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Operations Management, 12e (Heizer/Render/Munson)
Chapter 17 Maintenance and Reliability
Section 1 The Strategic Importance of Maintenance and Reliability
1) The Orlando Utilities Commission uses a computerized maintenance management program and
devotes significant dollar and labor resources to power plant maintenance, because the costs of
unexpected failure are incredibly high.
2) Maintenance includes all activities involved in keeping a system's equipment in working order.
3) Reliability is the probability that a machine part or product will function properly for a specified time
regardless of conditions.
4) The objective of maintenance and reliability is to maintain the capability of the system.
5) Which of the following statements about maintenance at the Orlando Utilities Commission is FALSE?
A) There are at least two types of preventive maintenance, including an annual maintenance and a less
frequent overhaul schedule.
B) Its preventive maintenance program has earned the company top rankings and its competitive
advantage.
C) Each power-generating unit is taken off-line every three years for a complete overhaul.
D) Each of its power-generating units is taken off-line for maintenance every one to three weeks.
E) Costs associated with breakdowns are several times higher than costs arising from preventive
maintenance.
6) The objective of maintenance and reliability is to:
A) ensure that breakdowns do not affect the quality of the products.
B) ensure that no breakdowns will ever occur.
C) ensure that preventive maintenance costs are kept as low as possible.
D) maintain the capability of the system.
E) ensure that maintenance employees are fully utilized.
7) What is the probability that a product will function properly for a specified time under stated
conditions?
A) functionality
B) maintenance
C) durability
D) reliability
E) fitness for use
8) Which of the following is a reliability tactic?
A) improving individual components
B) increasing repair speed
C) providing redundancy
D) A and C
E) A, B, and C
9) ________ consists of all activities involved in keeping a system's equipment in working order.
10) ________ is the probability that a machine part or product will function properly for a specified time
under stated conditions.
11) Describe how the Orlando Utilities Commission obtains competitive advantage through its
maintenance practices.
12) Identify the two reliability tactics and the two maintenance tactics.
13) Define reliability.
14) Define maintenance.
Section 2 Reliability
1) The product failure rate FR(%) is the percent of failures among the total number of products tested.
2) The MTBF (mean time between failures) is calculated as the reciprocal of the number of failures during
a period of time.
3) If the mean time between failures has been calculated to be 2,000 hours, then FR(N) = 0.0005
failures/unit-hour.
4) The reliability of a system in which each individual component must function in order for the entire
system to function, and in which each component has its own unique reliability, independent of other
components, is the product of the probabilities of each of those components.
5) Adding an additional part to a component or product ordinarily reduces reliability by introducing an
additional source of failure.
6) A redundant part or component increases reliability because it is connected in parallel, not in series.
7) A redundant part decreases reliability if the reliability of the redundant part is lower than that of the
part it is backing up.
8) What is the reliability of a four-component product, with components in series, and component
reliabilities of .90, .95, .98, and .99?
A) 0.8295
B) 0.90
C) 0.955
D) 0.99
E) 0.945
9) A system is composed of three components A, B, and C. All three must function for the system to
function. There are currently no backups in place. The system has a reliability of 0.966. If a backup is
installed for component A, the new system reliability will be:
A) unchanged.
B) less than 0.966.
C) greater than it would be if a backup were also installed for component B.
D) greater than 0.966
E) none of the above
10) A system has three components in series with reliabilities 0.9, 0.7, and 0.5. What is the system
reliability?
A) 0.315
B) 0.500
C) 0.700
D) 0.900
E) 2.100
11) A system has three components in parallel with reliabilities 0.9, 0.7, and 0.5. What is the system
reliability?
A) 0.315
B) 0.700
C) 0.900
D) 0.985
E) 2.100
12) Components A, B, and C are connected in series. Component D is connected in parallel to component
B. Which of the following statements is true?
A) The system works only if A works, B or C works, and D works.
B) Component B must work for the system to work.
C) The system works when A works, C, works, and either B or D works.
D) Components B and C are backups to A.
E) The system works if D works, and any of A, B, or C works.
