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There are three types of maintenance tasks: (1) breakdown, (2) corrective, and (3) preventive. The main difference in these types occurs at the point when the repair or maintenance task is performed. In breakdown maintenance, repairs don’t occur until the machine fails to function. Preventive maintenance tasks are implemented before a problem is evident and corrective tasks are scheduled to correct specific problems that have been identified in plant systems.
A comprehensive maintenance program should use a combination of all three. However, most domestic plants rely almost exclusively on breakdown maintenance to maintain their critical plant production systems.
WHAT IS BREAKDOWN MAINTENANCE?
In these programs, less concern is given to the operating condition of critical plant machinery, equipment, or systems. Since most of the maintenance tasks are reactive to breakdowns or production interruptions, the only focus of these tasks is how quickly the machine or system can be returned to service. As long as the machine will function at a minimum acceptable level, maintenance is judged to be effective. This approach to maintenance management is both ineffective and extremely expensive. Breakdown maintenance has two factors that are the primary contributors to high maintenance costs: (1) poor planning and (2) incomplete repair.
The first limitation of breakdown maintenance is that most repairs are poorly planned because of the time constraints imposed by production and plant management. As a result, manpower utilization and effective use of maintenance resources are minimal. Typically, breakdown or reactive maintenance will cost three to four times more than the same repair when it’s well planned.
The second limitation of breakdown maintenance is that it concentrates repair on obvious symptoms of the failure, not the root cause. For instance, a bearing failure may cause a critical machine to seize and stop production. In breakdown maintenance, the bearing is replaced as quickly as possible and the machine is returned to service. No attempt is made to determine the root cause of the bearing failure or to prevent a recurrence of the failure. As a result, the reliability of the machine or system is severely reduced. This normal result of breakdown maintenance is an increase in the frequency of repairs and a marked increase in maintenance costs.
WHAT IS PREVENTIVE MAINTENANCE?
The concept of preventive maintenance has a multitude of meanings. A literal interpretation of the term is a maintenance program that's committed to the elimination or prevention of corrective and breakdown maintenance tasks. A comprehensive preventive maintenance program will utilize regular evaluation of critical plant equipment, machinery, and systems to detect potential problems and immediately schedule maintenance tasks that will prevent any degradation in operating condition.
In most plants, preventive maintenance is limited to periodic lubrication, adjustments, and other time-driven maintenance tasks. These programs are not true preventive programs. In fact, most continue to rely on breakdowns as the principal motivation for maintenance activities.
A comprehensive preventive maintenance program will include predictive maintenance, time-driven maintenance tasks, and corrective maintenance to provide comprehensive support for all plant production or manufacturing systems.
WHAT IS CORRECTIVE MAINTENANCE?
The primary difference between corrective and preventive maintenance is that a problem must exist before corrective actions are taken. Preventive tasks are intended to prevent the occurrence of a problem. Corrective tasks correct existing problems.
Corrective maintenance, unlike breakdown maintenance, is focused on regular, planned tasks that will maintain all critical plant machinery and systems in optimum operating conditions. Maintenance effectiveness is judged on the life cycle costs of critical plant machinery, equipment, and systems, not on how fast a broken machine can be returned to service.
Corrective maintenance, as a subset of a comprehensive preventive maintenance program, is a proactive approach toward maintenance management. The fundamental objective of this approach is to eliminate breakdowns, deviations from optimum operating condition, and unnecessary repairs and to optimize the effectiveness of all critical plant systems.
The principal concept of corrective maintenance is that proper, complete repairs of all incipient problems are made on an as-needed basis. All repairs are well planned, implemented by properly trained craftsmen, and verified before the machine or system is returned to service. Incipient problems aren’t restricted to electrical or mechanical problems. Instead, all deviations from optimum operating condition, i.e., efficiency, production capacity and product quality, are corrected when detected.
CORRECTIVE MAINTENANCE: PREREQUISITES:
Corrective maintenance can't exist without specific support efforts. A number of prerequisites must exist before corrective maintenance can be properly implemented.
