MAINTENANCE MANAGEMENT PHILOSOPHY -1979
Alumax Mt Holly
by John Day PE, Maintenance and Engineering Manager
(1 of 3 plants worldwide ever to be certified as ”World Class Maintenance”)
Alumax of SC began development of the maintenance management concept with the idea that maintenance would be planned and managed in a way that provides an efficient continuous operating facility at all times. Add to this that maintenance would also be treated as an investment rather than a cost, and you have the comprehensive philosophy on which the maintenance management system was built. An investment is expected to show a positive return, and so should maintenance be expected to improve the profitability of an operation. The management philosophy for maintenance is just as important as the philosophy established for any business operation. For most industry, maintenance is a supervised function at best, with little real cost control. But it must be a managed function employing the best methods and systems available to produce profitable results that have a positive effect on profitability.
The development of a philosophy to support the concept of proactive planned maintenance is important. It is believed that many maintenance management deficiencies or failures have resulted from having poorly constructed philosophies or the reliance upon procedures, systems, or popular programs that have no real philosophical basis.
A Maintenance Philosophy
by “Nowlan and Heap RCM Study“
(1978 Report – United Airlines and US Department of Defense)
(The Foundation of Modern Maintenance and Reliability Programs)
A. Operator’s maintenance program has four objectives:
1. To ensure realization of the inherent safety and reliability levels of the equipment
2. To restore safety and reliability to their inherent levels when deterioration has occurred
3. To obtain the information necessary for design improvement of those items whose inherent reliability proves inadequate
4. To accomplish these goals at a minimum total cost, including maintenance costs and the costs of residual failures
B. Maintenance and Reliability is based on the following precepts:
1. A failure is an unsatisfactory condition
2. There are two types of failures:
a. Function Failures, usually reported by operating crews
b. Potential Failures, usually discovered by maintenance crews.
C. The consequences of a functional failure determine the priority of maintenance effort.
1. These consequences fall into four categories:
a. Safety consequences – Involving possible loss of the equipment and its occupants
b. Operational consequences – Which involve an indirect economic loss as well as the direct cost of repair
c. Nonoperational consequences – Which involve only the direct cost of repair
d. Hidden-failure consequences – Which involve exposure to a possible multiple failure as a result of the undetected failure of a hidden function
D. Scheduled maintenance is required for any item whose loss of function or mode of failure could have safety consequences.
1. If preventive tasks cannot reduce the risk of such failures to an acceptable level, the item must be redesigned to alter its failure consequences.
2. Scheduled maintenance is required for any item whose Functional failure will not be evident to the operating crew, and therefore reported far corrective action.
E. In all other cases the consequences of failure are economic, and maintenance tasks designed at preventing such failures must be justified on economic grounds.
1. All failure consequences, including economic consequences, are established by the design characteristics of the equipment and can be altered only by basic changes in the design:
2. Safety consequences can in nearly all cases be reduced to economic consequences by the use of redundancy.
3. Hidden functions can usually be made evident by instrumentation or other design features.
F. The feasibility and cost effectiveness of scheduled maintenance depend on the inspect-ability of the item and the cost of corrective maintenance depends on its failure modes and inherent reliability.
G. The inherent reliability of the equipment is the level of reliability achieved with an effective maintenance program”. This level is established by the design of each item and the manufacturing processes that produced it.
1. Scheduled maintenance can ensure that the inherent reliability of each item is achieved, but no form of maintenance can yield a level of reliability beyond that inherent in the design.
2. A Reliability Centered Maintenance Program includes only those tasks which satisfy the criteria for both applicability and effectiveness. The applicability of the item and its effectiveness is defined in terms of consequences the task is designed to prevent.
H. There are four basic types of tasks that mechanics can perform, each of which is applicable under a unique set of conditions. The first three tasks are directed at preventing functional failures of the items to which they assigned and the fourth is directed at preventing multiple failures involving that item:
1. On-Condition inspections of an items and correct any potential failures
2. Overhaul of an item at or before some specific age limit
3. Discard of an item (or one of its parts) at or before some specific life limit
4. Failure-finding inspections of a hidden-function item to find and correct functional failures that have already occurred but were not evident to the operating crew
I. A simple item, one that is subject to only one or a very few failure modes, frequently shows a decrease in reliability with increasing operating age. An age limit may be useful in reducing the overall failure rate of such items, and safe-life limits imposed on a single part play a crucial role in controlling critical failures.
J. A complex item, one whose functional failure may result from many different failure modes, shows little or no decrease in overall reliability with increasing age unless there is a dominant failure mode. Age limits imposed on complex components and systems (including the equipment itself) therefore have little or no effect on their overall failure rates.
K. The RCM decision diagram provides a logical tool for determining which scheduled tasks are either necessary or desirable to protect the safety and operating capability of the equipment.
1. The resulting set of RCM tasks is based on the following considerations:
2. The consequences of each type of functional failure
a. The visibility of a function..l failure to the operating crew (evidence that a failure has occurred)
b. The visibility of reduced resistance to failure (evidence that failure is imminent)
c. The age-reliability characteristics of each item
d. The economic tradeoff between the cost of scheduled maintenance and the benefits to be derived from it
3. A multiple failure, resulting from a sequence of independent failures, may have consequences that would not be caused by any one of the individual failures alone. These consequences are taken into account in the definition of the failure consequences for the first failure.
4. A default strategy governs decision making in the absence of full information or agreement. This strategy provides for conservative initial decisions, to be revised on the basis of information derived from operating experience.
L. A scheduled-maintenance program must be dynamic. Any prior-to service program is based on limited information, and the operating organization must be prepared to collect and respond to real data throughout the operating life of the equipment.
1. Management of the ongoing maintenance program requires an organized information system for surveillance and analysis of the performance of each item under actual operating conditions.
2. This information is needed for two purposes:
a. To determine the refinements and modifications to be made in the initial maintenance program (including the adjustment of task intervals)
b. To determine the needs for product improvement
3. The information derived from operating experience has the following hierarchy of importance:
a. Failures that could affect operating safety
b. Failures that have operational consequences
c. The failure modes of units removed as a result of failures
d. The general condition of un-failed parts in units that have failed
e. The general condition of serviceable units inspected as samples
M. At the time an initial program is developed information is available to determine the tasks necessary to protect safety;, and operating capability.
1. However, the information required to determine optimum task intervals and the applicability of age limits can be obtained only from age exploration after the equipment enters service.
2. With any new equipment there is always the possibility of unanticipated failure modes. The first occurrence of any serious unanticipated failure immediately sets in motion the following product-improvement cycle:
a. An on-condition task is developed to prevent recurrences while the item is being redesigned.
b. The operating fleet is modified to incorporate the redesigned part.
c. After the modification has proved successful, the special task is eliminated from the maintenance program.
d. Product improvement, based on identification of the actual reliability characteristics of each item through age exploration, is part of the normal development cycle of all complex equipment.
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4200 Faber Place Drive
Charleston, SC 29405