13) A job consists of a series of three tasks. Task 1 is performed correctly 98% of the time, task 2 is
performed correctly 99% of the time, and task 3 is performed correctly 97% of the time. What is the
reliability of this job?
A) 91.27%
B) 94.11%
C) 97.00%
D) 98.00%
E) 99.00%
14) As the number of components in a system connected in a series decreases, all other things being
equal, the reliability of the system usually:
A) increases.
B) stays the same.
C) decreases.
D) increases, then decreases.
E) decreases, then increases.
15) What is the reliability of the three components connected in series shown below?
A) 0.799425
B) 0.85000
C) 0.91333
D) 0.95000
E) 2.79
16) Consider a product with three components in series, with reliabilities of 0.90, 0.80, and 0.99 for
components A, B, and C, respectively. Furthermore, component B uses a backup that also has a reliability
of 0.80. What is the reliability of the system?
A) 0.50000
B) 0.71280
C) 0.80000
D) 0.85536
E) 3.49000
17) Ten high-technology batteries are tested for 200 hours each. One failed at 20 hours; all others
completed the test. FR(%) is ________ and MTBF is ________.
A) 10%; 1/1820
B) 90%; 1/1820
C) 10%; 1820 hours
D) 10%; 1980 hours
E) 10%; 2000 hours
18) Ten high-technology batteries are tested for 200 hours each. One failed at 50 hours; all others
completed the test. FR(%) is ________ and FR(N) is ________.
A) 10%; 1/1850
B) 10%; 1/2000
C) 25%; 1850 hours
D) 90%; 1/2000
E) 10%; 1/1950
19) Ten high-technology batteries are tested for 200 hours each. One failed at 20 hours; another failed at
140 hours; all others completed the test. FR(%) is ________ and MTBF is ________.
A) 20%; 880 hours
B) 10%; 1980 hours
C) 20%; 1760 hours
D) 20%; 1000 hours
E) 80%; 920 hours
20) MTBF measures the average:
A) calendar time between failures.
B) operating time between failures.
C) number of failures per unit time.
D) number of operations between failures.
E) downtime per breakdown.
21) A system is composed of four parts, J, K, L, and M. All four must function for the system to function.
The four component reliabilities are .99, .98, .992, and .998. The designers are considering putting an 80%
reliable backup at K. This backup will change the system reliability from ________ to ________.
A) 0.9762; 0.9605
B) 0.9605; 0.9762
C) 0.9605; some smaller value
D) 0.9605; 0.996
E) 0.98; 0.99
22) Suppose that a process is comprised of five components, each having a 75% success rate. By how
much would reliability change if the process switched from having the components in series to having the
components in parallel?
A) no change
B) 100% increase
C) 76.2% increase
D) 76.2 % decrease
E) 50% increase
23) A two-component process has an 81% success rate in series and a 99% success rate in parallel. If each
component has the same reliability, what is the reliability of an individual component?
A) 50%
B) 90%
C) 99%
D) 18%
E) 81%
24) How is MTBF related to FR(N)?
A) MTBF is measured in hours, while FR(N) is measured in years.
B) MTBF is normally distributed, with FR(%) as its mean and FR(N) as its standard deviation.
C) MTBF is the reciprocal of FR(N).
D) Both MTBF and FR(N) increase when breakdown maintenance is replaced by preventive maintenance.
E) MTBF and FR(N) are unrelated concepts.
25) ________ is the expected time between a repair and the next failure of a component, machine, process,
or product.
26) The inverse of the mean time between failures is the ________.
27) ________ is the use of a component in parallel to raise reliabilities.
28) What is the impact on system reliability of adding parts or components in parallel?
29) Increasing the number of parts or components in a product tends to reduce its reliability. Why is this
true only when adding components in series?
30) Explain carefully how redundancy improves product reliability.
31) "High reliability can be achieved in a product without having high reliability in the component parts.
In fact, any reliability target, no matter how high, can be achieved with only mediocre parts, so long as
enough of them are present." Discuss; an example may help.
