Maintenance is the combination of all technical and associated administr ative actions intended to retain an item in, or restore it to, a state in which it can perform its required function. Many companies are seeking to gain competitive advantage with respect to cost, quality, service and on-time deliveries. The effect of maintenance on these variables has prompted increased attention to the maintenance area as an i ntegra l part of productivity improvement. Maintenance is rapidly evolving into a major contributor to the performance and profitability of manufacturing systems. In fact, some see maintenance as the "last frontier" for manufacturing. In their article "Make Maintenance Meaningful" P.K. Kauppi and Paavo Ylinen describe the bulk of maintenance proce dures as being as: yPreventive maintenance²the prevention of equipment breakdowns before theyhappen. This includes inspections, adjustme nts, regular service and planned shutdowns. yRepair work²repairing equipment and troubleshooting malfunction s in an effort to return the equipment to its previous condition. These repairs may be reactive or preventive. yImprovement work²searching for better materials and improved design changes to facilitate equipment reliabilit y. Repair work is often a part of improvement work. As shown in Figure 1, six maintenance programs are identified within the maintenance hierarchy, each representing an increased level of sophistication. Figure 1 Maintenance Hierarchy
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Essentially, OEE offers a measurement tool that helps identify the real areas of
opportunity within an operation. These areas have been termed the "six big losses." OEE
allows the firm to break these losses into smaller components to better evaluate the
impact the maintenance program is making on the operation. The six losses are:
1. Breakdowns from equipment failure (unplanned downtime)
2. Setup and adjustments from product changes and minor adjustments necessary
to get the equipment operating properly after the line change
3. Idling and minor stoppages due to abnormal operation of the equipment causing
momentary lapses in production, but not long enough to track as downtime
4. R educed speeds, the discrepancy between design and actual speed the equipment
operates
5. Process defects due to scrapped production and defects needing rework
6. R educed yield and lost materials during the manufacturing process, from start-
up to end of production run
If a company has an OEE of 85 percent or more, then it is considered to be a world-class
company.
TRENDS INM AINTENANCE
Two major trends in the development of maintenance management research have been
identified: (1) emerging developments and advances in maintenance technology,
information and decision technology, and maintenance methods; and (2) the linking of
maintenance to quality improvement strategies and the use of maintenance as a
competitive strategy.
The first major trend has to do with the impact of artificial intelligence techniques, such
as expert systems and neural networks, on the formation of maintenance knowledge inindustrial organizations. There is a diverse application of expert systems within the
maintenance area. A number of these systems and their applications are listed below:
y CATS²an expert maintenance system for detecting sudden failures in diesel-
y INNATE²an expert system used for electronic circuit diagnosis
y FSM²an expert system used by Boeing for continuous condition monitoring of
aircraft alarms
y RL A²an expert system developed by Lockheed for repair-level analysis for major
parts in an aerospace system
y GEMS-TTS²an expert system used by AT&T maintenance specialists to isolate
faults in communication links
y TOPAS²an expert system that diagnoses transmission and signaling problems in
real time that may arise on switched circuits.
y CHA RLE Y²an expert system used by General Motors to diagnose problems with
broken machine tools and to instruct less experienced individuals by providing
explanationsy XCON²an expert system developed by Digital Equipment Corporation (now part
of Compaq) for product configuration
The second major trend is typified by the emergence of total productive maintenance,
which must be incorporated into the firm's strategy. In the quest for world-class
manufacturing, many industries are appreciating the need for efficient maintenance
systems that have been effectively integrated with corporate strategy. It is vital that
maintenance management becomes integrated with corporate strategy to ensureequipment availability, quality products, on-time deliveries, and competitive pricing.
Managerial attitudes have changed toward maintenance because of the emergence of
new management philosophies. In addition, social trends such as lack of capital,
fluctuations in currencies, competition, quality, and environmental consciousness, have
also encouraged a new focus on maintenance.
Maintenance will continue to be a major area of concern for manufacturers and other
forms of business. A study of some seventy manufacturing plants found that over 50percent of the maintenance work performed by these firms was reactive (run to failure,
emergency breakdown). The balance of maintenance work was preventive or period
based (25 percent), predictive or condition based (15 percent), and proactive or root-
caused based (10 percent). A strong correlation has been found to exist between
manufacturing cost reduction and preventive/predictive maintenance. Over a five-year
It¶s not sufficient to evaluate only the units needing maintenance. Study the system, the workingenvironment in which the units operate and how the unit or system might affect the operation of theplant itself. Identify bottlenecks that can harm the whole system ² or the whole plant. Checkbottlenecks with extra care and evaluate their condition to ensure that no unforeseen problems canthreaten plant operation.
Keep in mind that the plant¶s designers probably didn¶t have maintenance foremost in their mind.Instead, they probably had instructions to design and build it as cheaply as possible. Also, the plantprobably was erected by workers having little knowledge about maintenance. Most plants can showhorrendous examples of the effect of this lack of knowledge. There¶s no doubt that plantmaintenance often can benefit from a bit of a redesign, an option that shouldn¶t be ruled out.
Maintenance requirements typically come in three varieties: condition-based, time-based or run tofailure. These classifications are by no means exhaustive and they¶re only meant to provideguidance when evaluating the kind of maintenance suitable for each unit evaluated.
Conditi on-based maintenance ( CBM)
This can be the most economical choice, but only if the cost of the monitoring devices isn¶t too highand extensive dismantling to check the condition of the equipment isn¶t required. CBM works bestwhen simple checks are sufficient to get an indication of the equipment¶s condition.
