Learn How to Apportion Weight Factors to Achieve a Failure Rate Goal

You use allocations in reliability predictions to set reliability goals. Allocations allow you to assign a failure rate to an assembly and then, based on weight factors, calculate the projected failure rate of components and/or subassemblies within that assembly.

Using the apportionment method for allocation calculations, you enter the desired failure rate of a particular assembly as a failure rate goal or MTBF (Mean Time Before Failures) goal. You then assign weight factors for each assembly. The allocated values are then determined by totaling the weight factors and assigning failure rates based on percentages.

For example, let's assume that you have a multi-level system tree and want to enter a System Failure Rate Goal with allocations for two subsystems and several assemblies as follows:

System Failure Rate Goal: 1000 FITs.

Subsystem Allocations:
      A = 60% of the system = 600 FITs.
      B = 40% of the system = 400 FITs.

Assembly Allocations:
      A1 = 50% of Subsystem A = 300.
      A2 = 50% of Subsystem A = 300.
      B1 = 75% of Subsystem B = 300.
      B2 = 25% of Subsystem B = 100.

The system tree for this scenario would look like this:

1. System (1000)
      a. Subsystem A (60%)
            i. Assembly A1 (50%)
            ii. Assembly A2 (50%)
      b. Subsystem B (40%)
            i. Assembly B1 (75%)
            ii. Assembly B2 (25%).

Assembly B2 is 25% of the Subsystem B, and Subsystem B is 40% of the system total. Therefore, Assembly B2 is 10% of the system total.

Allocation Calculations

The equation used for calculating an allocated failure rate is:

Failure Rate Goal * Weight Factor for individual part or assembly / Total of all Weight Factors

For example, if an assembly has a specified failure rate of 100, and 4 parts are in the assembly with weight factors 1, 2, 3, and 4, the allocated failure rate for part 1 = 100 * (1/(1+2+3+4)) = 10. The other allocated failure rates are 20, 30, and 40 respectively.

If other parts are present in the assembly with calculated or specified failure rates (non-allocated), these failure rates are subtracted from the total. Using the above example, if other parts in the assembly have a total failure rate of 50, then the allocated failure rate of part 1 = (100-50) * (1/1+2+3+4)) = 5. The other parts would have failure rates of 10, 15, & 20 respectively.

If you would like information about how the Relex Reliability Prediction Module uses the apportionment method for allocation calculations, please email info@relexsoftware.com.