Operations management by stevenson 9th student slides supplement 15

Supplement 15
Maintenance

McGraw-Hill/Irwin

Copyright © 2009 by The McGraw-Hill Companies, Inc. All Rights Reserved.


Supplement 15: Learning Objectives
• You should be able to:
– Explain the importance of maintenance in production systems
– Describe the range of maintenance activities
– Discuss preventive maintenance and the key issues associated
with it
– Discuss breakdown maintenance and the key issues associated
with it
– State how the Pareto phenomenon pertains to maintenance
decisions

15S-2


Maintenance
• Maintenance

– All activities that maintain facilities and equipment in
good working order so that a system can perform as
intended
– Maintenance activity categories:
• Buildings and grounds
• Equipment maintenance

15S-3


Maintenance Goals
• The goal of maintenance is to keep the production
system in good working order at minimal cost
• Reasons for wanting to keep equipment and machines
in good operating condition:
–
–
–
–

Avoid production or service disruptions
Not add to production or service costs
Maintain high quality
Avoid missed delivery dates

15S-4


Breakdown Consequences
• Operations capacity is reduced, and orders are delayed
• There is no output, but overhead continues, increasing
the cost per unit
• There are quality issues
– output may be damaged

• There are safety issues

– employees or customers may be injured

15S-5


Maintenance Options
• Reactive:

– Breakdown maintenance
• Dealing with breakdowns or problems when they occur
• Proactive:

– Preventive maintenance
• Reducing breakdowns through a program of lubrication,
adjustment, cleaning, inspection, and replacement of worn
parts

15S-6


Total Maintenance Cost

15S-7


Preventive Maintenance
• The goal of preventive maintenance is to reduce the
incidence of breakdown or failures in the plant or
equipment to avoid the associated costs
–

–
–
–
–
–

Loss of output
Idle workers
Schedule disruption
Injuries
Damange to other equipment, products, or facilities
Repairs

15S-8


Preventive Maintenance Schedule
• It is periodic
• It is scheduled according to the availability of
maintenance personnel and to avoid interference with
operating schedules
• Maintenance is scheduled using some combination of
– The result of planned inspections that reveal a need for
maintenance
– According to the calendar (passage of time)
– After a predetermined number of operating hours

15S-9

Preventive Maintenance: Frequency
• The goal is to strike a balance between the cost of preventive
maintenance and the cost of breakdown
• The amount of preventive maintenance, therefore, is a function of
the expected frequency of breakdown, the cost of breakdown, and
the cost of preventive maintenance

15S-10


Frequency: Example 1
• The frequency of breakdown of a machine per month is $1,000 and
the cost of preventive maintenance is $1,250 per month. If
preventive maintenance is performed, the probability of a machine
breakdown is negligible. Should the manager use preventive
maintenance, or would it be cheaper to repair the machine when it
breaks down?
Number of breakdowns
Frequency of occurrences

0

1

2

3

.20

.30

.40

.10

15S-11


Frequency: Example 1
Number of breakdowns
Frequency of occurrences

0

1

2

3

.20

.30

.40

.10

Expected number

= 0(.20) + 1(.30) + 2(.40) + 3(.10) = 1.4
of breakdowns

Expected benefit of
Expected cost of Cost of preventive
=
−
preventive maintenance
breakdown
maintenance
= 1.4($1,000) - $1,250
= $150

15S-12


Frequency: Example 2
• Suppose the average time before breakdown is normally distributed
with a mean of 3 weeks and a standard deviation of .60 weeks. If
breakdown cost average $1,000 and preventive maintenance costs
$250, what is the optimal maintenance interval?

Preventive cost
$250
=
= .25
Breakdown cost $1,000
• Find the number of standard deviations from the mean represented by
an area under the normal curve of .25
• Using the standard normal table, it is -0.67

• Use this value to determine the maintenance interval

Mean + z (standard deviation) = 3 + (-.67)(.60) = 2.598 weeks

15S-13


Maintenance Approaches
• Predictive maintenance
– An attempt to determine when best to perform preventive
maintenance activities
• Ideally, preventive maintenance will be performed just prior to
a breakdown or failure because this will result in the longest
possible use of facilities or equipment without a breakdown

• Total productive maintenance
– JIT approach where workers perform preventive maintenance on
the machines they operate

15S-14


Maintenance Problems
• Maintenance problems are sometimes designed into products:

– Poor design
– Designer have accorded other aspects of design
greater importance
• Cost
• Appearance is more important than maintenance activities

15S-15