OP 17 – 1 Maintenance and Reliability 17 – 2 The objective is maintain the capability of the system 17 – 3 Strategic Importance of Mainten

OP 17 – 1

Maintenance
and Reliability

17 – 2

The
objective is

maintain the
capability
of the system

17 – 3

Strategic Importance of Mainten

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OP 17 – 1

Maintenance
and Reliability

17 – 2

The
objective is

maintain the
capability
of the system

17 – 3

Strategic Importance of Maintenance
and Reliability

Reduced value of investment in plant and equipment

Profits becoming losses

idle employees

Dissatisfied customers

Profitability

Reputation

Operation

Failure has far reaching effects on:

17 – 4

Maintenance and Reliability

Maintenance
is

all activities involved in
keeping a system’s

equipment in working
order

17 – 5

Maintenance and Reliability

Reliability is

the probability

that a machine will function
properly for a specified time

17 – 6

Important Tactics

Reliability

Improving individual components

Providing redundancy

Maintenance

Implementing or improving preventive
maintenance

Increasing repair capability or speed

17 – 8

Designing for
reliability is an
excellent place

to start reducing
variability.

17 – 9

Systems are composed of a
series of individual interrelated
components, each performing a

specific job.

If any one component fails to
perform, for whatever reason,

the overall system (e.g., an
airplane or machine) can fail

17 – 10

Because failures do occur in the real
world,

Understanding their occurrence is an
important reliability concept.

17 – 11

As the number of
components in

a series increases,

The reliability of the
whole system

declines very quickly

17 – 13

Reliability

and so on

R2 = reliability of component 2

where R1 = reliability of component 1

Improving individual components

Rs = R1 x R2 x R3 x … x Rn

a system in which each individual part or component may have its own unique rate of reliability

17 – 14

Example 1 Reliability
in a Series

The Bank of Montreal’s loan-processing centre
processes loan applications through

three clerks set up in series, with reliabilities of

0.90, 0.80, and 0.99.

The bank wants to find the system reliability.

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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%

17 – 16

A machine of n=50
interacting parts,

each of which has a 99.5%
reliability,

overall reliability is 78%.

A machine has 100 interacting
parts,

each with an individual
reliability of 99.5%,

the overall reliability will be
only about 60%!

Copyright © 2020 Pearson Canada Inc. 17 – 17

Product Failure Rate (FR)

Basic unit of measure for reliability

FR(%) =
Number of failures

Number of units tested
× 100%

FR( ) =
Number of failures

Number of unit−hours of operating time

Percent of failures

Number of failures during a period of time

Mean Time Between Failure MTBF

MTBF
FR

17 – 18

Failure Rate Example

20 air conditioning units designed for use in NASA space shuttles operated for 1000 hours

One failed after 200 hours and

one after 600 hours

Find:

Failure rate and
MTBF

17 – 19

Failure Rate Example

FR

FR(%)=Number of failures/Number of units tested %

17 – 20

Failure Rate Example (1 of 2)

FR( ) =
2

20 000 − 1200
= .000106 failure/unithr

MTBF =
1

.000106
= 9434hrs

17 – 21

Failure Rate Example (1 of 2)
Number of failures per operating hour:

17 – 22

Failure Rate Example (1 of 2)

17 – 23

Failure Rate Example

If the typical space
shuttle trip lasts six days,
NASA may be interested
in the failure rate per trip:

• =(Failures/unit-hr)(24 hr/day)(6 days/trip)
• =(0.000106)(24)(6)
• =0.0153 failure/trip

Failure rate

17 – 24

The technique here is to “back up” components with additional
components.

This is known as putting units in parallel and is a standard operations
management tactic.

17 – 25

Providing Redundancy

Provide backup components to increase reliability

(Probability that first component works)

+

[(Probability that backup works) × (Probability that first component fails)

The resulting reliability

17 – 26

Providing Redundancy

Provide backup components to increase reliability

of first
component

working

+

Probability
of second

component
working

×

Probability
of needing

second
component

If first component 80% and backup component 80%

The resulting reliability?

17 – 27

Maintenance

Preventive
maintenance –
• routine inspection and

servicing to keep
facilities in good repair

Breakdown
maintenance –
• emergency repairs on

failed equipment

Two types of
maintenance

17 – 28

Implementing Preventive Maintenance

Need to know when a system
requires service or is likely to fail

17 – 29

Implementing Preventive Maintenance

High initial
failure rates….

infant mortality

17 – 30

Implementing Preventive Maintenance

Once a product settles in,

MTBF (mean time between failures) generally
follows a normal distribution

17 – 33

Maintenance Costs

The traditional
view attempted

to balance

preventive and
breakdown

maintenance
costs

17 – 34

Maintenance Costs

17 – 35

Maintenance Costs

Typically the previous approach fails to
consider the true total cost of breakdowns

• Inventory
• Employee morale
• Schedule unreliability

17 – 36

Maintenance Costs (3 of 3)

Figure 17.4b

17 – 37

Example

Comparing Preventive and Breakdown Maintenance Costs

Farlen & Halikman is a CPA firm specializing in payroll preparation.

The firm has been successful in automating much of its work, using high-speed
printers for cheque processing and report preparation.

The computerized approach, however, has problems. Over the past 20 months,
the printers have broken down at the rate indicated in the following table:

Copyright © 2020 Pearson Canada Inc. 17 – 38

Maintenance Cost Example (1 of 4)

Number of
Breakdowns

Number of Months That Breakdowns
Occurred

0 Blank 2

1 Blank 8

2 Blank 6

3 Blank 4

Blank Total : 20

Copyright © 2020 Pearson Canada Inc. 17 – 39

Each time the printers break down, Farlen &
Halikman estimates that it loses an average of $300
in production time and service expenses.

One alternative is to purchase a service contract for
preventive maintenance.

Even if Farlen & Halikman contracts for preventive
maintenance, there will still be
breakdowns, averaging one breakdown per month.

The price for this service is $150 per month

Copyright © 2020 Pearson Canada Inc. 17 – 40

Maintenance Cost Example (1 of 4)

Should the firm contract for maintenance on their printers?

17 – 41

Maintenance Cost Example (2 of 4)

Compute the expected number of breakdowns

Number of
Breakdowns

Frequency Number of
Breakdowns

Frequency

0 2/20 = .1 2 6/20 = .3

1 8/20 = .4 3 4/20 = .2

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Compute the expected number of breakdowns

17 – 43

Maintenance Cost Example (3 of 4)

Compute the expected breakdown cost per month with no
preventive maintenance

Copyright © 2020 Pearson Canada Inc. 17 – 44

Maintenance Cost Example (4 of 4)

Compute the cost of preventive maintenance

Copyright © 2020 Pearson Canada Inc. 17 – 45

Maintenance Cost Example (4 of 4)

Hire the service firm; it is less expensive

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