubes (i.e., w hether th ey w ere fouling).

4. Electrical. Motor power consumpt ion w ould be used to assess th e condition of the

motor w indings.

5. U l tr asoni c Testi ng. P ipe w a ll thickness would be meas ured to determine erosion

and corrosion degradation.

6. Vibration. Vibra tion monitoring would be used to assess th e condition of rota ting

equipment (such a s pumps a nd m otors). Additiona lly, str uctura l problems can be

identified through resonance and model testing.

7. Lubri cant Analysis. Oil condition and w ear par ticle ana lysis would be used to

identify problems with the lubricant, and to correlate those problems with vibra-

tion when w ear pa rt icle concentra tions exceed pre-esta blished limits.

8. Fi ber Opti cs. Fiber optic inspections would be used to determine component

w ear, tube fouling, etc.

9. Thermography. Thermogra phy scans check motor control centers a nd electr ica l

distr ibution junction boxes for high tempera tur e conditions. High t empera tur e is

indicative of loose connections, shorts , or fa iling conductor insula tion. P iping in-

sula tion should be checked for porosities. Her e, high temperat ures are indica -

tive of failed/fa iling a rea s in t he pipe insula tion.


30/97

30 USACERL TR 99/41

10.Eddy Curr ent. Eddy current testing is used to determine and loca te leakin g

tubes.

11.Ai rborne Ul tr asoni cs. Airborne ultr a sonics indica te air leakin g from contr ol sys-

tem piping a nd compressors.

Proactive Maintenance

A proactive maint enan ce program is the capstone of RCM philosophy. It provides

a logical culmination to the other types of maintenance described above (reac-

tive, preventive, and predictive). Proactive maintenance improves maintenance

through better design, installation, maintenance procedures, workmanship, and

scheduling.

P roactive ma intenance is char a cterized by the following at t itudes:

Ma inta ining a feedback loop from ma intena nce technician s to building a rchi-

tects, engineers, and designers, in a n a t tempt t o ensure tha t design mista kes

ma de in the past a re not repeated in future designs.

Viewing ma intena nce an d su pporting functions from a life-cycle perspective.

This perspective will often show tha t cuttin g ma intena nce a ctivities to sa ve

money in the sh ort term often costs more money in the long term.

Constantly re-evaluating established maintenance procedures in an effort toimprove them a nd ensure tha t th ey ar e being applied in the proper mix.

Proactive maintenance uses the following basic techniques to extend machinery

life:

proper insta llat ion and precision r ebuild

fa iled-parts an alysis

root-cause fa ilure ana lysis

reliability engineering

rebuild certificat ion/verificat ion

a ge explora tion

recurren ce contr ol.

These proactive main tena nce stra tegies a re explain ed in th e follow ing sections.


31/97

USACERL TR 99/41 31

Specifications for New/Rebuilt Equipment

Equipment requires proper installation to control life cycle costs and manmade

reliability. Poor installation often results in problems routinely faced by both

ma intena nce personnel a nd operat ors. Rotor ba lan ce a nd alignment , tw o com-

mon rework items, are often poorly performed or neglected during initial instal-

lat ion. Adopting a nd enforcing of precision sta nda rds can more tha n double the

life of a ma chine. For exam ple, the contr a ct s pecificat ion for leveling equipment

being installed should include a maximum acceptable slope of the base and the

fra me; e.g., a ma ximum slope of 0.001 in./ft. The specifica tion a lso should include

the type and accuracy of the instrument used for measuring the slope; e.g., a 12-

in. machin ists level gra dua ted to 0.0002 in./ft. After th e criteria h a ve been in-

cluded in t he contr a ct specificat ions, t he inst alla tion should be checked to ensure

tha t t he mecha nic has complied w ith t he specifica tion.

Eq uipment is often procured using ina dequa te specificat ions. Exist ing sta n-

dards, often 25 to 30 years old, do not reflect current changes in building tech-

nology, but usually address only general or minimal performance criteria. Addi-

tionally, the life cycle costs and failure histories of families of equipment are

rarely documented for purchasing and contract personnel who (by regulation)

must procure conforming products solely ba sed on initia l least cost.

