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Maintenance
and Reliability
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The
objective is
maintain the
capability
of the system
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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:
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Maintenance and Reliability
Maintenance
is
all activities involved in
keeping a system’s
equipment in working
order
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Maintenance and Reliability
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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Designing for
reliability is an
excellent place
to start reducing
variability.
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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
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Because failures do occur in the real
world,
Understanding their occurrence is an
important reliability concept.
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As the number of
components in
a series increases,
The reliability of the
whole system
declines very quickly
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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
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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%
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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%!
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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
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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
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Failure Rate Example
FR
FR(%)=Number of failures/Number of units tested %
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Failure Rate Example (1 of 2)
FR(?) =
2
20 000 − 1200
= .000106 failure/unithr
MTBF =
1
.000106
= 9434hrs
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Failure Rate Example (1 of 2)
Number of failures per operating hour:
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Failure Rate Example (1 of 2)
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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
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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.
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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
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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?
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Maintenance
Preventive
maintenance –
• routine inspection and
servicing to keep
facilities in good repair
Breakdown
maintenance –
• emergency repairs on
failed equipment
Two types of
maintenance
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Implementing Preventive Maintenance
Need to know when a system
requires service or is likely to fail
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Implementing Preventive Maintenance
High initial
failure rates….
infant mortality
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Implementing Preventive Maintenance
Once a product settles in,
MTBF (mean time between failures) generally
follows a normal distribution
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Maintenance Costs
The traditional
view attempted
to balance
preventive and
breakdown
maintenance
costs
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Maintenance Costs
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Maintenance Costs
Typically the previous approach fails to
consider the true total cost of breakdowns
• Inventory
• Employee morale
• Schedule unreliability
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Maintenance Costs (3 of 3)
Figure 17.4b
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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:
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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
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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
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Maintenance Cost Example (1 of 4)
Should the firm contract for maintenance on their printers?
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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
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Maintenance Cost Example (3 of 4)
Compute the expected breakdown cost per month with no
preventive maintenance
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Maintenance Cost Example (4 of 4)
Compute the cost of preventive maintenance
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Maintenance Cost Example (4 of 4)
Hire the service firm; it is less expensive