A simple visual inspection can detect leaks or other mechanical faults. You can use touch to detectheat and vibrations. You can listen for damaged bearings and even smell to detect overheating or oilleaks. Also, trending the recorded process variables can be used to detect the need for maintenance. However, for vital or costly equipment, use instruments because they detect faultslong before an operator can using only the five senses.
Time-based maintenance
This is the type that manufacturers normally recommended. It¶s based solely on the number of operating hours or calendar days a unit has been in operation. This method normally is used whencondition monitoring is too difficult or when there¶s a clear correlation between operating time andmechanical failure. When using time-based maintenance, consider how the unit is used. Therecommended maintenance intervals normally apply to units in fairly constant operation. It¶s notuseful for units that are mainly in standby mode.
For the latter type of equipment it¶s better to multiply the number of starts by an ³equivalent´ number of operating hours. For example, an emergency generator is tested every week (52 starts per year)and each start can be considered equal to 20 running hours. Multiplying the number of starts by 20gives a value that should be added to the true running hours to arrive at the maintenance trigger point. This method is recommended because each start causes more wear than an hour in normaloperation.
R un t o failure
This maintenance approach is used for highly reliable equipment, when it¶s difficult to performcondition monitoring or when instrumentation is costly in comparison to the equipment value.
Although not normally recommended, run to failure can be used if the unit in question won¶t interferewith plant operation should it cease to function, assuming it can be replaced easily and rapidly. If not, it might be possible to alter the installation to enable a rapid replacement. This maintenancetype can be tied to the unit ² if the unit needs to be included in the maintenance system at all.
The design of any vital system should be reconfigured if some run-to-failure component can fail often
and without warning.
Writing the pr ocedures
After the relevant data for each applicable unit has been captured and its maintenance requirementanalyzed, it¶s time write procedures. Enter them directly into the computerized maintenance systemor write them by hand on a note pad ² the important thing is to have the procedures written.
Although it¶s essential to involve plant personnel in the equipment analysis, it¶s probably unfair toforce them to enter data into the computer system. As one operator put it, ³If I wanted to sit and writeall day, I would have gotten me an office job.´ Management should consider outsourcing the initialmaintenance procedure drafting and development using a competent technical writer with
knowledge about maintenance and producing suitable procedures.
If plant personnel follow the advice given above, the result will be a valuable information resourcethat forms the basis of the comprehensive set of maintenance procedures. It would reduce the labor required to write the procedures, thus saving a substantial amount of money.
Gunnar Gustafsson is educated as a Marine Engineer with a long experience from operating and maintaining ships, oil rigs and power plants of various types, He now puts his education and experience to good use as a technical writer. Contact him at
The Maintenance Information Loop
Figure 1 below describes the flow of maintenance information and how the various aspects
In addition to that, there is value in constructing a hierarchy of the equipment system
showing assemblies, subassemblies and individual components. This helps to keep track of
which section of the system is being considered at any time, and the list of components also
helps to identify the spare parts requirements for the system.
Of vital importance is the clear identification of the root cause of each failure, as this willaffect the selection of a suitable maintenance task. To illustrate this point, consider for
example, a seized gearbox. ³Seized´ is an effect. There could be several root causes of this
failure mode that can be addressed in different ways through the maintenance program.
There is usually no value in aiming maintenance at the effect of a failure.
Also important from a planning perspective is to identify the time it will take to carry out
each task independently. The sum total of these task times gives a good indication of how
long the total work order will take.
All of the above depends on the production process and the site¶s operating context, so
these comments should be taken simply as a guideline.
The following are a few points to consider when constructing a preventive maintenance
program:
Preventive maintenance tasks must:
y aim at the failure process
y be specific
y include specifications or tolerances
Wherever possible, aim for predictive rather than preventive tasks
y measure or check for conditions against a standard
y report the results
y create a follow-on task to repair or replace at the next opportunity
³Check and replace, if necessary´ tasks destroy planned times
Frequencies and estimated times for each task must be accurate and meaningful
Try wherever possible to only plan shutdown time for ³non-running´ tasks. Keep ³running´
tasks to be done during periods of normal production. Structure the maintenance program
After analysing all the maintenance requirements for the equipment system, these
individual tasks would be grouped together to create the checklists, based on common
criteria for:
y Craft
y Frequencyy Safety / Non-safety tasks
y Running / Non-running checks and sensible
y Timing, etc. «
Smoothing the PM workload
In order to smooth the PM workload, a robust approach is to base the spread of PM
activities on the checklists arising from the RCM-style analysis. This assumes that the
analysis has been conducted thoroughly and that it is in a format that can be amended
easily.
The graph in Figure 2 below illustrates how it is possible to arrange the occurrence of the
PM work orders in such a way to create the smoothest possible flow of regular preventive
maintenance work, while still leaving enough time to carry out those ³follow-on´ corrective
maintenance tasks that were identified from conducting the preventive/predictive checks
during the last maintenance stop.
It is important to notice that just because two checklists may have the same frequency, it is
not necessary to schedule them to be done at the same time. Sometimes, of course, it does
make practical sense to schedule PMs for the same day, but don¶t assume that this is always
true. As a general rule, in an automated or continuous process production environment, thetotal amount of work on one checklist or work planned for one maintenance period should
not exceed 80 percent of the total time available.