Optimum Replacement Window

Preventive maintenance is a logical choice if, and only if, the following two conditions are met:

Condition #1: The failure rate of the component in question increases with time, implying wearout.
Condition #2: The overall cost of the preventive maintenance action is less than the overall cost of a corrective action. (The overall cost for a corrective action should include ancillary tangible and/or intangible costs, such as downtime costs, loss of production costs, lawsuits over the failure of a safety-critical item, loss of goodwill, etc.)

If both of these conditions are met, then preventive maintenance makes sense. The Optimum Replacement window allows you to specify costs for planned (i.e., preventive) and unplanned (i.e., corrective) replacement tasks. It then uses this information to determine the most cost-effective time to perform the replacements, either individually or as a group.

What's Changed? In previous versions, optimum replacement intervals were calculated for each individual replacement task, via the task's properties window. Version 8 now allows you to calculate optimum replacement intervals at any level with an associated URD, either individually or in groups.

To access the Optimum Replacement window, choose FMRA > RAM Analysis > Optimum Replacement.

The Available Items area on the left side of the window shows, for the current branch of the system hierarchy, any item, function, failure or cause that has an associated URD. The following requirements must be met for an item to be available for inclusion in the optimization calculations:

Must have a failure distribution.
The failure model must meet the following requirements.
- Fixed reliability and exponential distributions are not allowed.
- Weibull distributions must have beta > 1.
- Gamma distributions must have k > 1.
- Loglogistic distributions must have std < 1.
- For generalized gamma distributions and mixed Weibull distributions, there must be an increasing failure rate within at least one time period. A local optimum can then be found within intervals with an increasing failure rate.

You can select an individual item's check box or drag it into the table on the right side of the window to include the item in the optimization calculations. You can also select the check box in the Available Items header to select all check boxes. To remove items from the optimization calculations, you must clear the check box(es); you cannot drag items out of the table.

For each item that is included in the optimization calculations, you will need to specify the cost for a planned replacement and the cost for an unplanned replacement. This is done by entering the projected costs in the Planned Replacement Cost and Unplanned Replacement Cost columns of the table.

When you click Calculate, you can choose from the following options:

Calculate the individual optimum replacement time for each item.
Calculate a common optimum replacement time, which is the most cost-effective time to replace all of the selected items concurrently.
Calculate clustered optimum replacement times, which allows you to specify a number of times to replace groups of the selected items. For example, if you enter 3 in the Number of clusters field, the software will determine the three best times to replace groups of items such that all of the selected items are replaced at one of the three times.

The software will calculate the optimum replacement time for the items based on their failure characteristics and on the defined costs. This value will be displayed in the Optimum Replacement Time column of the table. In addition, the Cost per Unit Time column will display the estimated cost per unit time for a replacement performed at the calculated optimum replacement time. Multiply the replacement time by the cost per unit time to obtain the cost of replacing the item at the optimum replacement time.

You can click Create Maintenance Task(s) at Replacement Time to create a preventive task for one or more of the items. The window that appears allows you to select which items to create tasks for. The preventive tasks created in this way restore the item to as good as new condition and are scheduled at fixed intervals based on item age, equal to the optimum replacement time.

You can click Cost vs. Time Plot to show a plot of the results. If you have calculated individual optimum replacement times, the plot will show the cost for each item as a function of time. If you have calculated a common optimum replacement time, the plot will show the common cost as a function of time. If you have calculated clustered optimum replacement times, the plot will show the cost for each cluster as a function of time.