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Maintenance and Reliability

PowerPoint presentation to accompany

Heizer and Render

Operations Management, Eleventh Edition

Principles of Operations Management, Ninth Edition

PowerPoint slides by Jeff Heyl

Additional content from Gerry Cook

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Outline

Global Company Profile:
Orlando Utilities Commission

The Strategic Importance of Maintenance and Reliability
Reliability
Maintenance
Total Productive Maintenance

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Learning Objectives

When you complete this chapter you should be able to:

Describe how to improve system reliability
Determine system reliability
Determine mean time between failure (MTBF)
Distinguish between preventive and breakdown maintenance

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When you complete this chapter you should be able to:

Learning Objectives

Describe how to improve maintenance
Compare preventive and breakdown maintenance costs
Define autonomous maintenance

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Orlando Utilities Commission

Maintenance of power generating plants
Every year each plant is taken off-line for 1-3 weeks maintenance
Every three years each plant is taken off-line for 6-8 weeks for complete overhaul and turbine inspection
Each overhaul has 1,800 tasks and requires 72,000 labor hours
OUC performs over 12,000 maintenance tasks each year

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Orlando Utilities Commission

Every day a plant is down costs OUC $110,000
Unexpected outages cost between $350,000 and $600,000 per day
Preventive maintenance discovered a cracked rotor blade which could have destroyed a $27 million piece of equipment

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Strategic Importance of Maintenance and Reliability

The objective of maintenance and reliability is to maintain the capability of the system

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Strategic Importance of Maintenance and Reliability

Failure has far reaching effects on a firm’s
Operation
Reputation
Profitability
Customer satisfaction
Reducing idle time
Protecting investment in plant and equipment

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Maintenance and Reliability

Maintenance is all activities involved in keeping a system’s equipment in working order
Reliability is the probability that a machine will function properly for a specified time

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Important Tactics

Reliability

Improving individual components

Providing redundancy

Maintenance

Implementing or improving preventive maintenance

Increasing repair capability or speed

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Maintenance Management

Figure 17.1

Partnering with maintenance personnel

Skill training

Reward system

Employee empowerment

Employee Involvement

Clean and lubricate

Monitor and adjust

Make minor repair

Keep computerized records

Maintenance and Reliability Procedures

Reduced inventory

Improved quality

Improved capacity

Reputation for quality

Continuous improvement

Reduced variability

Results

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Reliability

System reliability
Improving individual components

Rs = R1 x R2 x R3 x … x Rn

where R1 = reliability of component 1

R2 = reliability of component 2

and so on

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Overall System Reliability

Figure 17.2

Reliability of the system (percent)

Average reliability of each component (percent)

| | | | | | | | |

100 99 98 97 96

100 –

80 –

60 –

40 –

20 –

0 –

n = 10

n = 1

n = 50

n = 100

n = 200

n = 300

n = 400

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Reliability Example

Reliability of the process is

Rs = R1 x R2 x R3 = .90 x .80 x .99 = .713 or 71.3%

.99

.80

.90

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Product Failure Rate (FR)

Basic unit of measure for reliability

Number of failures

Number of units tested

FR(%) = x 100%

Number of failures

Number of unit-hours of operating time

FR(N) =

FR(N)

MTBF =

Mean time between failures

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Failure Rate Example

20 air conditioning units for use in the international space station operated for 1,000 hours

One failed after 200 hours and one after 600 hours

FR(%) = (100%) = 10%

20,000 - 1,200

FR(N) = = .000106 failure/unit hr

.000106

MTBF = = 9,434 hrs

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Failure Rate Example

20 air conditioning units for use in the international space station operated for 1,000 hours

One failed after 200 hours and one after 600 hours

FR(%) = (100%) = 10%

20,000 - 1,200

FR(N) = = .000106 failure/unit hr

.000106

MTBF = = 9,434 hrs

Failure rate per trip

FR = FR(N)(24 hrs)(6 days/trip)

FR = (.000106)(24)(6)

FR = .0153 failure/trip

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Providing Redundancy

Provide backup components to increase reliability

Probability of first component working

Probability of needing second component

Probability of second component working

RS =

(.8)

(1 - .8)

= .8

.16 = .96

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Redundancy Example

A redundant process is installed to support the earlier example where Rs = .713

RS = [.9 + .9(1 - .9)] x [.8 + .8(1 - .8)] x .99

= [.9 + (.9)(.1)] x [.8 + (.8)(.2)] x .99

= .99 x .96 x .99 = .94

Reliability has increased from

.713 to .94

0.90

0.80

0.99

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Parallel Redundancy

Increased reliability

through parallel

redundancy

Reliability of new design = 1 – .00012 = .99988

0.95

0.975

Reliability for

the middle path

= R2 x R3 = .975 x .975 = .9506

Probability of failure

for all 3 paths

= (1 – 0.95) x (1 – .9506) x (1 – 0.95)