To solve this pr oblem, design engineers m ust w rit e proper specificat ions, re-

search (and test if possible) the equipment of different vendors, and document

problems. These specifications should include, as a minimum, vibrat i on, al ign-

ment, and balan cin g cr it er ia. If these criteria a re included in the pla ns and

specifications for new construction or major building renovations, they become

par t of th e contr a ctual documents t ha t the contra ctor must fulfill. This gives the

building ow ner some solid recourse to make th e contra ctor prove tha t t he equip-

ment in the building is oper ati ng pr operl y a nd not just tha t it is operat ing. At

the t urnover of the facility to th e owner/tena nt, if t he contr a ctor does not fulfill

the requirements listed in these vibration, alignment, and balancing criteria

listed in t he specificat ions, t hen t he owner ca n r efuse accepta nce of th e buildingunt il the contra ctor does so. For exa mple, rota ting equipment ha s some sort of

sha ft(s) and bearings. Vibrat ion ana lysis can determine if the shafts a re aligned

an d if the bearings have been dama ged in shipment or insta llat ion. Finding

alignment and bearing problems before turnover is preferable to the more usual

scenario, in which a certain number of bearings fail after turnover well before

an y normal w ear or operat ion would have caused them to fail .

Local companies that specialize in vibration measurement (and work with vibra-

tion daily) and shaft alignment should be consulted to assist in writing the sec-


32/97

32 USACERL TR 99/41

tions of the contra ct specifica tions tha t deal w ith vibra tion/a lignment. Tha t w a y,

the designer ca n be sure th at a ll pertinent vibrat ion/a lignment criteria a re in-

cluded.

Performance test ing is another proactive maintenance strategy that must be

conducted. P erforma nce testing can occur in severa l places: (1) in the fa ctory

prior to shipment, (2) after the equipment is installed and immediately prior to

acceptance, and (3) at the beginning of the daily operation of the equipment, to

esta blish a performa nce baseline as t he equipment begins operat ion.

Balance

B earings a re the ma chine components th at support a nd t ra nsfer t he forces from

the rota ting element to the ma chine fra me. The fact tha t only 10 to 20 percent ofrolling element bearings achieve their design life results in the perception that

bearing s inherently pose a r eliability problem. One of the leading causes of pre-

ma tu re rolling element /bea ring fa ilure is pa ra sitic loa d due to excessive forces

imposed by imbala nce an d misalignment. P ar asit ic loa ds result in increased dy-

na mic loads on the bear ings. The design formula s (SK F, 1973) used to ca lculat e

th eoretical rolling element /bea rin g life a re:

a . for ball B earings:

L10

Life Hours =(16,667/RPM) x (C/P)3

b. for roller Bearings:

L10

Life Hours = (16,667/RPM) x (C/P)10/9

Where:

L10

= the number of hours 90 percent of a group of bear ings should a t ta in or

exceeded under a const ant load (P ) prior t o fatigue fa ilure

C = the bear ing load tha t w i ll result in a l ife of 1 mill ion revolut ions

P = t h e a ct u a l bea r in g loa d , s t a t ic a n d dy n a m ic.

As shown, bearing life is inversely proportional to speed and more significantly,

invers ely proport iona l to the th ird power of load for ba ll an d to th e 10/9 power

for roller bear ings.


33/97

USACERL TR 99/41 33

Balance Calculations

Precision balance of motor rotors, pump impellers, and fans is one of the most

critical and cost effective techniques for achieving increased bearing life and re-

sulta nt equipment relia bility. It is not usually sufficient to perform a single

plan e bala nce of a rotor t o a level of 0.10 in/sec, nor is it sufficient t o bala nce a

rotor until it a chieves seemingly low vibra tion levels. When a na lyzing a piece of

rotating equipment, the vibration technician should take readings in the vertical

an d horizonta l planes, and at a ll bea ring points. He should also ta ke axia l

rea dings. P recision bala nce methods should also include the ca lculat ion of re-

sidual imbalance.* Residua l imba lan ce calculates the imba lan ce left in the

piece of equipmen t once th e ba la ncing procedure ha s been executed. Nothin g is

perfect. A piece of rota ting equipment w ill never be bala nced so tha t a ll of the

ma ss is distributed evenly around t he axis of rota tion. As the piece of equipmentrotates the additional force imposed by this extra piece of mass will be slung

ar ound the a xis.

The following equa tion can be used t o calculat e residual im bala nce:

MVe

VrU Eq. 1

Where:

U = am ount of residual imbala nce

Vr = actua l imbala nce

Ve = tria l mass imbala nce

M = t r i a l mass .

This equa tion can be expressed as:

EffectWeightTrialBalance)After(AmplitudeRadius)Weight(TrialWeight)(TrialImbalanceResidual =

*Note: The following equations and discussion of permissible imbalance is based on ISO 1940/1, Mechanical Vi-

bration-Balance Quality Required of Rigid Motors(1986).


