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In life data analysis, it is assumed that the components being analyzed are non-repairable; that is, they are either discarded or replaced upon failure. When non-repairable components are assembled as part of a system, the system's maintenance activities will involve replacing those worn or failed components. With the Maintenance Planning tool, you can use the information obtained from life data analysis to determine the most cost-effective time to replace an individual component.
The Maintenance Planning tool offers two strategies:
Planned replacement is the practice of replacing components before they fail in order to promote continuous system operation or to avoid dangerous or costly failures. Cost is always a factor with scheduled replacements: replace components too often and you increase the maintenance costs; schedule replacements too far apart and you increase the risk of failures. The goal, then, is to minimize overall costs by finding the optimal time to replace components.
Inspections are used to reveal impending failures, such as corrosion or other forms of degradation. In general, a component is not replaced during inspection unless it is found failed, but if there is evidence that the component is approaching the end of its life, then it may be possible to schedule a replacement at the earliest convenience rather than allowing the failure to occur and possibly cause severe consequences. The goal, then, is to find the optimal time to perform the inspections such that oncoming failures are detected and the component can be scheduled for replacement before any failures occur.
The tool uses the optimum age replacement model to estimate the ideal planned replacement and inspection intervals. The model uses the following assumptions: a) the failure rate of the component increases with time, implying wearout, and b) the cost of replacing the component before failure occurs is significantly less that the cost of replacing the component at the time of failure.
Follow the steps outlined below to generate a cost vs. time plot.
Open the tool by choosing Insert > Tools > Maintenance Planning.
Select the data sheet to use for the analysis. Note that the data sheet must have already been analyzed (i.e., the parameters of the distribution have been calculated) in order for you to select it.
Click the Plot Type drop-down list on the control panel and select which plot to generate. You can choose to create a Planned Replacement plot, an Inspection Replacement plot, or place both on the same plot.
In the Costs area, enter the required inputs. If you have selected the Planned Replacement plot, only the first two inputs are required. For the two other plots, all four inputs are required. These are:
Planned Replacement is the average cost of replacing the component before failure occurs. This includes downtime costs and other associated costs.
Unplanned Replacement is the average cost of replacing the component due to failure. This includes downtime costs and other costs associated with the risk, such as lawsuits over the failure of a safety-critical item, loss of goodwill, etc.
Inspection is the average cost of each inspection procedure.
Failure Detection is a value between 0 and 1. It indicates the percentage of the component's life that must have elapsed in order for the oncoming failure to be detected. For example, if the component is estimated to fail in 1,000 days and the failure detection threshold value is 0.9, then the oncoming failure can begin to be detected at 900 days.
Note: By default, the planned and unplanned replacement costs are based on the average long-term costs, but there may be cases where the average costs change with time, or when components are replaced with either costlier or less expensive versions. In this situation, you may wish to compute for the optimal time based on the average costs within one replacement cycle (defined as the time when a failure occurs or the time of the scheduled replacement when no failure occurs). To make this change, select the Plot One Cycle Cost/Time option on the Settings page of the control panel.
In the Scaling area, the X and Y boxes show the minimum and maximum values for the x- and y-axes. To change these values, clear the check box beside the value range. If it is selected, the application will automatically choose appropriate values for the range.
To calculate the results and generate the plot, choose Maintenance Planning > Analysis > Plot or click the icon on the control panel.
The Parameters area of the control panel shows the parameters of the distribution of the data set, while the Results area shows the estimated minimal cost and optimal time for the replacement. To see a summary report of the analysis, click anywhere within these two areas. To open the standard folio that contains the data set, click the blue link that displays the name of the folio and data sheet.
You can use the results from the maintenance plan to create preventive and/or inspection tasks for use in BlockSim simulation diagrams. In this case, the tasks represent the maintenance activities for non-repairable components (i.e., the tasks are configured to 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).
To create a task, choose Maintenance Planning > Analysis > Create Task.
This opens a Maintenance Task window that is identical to the interface used in the BlockSim application. The Task Scheduling and Restoration properties are pre-configured based on the analysis results. You can edit these properties and/or define all the other properties of the task, if desired.
When you create a task, the task's name will appear as a link on the control panel, under the Results area. You can click the link to view its properties.
The tasks you create can also be accessed by both applications via the Scheduled Tasks page of the Resource Manager by choosing Project > Synthesis > Resource Manager.
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