Maintenance Frequency
This topic has a brief overview and covers:
The objective of establishing a maintenance frequency is to preserve the inherent levels of reliability and safety of equipment at minimum cost. There usually is no precise method of establishing the "correct" maintenance interval due to:
- Lack of detailed performance data
- Differences in the use of similar items
- Lack of complete understanding about the behavior of the item.
The process involves setting an initial interval following a Failure Mode, Effects and Criticality Analysis, and progressively refining the interval based on maintenance and performance data collected for the item.
There are two intervals that are considered:
- Fixed Interval which is used to express the maximum interval between maintenance tasks
- On-Condition Interval where the maintenance action depends upon the item reaching some pre-determined condition.
Fixed intervals include:
- Elapsed calendar time
- Number of operations
- Operating hours
- Any other measured parameter.
It is appropriate for items that exhibit a wear pattern for which an effective preventive or condition monitoring task has been identified. It may be appropriate if the item is:
- Safety critical and the task will prevent personnel injury or property damage
- Environmentally critical and the task will be effective in maintaining a high level of public confidence
- Function critical and the task will maintain a high level of reliability
- Likely to damage other equipment (secondary damage) if it fails
- Subject to some economic consideration which justifies doing the task at a fixed interval e.g. Lubrication.
Preventive or condition monitoring tasks at a fixed interval ensures the reliability of certain items. The combination of an effective maintenance task with an inappropriate maintenance interval (frequency) will result in an ineffective and inefficient operation. Some considerations when determining an interval are as follows:
Level of Reliability Required
In the case of safety critical and environment critical items and to a lesser extent function critical items, the interval must be set to ensure that the number of failures is held to an acceptable level.
Frequency Parameter
The particular frequency parameter used, e.g. operating hours or elapsed time, must relate to the actual cause of failure. For example, if failures occur as the result of hours in operation, a maintenance interval based on calendar time will normally not be effective.
Level of Over-Servicing
Except for safety and environment critical items, where a level of over servicing can be tolerated, maintenance intervals should be set at the maximum figure consistent with the level of reliability required. In most cases, this will involve progressive extension of the interval based on analysis of operating and failure data using the Failure Mode Effects and Criticality Analysis process.
An interval of on-condition (OC) is used when the need for a maintenance process is to be determined through evaluation of the actual condition of an item. As a result, the intervals at which successive maintenance actions of the same type are performed on the item are of variable length.
An OC interval is appropriate for a wide range of items, and should be considered for all tasks. However, the use of an OC interval requires that some means of monitoring or establishing the condition of the item is available. This can be done either by routine monitoring of the performance of the item by operators or by condition monitoring of the item to establish actual physical condition.
The term on-condition does not mean on failure, even though in some cases the occurrence of a failure may be the only means of determining that some maintenance action is required. On-condition tasks are designed for performance before complete failure occurs, when either the performance of the item deteriorates to an unacceptable level, or the physical inspection detects an unacceptable condition. Even though an OC interval is applied, a specific task or process will still be performed when the condition of the item indicates that maintenance is warranted.
MTBF is defined as the average life of a group of items and is a useful measure of reliability for a number of engineering and maintenance planning functions. It is not appropriate to use the MTBF as the maintenance interval as it may lead to a progressive and unnecessary reduction in achieved life if the frequency is maintained to the MTBF.
Expressing Maintenance Intervals
The maintenance frequency should be expressed in a standard format to ensure consistency throughout and to meet the requirements of the maintenance management system e.g. 4 WEEKS, 12 WEEKS, 1 MONTH, 3 MONTHS, 100 hours, 200 hours etc.