System DFMEA:

The DFMEA will always take the feed from the Boundary and P-diagram or Parameter diagram. The ideal output from the boundary diagram will become the desired or required function in the DFMEA. In the case of System DFMEA, the cause will always be the inability to receive the ideal input/interface function of the surrounding sub system.

Every Item/Function should consist of all the attributes addressed in the Boundary diagram and all the ideal output function from the Parameter diagram (P-diagram). The signal factors are the ideal conditions under which input is provided to deliver the desired output.

The potential failure mode should consist of four anti-functions such as partial function, intermittent function, no-function and unintended function.

(Example:
Function of a transmission is to multiply and distribute torque and speed between upper and lower power train.

1. The potential failure modes for the above function would be,

• unintended torque and speed multiplication between upper and lower power train

This would assume that torque required is not as per specification, it could be more or way less than desired.

• inadequate torque and speed multiplication between upper and lower power train

This is assuming that torque requirement is not to the desired level, its way less than the specification.

• No torque  and speed multiplication between upper and lower power train

There is no supply of torque.

• intermittent torque and speed multiplication between upper and lower power train

This is assuming that there is fluctuation is supply of torque.

2. Now identify the effect of each failure mode identified above. There may be multiple effects with each failure mode.

a) failure mode:  unintended torque

Effect: Unintended transmission function (take from Parameter diagram's noise factors', it could be any thing in noise factor. If you do not find it in noise factor, then you missed adding the point to it. Add the newly thought of Effect of failure mode into proper column of noise factor.

b) failure mode:  inadequate torque

Effect: Low output from transmission (poor performance/ degraded performance should be picked from the noise factors in the Parameter diagram)

c) failure mode:  No torque           

Effect: Transmission does not operate, machine inoperable. This should also be a part of Noise factors in Parameter diagram)

d) failure mode:  intermittent torque

Effect: Sluggish performance, intermittent performance of the machine, this should be picked from the Noise factor in the Parameter diagram)

3. Brainstorm to get accurate causes of failure modes. There may be multiple causes to each of failure modes.
The causes at the system level should mode focus on the sub system failures. If doing the sub system, it should focus on the next level of assemble or if second last, then it should focus on the components. If the DFMEA is focusing on the component its causes should be the design geometry and features. Since this is the system level, lets focus on the sub system level failures as causes of System failures. This is derived from the Boundary diagram, where the system is interacting with the other surrounding sub system. During the interaction between the system and sub system you notice that there are features listed under each mating sub system. Brain storm and identify the function from the mating system, which if not performed will lead to the system failure.

a) failure mode:  unintended torque
Cause 1: Wrong Torque converter chosen with this transmission.(Torque converter is the mating component of Transmission)
Cause 2: Problem originating from the planetary system (Internal component of Transmission)
Cause 3: Weight of mating component used is heavier than anticipated.
b) failure mode:  inadequate torque
Cause 1: Internal component failure, please do internal system DFMEA to understand unintended torque.
Cause 2: Incorrect Engine size chosen for the transmission.(mating component from Boundary diagram)
Cause 3: Wrong Torque converter chosen with this transmission.(Torque converter is the mating component of Transmission)
c) failure mode:  No torque           
Cause 1: Axle failure (lower power train failure, this is also a mating part of transmission in the Boundary diagram)
Cause 2: Drive shaft failure.(internal connecting component from Boundary diagram)
d) failure mode:  intermittent torque
Cause 1: OIl system not delivering continuously (Internal component failure, this is taken from the Boundary diagram)
Cause 2: Planetary system failure.(internal component from Boundary diagram)

4. Controls factors are procedures applied in order to prevent the failure from occuring in the first place and if occurs for cause not identified, it should be easily detected. According to Ford motor corporation, detection is more important than preventive controls. The design controls include certain analysis test, simulations in form of preventive control and field test or other lab test in form of detective control. The main idea is to eliminate the failure from occuring in production of at customer. The controls for various products will be different, like for a transmission, DTS, Lab test or field simulation will be considered for a detection control and certain softwares to check efficiency and performance will be considered as preventive controls.