34/97

34 USACERL TR 99/41

P ermissible imbala nce is relat ed to equipment type a nd rotor ma ss. In general,

the grea ter t he rotor m ass, t he greater the permissible imbala nce.

Effect of Imbalance

As discussed earlier, imbalance forces make a major contribution to decreased

bearing life. For example, consider a rotor turn ing a t 3600 RP M w ith 1 oz. of un-

bala nce on a 12-in. ra dius.

Ca lcula ting t he amount of centrifuga l force due to imba lan ce:

22

21020

2mrf

g

fmrmrmAF .

)(====

Where:

F = Force

m = imba la nce (lb)

A = a ccelera tion (in.2/sec)

r = ra dius of imba la nce (in.)

= rota t ional veloci ty (radians/sec)

f = rot a t iona l freq uency (H z)

g = 386.4 in./sec.

S ubs tit ut ing 1 oz. (1/16 lb.), 12 in., 3600 RP M (60 Hz ) yield s:

( ) ( ) ( ) lb27560121610.102F 2 ==

Thus, 1 oz. of imbalance on a 12-in. radius at 3600 RPM creates an effective cen-

tr ifugal force of 275 lb. Now calcula te th e effect of this w eight on bearing life.

Su ppose tha t th e bea ring s wer e designed t o support a 1000-lb. rotor. The ca lcu-

lat ed bearing life is less th a n 50 percent of th e design life:

( ) ( )[ ]

LifeLDesign

LifeDesignLifeLActual

10

310

480

27510001000

.=

+=

Alignment

The forces of vibration from misalignment also cause gradual deterioration of

seals, couplings, drive windings, and other rotating elements with close toler-

a nces. Ma intena nce technicians should use precision equipment a nd alignment


35/97

USACERL TR 99/41 35

methods, (e.g., reverse dial or laser alignment system) to bring alignment toler-

a nces w ithin precision stan da rds. Contr a ry to popular belief, both laser a lign-

ment and reverse dial indicator equipment offer equal levels of precision; how-

ever, laser alignment is considerably easier to learn and much easier to execute

in t he field.

In a ddition to the a lignment specificat ions, Ta ble 1 conta ins th e add itional t oler-

a nce recommenda tions.

Alignment Effects

Based on data from a petrochemical industry survey, precision alignment prac-

tices achieve:

a vera ge bearing life increases by a factor of 8.0

ma intena nce costs decrease by 7 percent

ma chinery a va ilability increases by 12 percent.

Table 1. Recommended coupled alignment tolerances (General Motors, 1993).

Tolerance

Coupling Type

Maximum Speed

(RPM)

Horizontal & Vertical

Parallel Offset (IN).

Angularity (Inch/10

inch of Coupling Dia.)

600 0.005 0.010

900 0.0053 0.007

1200 0.0025 0.005

1800 0.002 0.003

3600 0.001 0.002

Short Coupling

7200 0.0005 0.001

600 0.005 N/A

900 0.0018 N/A

1200 0.0012 N/A

1800 0.0009 N/A

3600 0.0006 N/A

Coupling with

Spacer (Meas-

urement is per

inch of spacer

length)

7200 0.00015 N/ASource: NASA RCM Guide, page 3-14.


36/97

36 USACERL TR 99/41

Figure 9 presents t he effect of misalignment on bearing life of a cylindrica l roller

bearing and shows the drastic decrease in component life caused by a few min-

utes (i.e., micro-degrees) misalignment.

Misalignment can also cause a significant increase in the cost of energy con-

sumption. Consider t he case below. After r e-alignm ent, t he current dra w of 460V

motor decrea sed from 25 to 23 Amp. Assume th e motor ha s a pow er factor of

0.90, given in the relation:

( ) ( )1000

3 21

PFAVkW

=

Where:

kW = Change in power consumpt ion , k ilowa t t s

A = C ha nge in Ampera ge dra w

V = Ra t ed Volt a ge

P F = P ow er Fa ct or.

Figure 9. Decrease in life of cylindrical roller bearings as a function of

misalignment.


37/97

USACERL TR 99/41 37

Substituting the values given above:

( )( )( )

1.43kW

1000

0.90V460Amps21.732kW

=

=

Assuming the motor runs an average of 7,000 hours per year, and the average

cost of pow er d ur ing t he y ea r is $0.07/kW-hour :

( )( ) ar$700.00/ye70000.071.43trainmachinepersaved$ ==

Failed-Part Analysis

This proactive process involves visually inspecting failed parts after removal to

identify the ca use(s) of their failure. More deta iled technical an a lysis ma y be

conducted w hen necessa ry t o determin e

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Usacerl Tr-99 41 Rcm Guide 1999

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