mech826-week01-maintenancestrategies

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September 13, 2010 Page 1

Machine Condition Monitoring

and

Fault Diagnostics

Chris K Mechefske


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Introduction to Machine Condition Monitoring

and Condition Based Maintenance

Basics of Mechanical Vibrations

Vibration Transducers

Vibration Signal Measurement and Display

Machine Vibration Standards and Acceptance

Limits (Condition Monitoring)

Vibration Signal Frequency Analysis (FFT)

Course Overview


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Machinery Vibration Testing and Trouble Shooting

Fault Diagnostics Based on Forcing Functions Fault Diagnostics Based on Specific Machine

Components

Fault Diagnostics Based on Specific Machine Type

Automatic Diagnostic Techniques

Non-Vibration Based Machine Condition Monitoringand Fault Diagnosis Methods

Course Overview


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Current Topic







Limits (Condition Monitoring) Vibration Signal Frequency Analysis (FFT)


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What is Machine Condition Monitoring and

Fault Diagnostics?

basically it is a maintenance tool

also being applied in quality control, and

process control, process monitoring

Introduction


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Introduction

Why conduct Machine Condition Monitoring and

Fault Diagnostics?


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ISO definition:

a field of technical activity in which selected

physical parameters, associated with machinery

operation, are observed for the purpose of

determining machinery integrity

not just vibration based

Introduction


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Also includes:

oil analysis (oil quality, contamination)

wear particle monitoring and analysis

force

sound pressure (intensity)

temperature output (machine performance)

product quality

odour visual inspection and others

Introduction


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To place Machine Condition Monitoring and Fault

Diagnostics in context with the larger plant

maintenance picture:

Most machinery is required to operate within a close

set of limits.

operating speed (not variable speed machines) load (throughput)

product quality standards

Occasionally machinery is required to operate outside

these limits for short times (electric generators).

Machinery Failure


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The main reason for employing a Machine Condition

Monitoring and Fault Diagnostics program is to find

accurate, quantitative information on the present

condition of the machinery.

reasonable expectation of machine performance will a machine stand a required overload?

should equipment be serviced now or later?

what is the expected time to failure?

what is the expected failure mode?

Machinery Failure


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Machinery failure is the inabil ity of the machine

to perform its required function.

Failure is machinery specific.

Examples:

conveyor belt drive-end pulley bearings

failure = seized bearing (belt stops)

computer disk drivefailure = slow response, noisy

Machinery Failure


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Machinery failure may involve only a sub-

system of the machine or process.

Failure of a sub-system (or component) may result

in overall failure.

Example:

a flat tire on your car is a failure of the car (it

can no longer be driven)

Machinery Failure


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Other considerations which may dictate machineperformance:

economics (obsolescence - run to failure,

premature replacement)

safety (minimize risk of failure trains, planes

and automobiles).

Machinery Failure


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design deficiencies

material deficiencies processing deficiencies

improper assembly practices improper service conditions

inappropriate maintenance

excessive demands

Causes of Failures


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Catastrophic - sudden and complete.

Incipient - partial, usually gradual.

In most cases there is some advanced warning of

the onset of failure.

All failures pass through an incipient phase evenif they do so quickly.

Types of Failure


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detect onset of equipment deterioration

diagnose the condition

trend its progression over time

prognose (predict) when ultimate failure willoccur

allow time for maintenance planning

This excludes failures caused by unforeseen and

uncontrollable outside forces - earthquakes, etc.

The goal of Machine Condition Monitoring

and Fault Diagnostics is to:


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Bath-tub Curve (individual machine or population

of machines)

Frequency of Failure

Failure

Rate(prob.

of

failure)

Time In Service

Wear In

Normal Wear

Wear Out


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Typically high frequency of failures early in theexpected life of a machine due to:

design errors manufacturing defects

assembly mistakes

installation problems

commissioning errors

Wear In Failures


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Typically occur during the majority of the life ofa machine.

Relatively low failure rate when operating within

design specifications.

Normal Wear (Random Failures)


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Occur towards the end of a machines design

life.