= (.05) x (.0494) x (.05) = .00012

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Maintenance

Two types of maintenance
Preventive maintenance – routine inspection and servicing to keep facilities in good repair
Breakdown maintenance – emergency or priority repairs on failed equipment

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Implementing Preventive Maintenance

Need to know when a system requires service or is likely to fail
High initial failure rates are known as infant mortality
Once a product settles in, MTBF generally follows a normal distribution
Good reporting and record keeping can aid the decision on when preventive maintenance should be performed

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Computerized Maintenance System

Figure 17.3

Inventory and purchasing reports

Equipment parts list

Equipment

history reports

Cost analysis

(Actual vs. standard)

Work orders

Output Reports

Personnel data with skills, wages, etc.

Equipment file with parts list

Maintenance

and work order schedule

Inventory of spare parts

Repair history file

Data Files

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Maintenance Costs

The traditional view attempted to balance preventive and breakdown maintenance costs
Typically this approach failed to consider the full costs of a breakdown
Inventory
Employee morale
Schedule unreliability

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Maintenance Costs

Figure 17.4 (a)

Traditional View

Total costs

Breakdown maintenance costs

Costs

Maintenance commitment

Preventive maintenance costs

Optimal point (lowest

cost maintenance policy)

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Maintenance Costs

Figure 17.4 (b)

Full Cost View

Costs

Maintenance commitment

Optimal point (lowest

cost maintenance policy)

Total costs

Full cost of breakdowns

Preventive maintenance costs

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Maintenance Cost Example

Should the firm contract for maintenance on their printers?

Average cost of breakdown = $300

NUMBER OF BREAKDOWNS	NUMBER OF MONTHS THAT BREAKDOWNS OCCURRED
0	2
1	8
2	6
3	4
Total :	20

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Maintenance Cost Example

Compute the expected number of breakdowns

= (0)(.1) + (1)(.4) + (2)(.3) + (3)(.2)

= 0 + .4 + .6 + .6

= 1.6 breakdowns / month

NUMBER OF BREAKDOWNS	FREQUENCY	NUMBER OF BREAKDOWNS	FREQUENCY
0	2/20 = .1	2	6/20 = .3
1	8/20 = .4	3	4/20 = .2

Number of breakdowns

Expected number of breakdowns

Corresponding frequency

∑

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Maintenance Cost Example

Compute the expected breakdown cost per month with no preventive maintenance

= (1.6)($300)

= $480 per month

Expected breakdown cost

Expected number of breakdowns

Cost per breakdown

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Maintenance Cost Example

Compute the cost of preventive maintenance

= (1 breakdown / month)($300) + $150 / month

= $450 / month

Hire the service firm; it is less expensive

Preventive maintenance cost

Cost of expected breakdowns if service contract signed

Cost of

service contract

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Increasing Repair Capabilities

Well-trained personnel

Adequate resources

Ability to establish repair plan and priorities

Ability and authority to do material planning

Ability to identify the cause of breakdowns

Ability to design ways to extend MTBF

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Increasing Repair Capabilities

Figure 17.5

Operator

(autonomous maintenance)

Maintenance department

Manufacturer’s field service

Depot service

(return equipment)

Increasing Operator Ownership

Increasing Complexity

Preventive

maintenance costs less and

is faster the more we move to the left

Competence is higher as we

move to the right

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Autonomous Maintenance

Employees accept responsibility for
Observe
Check
Adjust
Clean
Notify
Predict failures, prevent breakdowns, prolong equipment life

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Total Productive Maintenance (TPM)

Designing machines that are reliable, easy to operate, and easy to maintain
Emphasizing total cost of ownership when purchasing machines, so that service and maintenance are included in the cost

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Total Productive Maintenance (TPM)

Developing preventive maintenance plans that utilize the best practices of operators, maintenance departments, and depot service
Training for autonomous maintenance so operators maintain their own machines and partner with maintenance personnel

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More on Maintenance –

A simple redundancy formula
Problems with breakdown and preventive maintenance
Predictive maintenance
Predictive maintenance tools
Maintenance strategy implementation
Effective reliability

Supplemental Material

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Providing Redundancy –
An Alternate Formula

The reliability of one pump =
The probability of one pump not failing = 0.8

P(failing) = 1- P(not failing) = 1 - 0.8 = .2

P(failure of both pumps) =

P(failure) pump #1 x P(failure) pump #2

P(failure of both pumps) = 0.2 x 0.2 = .04

P(at least one pump working) =

1.0 - .04 = .96

If there are two pumps with the same probability of not failing

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Problems With Breakdown Maintenance