32) What is FR(N)? How is it calculated? How are FR(N) and MTBF related?
33) Ten high-intensity bulbs are tested for 100 hours each. One failed at 40 hours; all others completed the
test. Calculate FR(%) and FR(N).
34) Ten high-intensity bulbs are tested for 100 hours each. One failed at 10 hours; all others completed the
test. Calculate FR(%), FR(N) and MTBF.
35) Ten high-intensity bulbs are tested for 100 hours each. One failed at 40 hours; another failed at 70
hours; all others completed the test. Calculate FR(%), FR(N), and MTBF.
36) A product is composed of a series connection of four components with the following reliabilities.
What is the reliability of the system?
Component
1
2
3
4
Reliability
.90
.95
.97
.88
37) A system has four components in a series. What is the reliability of the system?
Component
1
2
3
4
Reliability
.90
.95
.90
.99
38) A system consists of four components in series. The reliability of each component is 0.96. What is the
reliability of the system?
39) A system has six components in series. Each component has a reliability of 0.99. What is the reliability
of the system?
40) The diagram below identifies the elements of service as provided by a soft drink vending machine.
Each element has an estimate of its own reliability, independent of the others. What is the reliability of the
"system"?
41) Century Digital Phone advertises phone battery life (on standby) of up to three days. The standard
deviation is thought to be five hours. Tina Talbot, an employee at CDP, tested 10 of these batteries for 72
hours. One failed at 40 hours; one failed at 62 hours; one failed at 70 hours. All others completed the test.
Calculate FR(%), FR(N), and MTBF.
42) The Everstart is a battery with an intended design life of 72 months. Stephanie Bradley recently put
five of these batteries through accelerated testing (the company couldn't wait six years) to simulate failure
patterns. The test results had one failure at 26 months, one failure at 32 months, one failure at 50 months,
and one failure at 62 months. Calculate FR(%), FR(N), and MTBF.
43) The academic service commonly referred to as "registration" consists of several smaller components:
advising, registration for courses, fee assessment, financial aid calculations, and fee payment. Each of
these modules operates independently and has some probability of failure for each student. If the five
probabilities that accompany these services are 95%, 90%, 99%, 98%, and 99%, respectively, what is the
"reliability" of the entire product from the student's perspective, i.e., the probability that all five will work
according to plan?
44) A simple electrical motor has three components: windings, armature, and housing. These three
components have reliabilities of .9998, .9992, and .9999. There is no possibility of redundant parts. What is
the reliability of the motor? Round your answer to four decimal places.
45) A simple electrical motor has three components: windings, armature, and housing. These three
components have reliabilities of .97, .992, and .999. There is no possibility of redundant parts. The motor
must have an overall reliability of at least 0.980, according to the product line manager who will use the
motor as an input. What would you do to redesign the motor to meet this specification? Discuss,
including a recalculation to meet the standard.
46) A product has four components A, B, C, and D. The finished product must have a reliability of at least
.95. The first three components come from a supplier, and they have reliabilities of .99, .98, and .995,
respectively. The fourth component is being designed now. What must the reliability of component D be
in order to meet the product reliability condition?
47) A product has three components X, Y, and Z. Product X has reliability of 0.991, and Y has reliability of
0.993. If Z has reliability of 0.991, what is the reliability of the entire product? Can Z be redesigned to be
reliable enough for the entire product to have reliability of 0.99? Explain.
48) A component must have reliability of .9925. Two technologies are available for this component: one
produces a component with .999 reliability at a cost of $2000. Another produces a component with .73
reliability at a cost of $450. Which is cheaper: one high quality component or a parallel set of inferior
components? (Hint: for the inferior components, you first need to determine the number of them that
would be needed to ensure overall reliability of .9925.)
49) General Grant must send orders to General Butler. Carrier pigeons are the medium of choice. A single
pigeon has a .7 probability of arriving at the proper destination in a timely fashion. How many pigeons,
each carrying an identical set of orders, must Grant send in order for him to have 98% confidence that the
orders reached General Butler?
