Optimum Replacement Window

The Optimum Replacement window helps you to determine the best time to replace components, based on the costs for planned (i.e., preventive) and unplanned (i.e., corrective) maintenance.

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.)

To access the Optimum Replacement window, choose FMRA > Calculations > Optimum Replacement.

Available Items

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.

Planned and Unplanned Replacement Costs

In the table, the columns with white backgrounds require input. The Planned Replacement Cost and Unplanned Replacement Cost are always required. Simply type the cost directly into each cell.

Calculation Options

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.

Results and Plots

The table shows the results for each item. The total costs per unit time based on all included items are displayed under the table.

Click Cost vs. Time Plot to show a plot of the results.

Create Tasks

You can click Create Maintenance Task(s) > At Replacement Time to create preventive tasks for one or more of the items.