Maintenance Processes
This topic covers:
Maintenance is:
"All actions necessary for retaining an item as near as practical to its original condition, but excludes building cleaning and rehabilitation".
There are two categories of maintenance
Within each category of maintenance there are a number of processes:
Preventive maintenance actions are those which delay or prevent the occurrence of a known failure mode. Two conditions are necessary for effective preventive maintenance:
-
The item must exhibit a well defined wear characteristic such that the chance of failure increases with time
-
Some effective action must be available which will consistently delay, prevent or predict the occurrence of the identified failure mode.
Preventive maintenance is designed for performance either before an item fails or reaches the point where an unacceptable level of performance has occurred. However, because of the two above conditions, which must be present before effective preventive maintenance can be performed, this form of maintenance is only applicable to a limited class of items.
The concept of preventive maintenance can best be understood by reference to the following examples:
Lubrication - Items incorporating bearings or mechanical linkages require lubrication as a part of their basic design to reduce the wear rate. The lubricant deteriorates with time and use, which reduces its effectiveness. Renewal of that lubricant is a preventive task in that it is intended to restore design operating conditions and delay wear out or failure of the item.
Replacement - Brakes and rotating electrical machinery are representative of items on which a particular component “wears out”. The brake pads wear at a relatively high rate while other parts such as the discs wear at a lower rate. In the case of a motor, brushes exhibit a relatively high wear rate. In both cases, wear beyond a certain limit will cause failure of the item. Replacement of brake pads or brushes before the wear limit is reached may prevent failure and extend the operating life of the item.
Although preventive maintenance is intended to prevent or delay failures, through action designed to prevent an identified cause of failure before it occurs, it can never be completely effective. Some items, which are subject to preventive maintenance, will still fail before the preventive task is due to be performed. These failures take place because the rate of wear cannot be predicted with complete accuracy and because random failures not associated with the identified wear pattern, occur. This is apparent in the Bath Tub curve.
Servicing
Servicing tasks are usually associated with the replenishment of consumables to maintain equipment in operating condition and general observation tasks. Servicing is most effective when it is closely related to the failure being considered. Some servicing should be carried out while equipment is operating otherwise the fault cannot be detected, while other servicing is best done before or after operating.
Condition monitoring comprises those maintenance actions that serve to determine the condition and/or correct functioning of equipment. It includes inspection and testing. Condition monitoring tasks do not provide any inherent preventive effect, but serve to establish the condition of an item at the time the maintenance was performed. If condition monitoring leads to the detection of an unsatisfactory condition, either preventive or corrective maintenance action may be necessary. Condition Monitoring can be further subdivided into the following two categories:
-
Specific condition monitoring tasks
-
General condition monitoring tasks.
Specific Condition Monitoring
These tasks comprise a specific inspection or test, performed on a specified item or area to detect some specific condition. A specific task will always result in a maintenance instruction in the form of Inspect/Test Item X for Condition Y. A number of conditions must be met before a specific task can be considered to be effective:
-
There must be a reasonable chance that the condition for which the inspection is being performed will occur on the item being examined
-
The task prescribed must be capable of detecting the condition if it is present
-
It must be possible (usually by measurement) to predict when failure will occur or when the next condition assessment should be carried out.
General Condition Monitoring
In many cases, it is neither possible nor appropriate to define a specific condition-monitoring task for an item. This is because of insufficient information or history on possible failure modes, or because the most likely failure modes cannot be detected by inspection, e.g. failure of electronic equipment. However, in such cases it may be desirable to inspect the item to detect any unsatisfactory condition that is developing. This type of inspection is termed general and is usually carried out to detect obvious faults or developing problems not covered by specific condition monitoring or preventive tasks. Confidence checks are typical of this type of maintenance.
Corrective maintenance comprises those actions necessary to restore items or systems to a satisfactory condition or level of performance after a failure or other unsatisfactory condition has been detected. It may be performed on any item irrespective of whether another form of maintenance, i.e. preventive or condition monitoring, is also defined for the item. It can be planned or unplanned. In some cases, the corrective maintenance action necessary may be equivalent to a preventive task defined for the item.
Repair is a corrective maintenance process, involving detection and correction of a fault that has caused failure. Repair is applicable to both systems and items and is carried out on an unscheduled basis. The repair process is limited to fault isolation followed by disassembly of the item to that level necessary to restore or replace failed parts, reassembly and test. It does not include systematic inspection and restoration of those parts that have not failed but may lead to additional work if some obvious deficiency is noted during the course of repair activity. For some types of items, particularly electronic items, repair is the only effective maintenance process that can be performed.
In some instances a maintenance process that enables the system or item to meet its operational objective cannot be applied. The only effective solution is redesign. Some organizations class this as re-engineering.
Specific changes to an item to improve its characteristics or to add new ones are classed as modifications. They are usually carried out when preventive or condition-monitoring tasks identify a need.
The throw away process depends on the application of the item, and is appropriate when:
-
The use of any other maintenance process is uneconomical
-
The useful life of the item is limited for reason such as installed life or structural fatigue e.g. hydraulic hoses.