Gradually increasing failure rate at the expected

end of a machines useful life - primarily due to:

fatigue

wear mechanisms

corrosion

obsolescence

Wear Out Failures


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The slope of the wear-out part of the bath-tub-

curve is machine and operational history

dependent.If the machine design is such that the operational life ends abruptly

(due to fatigue for example) or the machine is under designed to meet

the load expected or the machine has endured a severe operational life

(experienced numerous over-loads) the slope of the curve in the wear-out section will increase sharply with time. If the machinery is over

designed or experiences a relatively light loading history the slope of

this part of the bath-tub-curve will increase only gradually with time.

This curve shape is generally true for individualmachines and populations of machines of the

same type.

Wear Out Failures


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Bath-tub Curve (individual machine or population

of machines)


Failure

Rate

Time In Service

Wear InNormal Wear Wear Out


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Bath-tub Curves (as a function of machine duty)


Failure

Rate

Time In Service

Wear In

Normal Wear

Wear Out

Increasing Duty


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History of Maintenance Expectations


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History of Maintenance Techniques


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History of Equipment Failure


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Mechanical Components

Expected Equipment Failure Rates


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Electronic Components

Expected Equipment Failure Rates


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Basic Maintenance Strategies

There are three basic categories ofmaintenance practise

Run to failure

Scheduled

Condition Based


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Reactive

Philosophy: Fix it when it breaks

Benefit:

Zero initial investment

Cost:

Costly catastrophic

breakdowns

Scheduled

Philosophy: Change it out every ____ hours

Benefit:

Reduced catastrophicbreakdowns

Cost:

Premature work

Condition

Based (CBM)

Philosophy: Does it need to be fixed?

Benefit:

Maintenance done when

needed

Cost:

Requires effective use of

information

Upfront cost


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Run to failure (Breakdown) Maintenance

maintenance performed only when machinery

has failed. Example: Burnt out light bulb.


Machine Duty (Load)

Estimated

Capacity

and Load

Time In Service

Machine Capacity

(Est.)Failures

Maintenance


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Scheduled (Preventive) Maintenance

specific maintenance tasks performed at set

time intervals (or duty cycles)

significant margin between machine capacity

and actual duty maintained.

Example: Oil changes on your car engine,

light bulbs above a shop floor.



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Scheduled Maintenance


Machine Duty (Load)

Estimated

Capacity

and Load

Time In Service

Machine Capacity(Est.)

Maintenance

Margin

Margin


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Condition Based (on-condition, predictive)

Maintenance

actual condition of the machinery is assessed

data used to optimally schedule maintenance

maximum production and avoidance of catastrophic

failures is achieved

Example: Tire changes on your car.



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Condition Based Maintenance


Machine Duty (Load)

Estimated

Capacity

and Load

Time In Service

Machine Capacity(Est.)

Maintenance

Minimum Margin

Reduced Load

i i S i


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Note: margin between duty and capacity is neverallowed to reach zero - breakdown

avoidance.

Results: longer time between maintenance tasks

than for scheduled maintenance.


B i M i S i


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Advantages and disadvantages do exist.

Situations exist where one or the other would be

appropriate.The maintenance engineer must decide and justifyaction.

Combinations of strategies may be required withina given plant on different machines.

Changes in maintenance strategies may berequired for given machines during the life of themachine or as operating conditions change.



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Summary of Maintenance Strategies


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Additional Maintenance Strategies

Proactive Maintenance:

Redundancy:

Factors hich Infl ence Maintenance Strateg


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classification of machine

- critical to production?

- high cost of replacement?

- long lead time for replacement?

manufacturers recommendations

failure data (history), MTTF, MTBF, failure modes

redundancy

safety (plant personnel, community, environment)

parts cost/availability costs (personnel, administrative, equipment)

running costs

Factors which Influence Maintenance Strategy



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In general the following rules apply.

Breakdown Maintenance

if equipment is redundant

low cost spares available

interruptible process, stockpiled product

safe failure modes

long MTTF/MTBF

low cost secondary damage quick repair or replacement (low cost of

interruption to production)




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Preventive Maintenance

statistical failure rate available narrow failure distribution (predictable MTBF)

maintenance restores full integrity

single failure mode (known)

low cost of regular overhaul/replacement

unexpected interruptions to production

expensive (scheduled interruptions not so bad)

low cost spares available reduced number of breakdowns required

costly secondary damage from failure




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expensive/critical machinery

long lead time for replacement (no spares)

uninterruptible process (both regular andunexpected) - costly

large/complex machinery

overhaul expensive/needs highly trained people reduced numbers of highly skilled maintenance

people




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Condition Based Maintenance (contd) costs of monitoring program acceptable.

safety is a priority (failures dangerous)

remote, mobile equipment

failure not indicated by operation degeneration

costly secondary damage.