Run it till it breaks”
Might be ok for low criticality equipment or redundant systems
Could be disastrous for mission-critical plant machinery or equipment
Not permissible for systems that could imperil life or limb (like aircraft)

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Problems With Preventive Maintenance

Fix it “whether or not it is broken”
Scheduled replacement or adjustment of parts/equipment with a well-established service life
Typical example – plant relamping
Sometimes misapplied
Replacing old but still good bearings
Over-tightening electrical lugs in switchgear

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Another Maintenance Strategy

Predictive maintenance – Using advanced technology to monitor equipment and predict failures
Using technology to detect and predict imminent equipment failure
Visual inspection and/or scheduled measurements of vibration, temperature, oil and water quality
Measurements are compared to a “healthy” baseline
Equipment that is trending towards failure can be scheduled for repair

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Predictive Maintenance Tools

Vibration analysis
Infrared Thermography
Oil and Water Analysis
Other Tools:
Ultrasonic testing
Liquid Penetrant Dye testing
Shock Pulse Measurement (SPM)

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Predictive Maintenance
Vibration Analysis

Using sensitive transducers and instruments to detect and analyze vibration
Typically used on expensive, mission-critical equipment–large turbines, motors, engines or gearboxes
Sophisticated frequency (FFT) analysis can pinpoint the exact moving part that is worn or defective
Can utilize a monitoring service

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Predictive Maintenance Infrared (IR) Thermography

Using IR cameras to look for temperature “hot spots” on equipment
Typically used to check electrical equipment for wiring problems or poor/loose connections
Can also be used to look for “cold (wet) spots” when inspecting roofs for leaks
High quality IR cameras are expensive – most pay for IR thermography services

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Predictive Maintenance
Oil and Water Analysis

Taking oil samples from large gearboxes, compressors or turbines for chemical and particle analysis
Particle size can indicate abnormal wear
Taking cooling water samples for analysis – can detect excessive rust, acidity, or microbiological fouling
Services usually provided by oil vendors and water treatment companies

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Predictive Maintenance
Other Tools and Techniques

Ultrasonic and dye testing – used to find stress cracks in tubes, turbine blades and load bearing structures
Ultrasonic waves sent through metal
Surface coated with red dye, then cleaned off, dye shows cracks
Shock-pulse testing – a specialized form of vibration analysis used to detect flaws in ball or roller bearings at high frequency (32kHz)

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Maintenance Strategy Comparison

MAINTENANCE STRATEGY	ADVANTAGES	DISADVANTAGES	RESOURCES/ TECHNOLOGY REQUIRED	APPLICATION EXAMPLE
Breakdown	No prior work required	Disruption of production, injury or death	May need labor/parts at odd hours	Office copier
Preventive	Work can be scheduled	Labor cost, may replace healthy components	Need to obtain labor/parts for repairs	Plant relamping, machine lubrication
Predictive	Impending failures can be detected & work scheduled	Labor costs, costs for detection equipment and services	Vibration, IR analysis equipment or purchased services	Vibration and oil analysis of a large gearbox

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Maintenance Strategy Implementation

Percentage of Maintenance Time by Strategy

Breakdown

Preventive

Predictive

1 2 3 4 5 6 7 8 9 10

Year

100%

80%

60%

40%

20%

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Is Predictive Maintenance
Cost Effective?

In most industries the average rate of return is 7:1 to 35:1 for each predictive maintenance dollar spent
Vibration analysis, IR thermography and oil/water analysis are all economically proven technologies
The real savings is the avoidance of manufacturing downtime – especially crucial in JIT

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Predictive Maintenance and Effective Reliability

Effective Reliability (Reff) is an extension of Reliability that includes the probability of failure times the probability of not detecting imminent failure
Having the ability to detect imminent failures allows us to plan maintenance for the component in failure mode, thus avoiding the cost of an unplanned breakdown

Reff = 1 – (P(failure) x P(not detecting failure))

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How Predictive Maintenance Improves Effective Reliability

Example: a large gearbox with a reliability of .90 has vibration transducers installed for vibration monitoring. The probability of early detection of a failure is .70. What is the effective reliability of the gearbox?

Reff = 1 – (P(failure) x P(not detecting failure))

Reff = 1 – (.10 x .30) = 1 - .03 = .97

Vibration monitoring has increased the effective reliability from .90 to .97!

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Effective Reliability Caveats

Predictive maintenance only increases effective reliability if:
You select the method that can detect the most likely failure mode
You monitor frequently enough to have high likelihood of detecting a change in component behavior before failure
Timely action is taken to fix the issue and forestall the failure (in other words you don’t ignore the warning!)

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Increasing Repair Capabilities

Well-trained personnel

Adequate resources

Proper application of the three maintenance strategies

Continual improvement to improve equipment/system reliability

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