Repairable System Allocation Method Calculations
The repairable system allocation method is similar to the Equal Apportionment method. However, instead of allocating failure rate (or reliability) equally for all subsystems, availability is allocated equally for all subsystems.
In the repairable system allocation method, the required availability goal is entered. No weight factors are entered. Instead, MTTF values are obtained from MTTR in the System Tree Items table. Only specified MTTR values are used to compute failure rate allocations.
The equation for computing availability is:
Where n equals the number of subsystems.
The equation for computing failure rate is:
Example
Required System Availability = 0.99 in a system with three subsystems. The MTTR values for the three subsystems are given in the table below.
Subsystem | MTTR |
|---|---|
Subsystem A | 0.2 |
Subsystem B | 0.5 |
Subsystem C | 0.3 |
For the top-most assembly (system), you enter an availability goal of 0.99 in its Allocation pane. For more information, see Repairable System Method. No other allocation data is required. However, to obtain the results shown below, under General in the Project properties, Repair Time units must be set to Hours. Additionally, under General in the System file properties, Failure Rate Multiplier must be set to 1000000 so units are failures per million hours. For more information, see:
Calculations for system availability and subsystem failure rates follow.
Because the failure rates of the three subsystems in the above calculations are in failures per hour, to get the same failure rate as the subsystem, they must be multiplied by 1000000. Thus, the failure rate of subsystem A is 16779, the failure rate of subsystem B is 6711, and the failure rate of subsystem C = 11186.