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When you select the Repairable Systems allocation model in the Allocation utility, the Elements field in the control panel will update automatically to display the number of blocks that you have selected to include in the analysis (i.e., the number of check boxes that are selected in the Include column of the table).
In the Availability Goal field in the control panel, enter the desired overall availability for the system, expressed as a decimal from 0 to 1, excluding 0. Then click Calculate to obtain the results.
The following results will be displayed in the table for each block selected to be included in the analysis:
MTTR is the mean time between repairs, expressed in hours.
Allocated Failure Rate is the recommended failure rate for the block, based on the allocation analysis. This calculated result is expressed in failures per million hours (FPMH) for MIL-217 and NSWC, and in failures per billion hours (FITS) for Bellcore/Telcordia and FIDES.
Allocated MTBF (hrs) is the recommended mean time between failures for the block, based on the allocation analysis. This calculated result is always expressed in hours.
Current Failure Rate is the current failure rate for the block based on the current reliability prediction so you can compare with the recommended value.
Description:
Allocates failure rates to subsystems in series with constant failure rates.
The allocated subsystem failure rates allow the system to meet an availability objective.
Assumes that all subsystems are identical and have a constant repair rate.
The steps are as follows:
Determine the steady-state availability for each subsystem using the expression:
where:
A = Required system availability.
k = Number of subsystems.
Determine the ratio of the allocated failure rate to the repair rate for each subsystem using the expression:
Determine the failure rate allocated to each subsystem with the expression:
where μi = Repair rate for each subsystem.
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