INCIPIENT PROBLEMS -- ACCURATE IDENTIFICATION:
Both preventive and corrective maintenance programs must be able to anticipate maintenance requirements before a breakdown can occur. A comprehensive predictive maintenance program that has the ability to accurately identify the root cause of all incipient problems is the first requirement of corrective maintenance. Without this ability, corrective actions can't be planned or scheduled.
All corrective repairs or maintenance must be well planned and scheduled to minimize both cost and interruption of the production schedule. Adequate time must be allowed to permit complete repair of the root cause and resultant damage caused by each of the identified incipient problems.
Proper maintenance planning is dependent on well-trained planners, a viable maintenance data base, and complete repair procedures for each machine train or system within the plant.
Trained Maintenance Planners
Many plants don’t have full-time maintenance planners or their planners lack the knowledge or skills that the job demands. It’s therefore imperative that proper training is provided to ensure that each planner has the skills necessary to properly plan repairs and maintenance tasks.
Maintenance History Database
The planner must have accurate maintenance history in order to properly plan repairs. As a mini mum, he must know the standard mean-time-to-repair for every recurring repair, rebuild, and maintenance task required to maintain optimum operating condition of critical plant machinery, equipment, and systems. Without this knowledge, he can't plan an effective repair.
In addition, the planner must know the specific tools, repair parts, auxiliary equipment, and craftsmen skills required to complete each maintenance task. This information, in conjunction with proper repair sequence, is an absolute requirement of a viable repair plan.
This type of information requires a comprehensive maintenance database that compiles actual mean-time-to-repair, standard repair procedures, and the myriad of other information required for proper maintenance planning.
PROPER REPAIR PROCEDURES:
Repairs must be complete and properly implemented. In many cases, poor maintenance or repair practices result in more damage to critical plant machinery than the observed failure mode.
A fundamental requirement of corrective maintenance is proper, complete repair of each incipient problem. To meet this requirement, all repairs must be made by craftsmen who have the necessary skills, repair parts, and tools required to return the machine or system to as-new condition.
A growing number of maintenance craftsmen don’t have the minimum skills required to properly maintain or repair plant equipment, machinery, or systems. In many cases, they can't properly install bearings, align machine trains, or even balance rotating equipment. Few have the knowledge and skills required to properly disassemble, repair, and reassemble the complex machinery or systems that comprise the critical production systems within plants.
A prerequisite of corrective maintenance is skilled craftsmen. Therefore, plants must implement a continuous training program that will provide the minimum craftsman skills required to support their production or manufacturing systems. The training program should include the means to verify craftsman skills and periodically refresh these skills.
Standard Maintenance Procedures
All recurring repairs and maintenance tasks should have a standard procedure that will specifically define the correct method required for competition. These procedures should include all of the information, such as tools, safety concerns, and repair parts, required for the task and a step-by-step sequence of tasks required to complete the repair.
Each procedure should be complete and contain all information required to complete the repair or recurring preventive maintenance task. The craftsman should not be required to find or have additional information in order to complete the repair.
SUFFICIENT TIME TO REPAIR
One of the fundamental reasons that most plants rely on breakdown maintenance is that tight production schedules and management constraints limit the time available for maintenance. The only way to reduce the number and frequency of breakdown repairs is to allow sufficient time for proper maintenance.
Plant management must permit adequate maintenance time for all critical plant systems before either preventive or corrective maintenance can be effective. In the long term, the radical change in management philosophy will result in a dramatic reduction in the downtime required to maintain critical production and manufacturing equipment. Machinery that's maintained in as-new condition and not permitted to degrade to a point that breakdown or serious problems can occur will require less maintenance than machinery maintained in a breakdown mode.
The final prerequisite of corrective maintenance is that all repairs or rebuilds must be verified before the machine train or system is returned to service. This verification process will ensure that the repair was properly made and that all incipient problems, deviations from optimum operating conditions, or other potential limitations to maximum production capacity and reduced product quality have been corrected.
CORRECTIVE MAINTENANCE: ROLE
Corrective maintenance will remain a critical part of a comprehensive plant maintenance program.
However, the objective of a viable preventive program is to eliminate all breakdown maintenance and severely reduce the number and frequency of corrective maintenance actions.
The ultimate objective of any maintenance program should be the elimination of machine, equipment, and system problems that require corrective actions.