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Finally:

Each case must be evaluated individually.

Principal considerations defined in economic terms.

Company policy considerations.


Machine Condition Monitoring and


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Potential advantages

increased machine availability and reliability

improved operating efficiency

improved risk management (less down time)

reduced maintenance costs (better planning)

reduced spare parts inventories

improved safety improved knowledge of machine condition

(safe overloading of machine possible)

Fault Diagnostics



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Potential advantages (contd)

extended operational life of machine

improved customer relations (less planned /

unplanned downtime)

elimination of chronic failures (root causeanalysis and redesign)

reduction of post overhaul failures due to

improperly performed maintenance orreassembly

Fault Diagnostics



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Potential disadvantages

monitoring equipment costs (high)

operational costs (running the program)

skilled personnel needed

needs strong management commitment

long run-in time to collect machine histories and

set trends reduced costs are harder to sell as direct

benefits to management than increased profits

Fault Diagnostics

MCMAD Philosophy


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Get useful information on the condition of equipment

to the people who need it.

- operators, maintenance, managers, etc.- these groups need different information at

different times

This means:- collect useful data

- change data into information in a form

required by and useful to others- timely reporting

p y

MCMAD Philosophy


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Get useful information regarding the condition of

the equipment to the people who need it in a

timely manner.

operators minute by minute (overall

vibration/temperature levels, alarms)

maintenance personnel weekly/monthly(trends, alarm reports, raw data, frequency

spectra)

managers quarterly/yearly (maintenance

history, failure rates, cost/benefit of monitoring)

p y

MCMAD Philosophy


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Types of data collected:

- vibration severity, frequency analysis,

temperature, oil analysis, etc.

Types of information gleaned:

- existing condition

- trends

- expected time to failure at a given load

- type of fault existing or developing

- type of fault which caused failure

p y

MCMAD Tasks


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Detection

Diagnosis Prognosis

Post Mortem Prescription

MCMAD Tasks


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Detection

data gathering comparison to standards

comparison to limits set in-plant for specificequipment

trending over time

MCMAD Tasks


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Diagnosis

recognising the type of fault developing(different fault types may be more or less

serious and require different action)

severity of fault

MCMAD Tasks


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Prognosis

expected time to failure trending/predicting

forecasting maintenance planning/timing

MCMaFD Tasks


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Post Mortem

root cause failure analysis research, laboratory / field tests

modeling of system and analysis

MCMAD Tasks


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Prescription (activity dictated by information

collected)

may be applied at any stage

alter operating conditions

alter monitoring strategy (frequency, type)

redesign process or equipment

MCMAD Strategies


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How much data to collect?

How much time to spend at data analysis?

These things dictate the MCMAD strategy(cost will always be a factor)

Consider:

equipment class, size, importance within

process, replacement cost and availability

safety

different pieces of equipment or processes may

require different monitoring strategies.

MCMAD Strategies


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No Monitoring

inexpensive, non-critical equipment in stock equipment (or readily accessible)

low load equipment

low failure rate known

failure modes well understood

MCMAD Strategies


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Periodic Monitoring

non-critical equipment failure modes known

historically dependable equipment

trending and severity levels checks only

problems trigger more rigorous investigations

MCMAD Strategies


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Continuous Monitoring

permanently installed monitoring system

samples and analyses data automatically

critical equipment (expensive to replace with

downtime (loss of production) beingexpensive)

changes in condition trigger more detailed

investigation or possibly automatic shutdown

What to Measure


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vibration levels (displacement, velocity, acceleration).

oil analysis (lubricating quality contamination)

wear particle monitoring and analysis (number, size,shape, composition)

force measurements

sound level odour

temperature

output quantity product quality

visual inspection, etc.

Next Time


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Limits (Condition Monitoring)

Vibration Signal Frequency Analysis (FFT)

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