FMEA - Rate all existing Design Controls, calculate the RPN, and establish corrective actions required

In this article I will discuss design controls and their effectiveness and look at how to calculate RPN and establish a priority for any necessary actions.

As I come to the end of this series of FMEA articles I would like to invite you to download a free copy of our FMEA Snap-Shot. The Snap-Shot series are key process documents that have been produced as a guide to the process steps of (in this case) FMEA. Our clients use these during FMEA meetings as an aid for the team. Please use the button at the end of this article.

Unlike previous articles I will discuss this activity in two sections and include assessing questions at the end of each section.

Introduction – Determine Existing Design Controls and Establish Rankings

At this stage we are considering the existing design controls, this will include all those that are part of the current design and design verification processes. These controls may fall into one of four categories: -

•  Detection controls that will detect the cause of failure

• Detection controls that will detect the failure mode

• Preventative controls that will prevent the cause from occurring

• Preventative controls that will prevent the failure mode from occurring

Design Controls are those activities within the design and design verification processes that are already in place to prevent or detect and might include: -

•  Life cycle testing

• Design verification plans

• Life cycle modelling

• Design validation

• Stress testing

• Design reviews

• Inspection of design

• Etc.

It is critical to ensure that the following are taken into account when ranking current design controls: -

• Product life cycle – if the failure mode happens during the useful life span of the product does the control (e.g. life testing) consider longevity?

• Have all customer usage profiles been considered in the control?

• Have all robustness issues been addressed?

• How representative is the control with respect to actual populations (product and customer/environment)?

NOTE: If you are new to FMEA and Reliability issues, please consider an FMEA Application Workshop. For more details please contact us at www.resultsresults.co.uk and we will be happy to help in any way we can.

Once established, we use the DFMEA rating tables to rate how likely these controls are to work, the higher the number the less likely the control will work. In the FMEA Flow diagram it can be seen that the flow is important to establish which control detects which cause and which cause is associated with which failure mode.

For example, if a design verification test does not consider or test for the failure mode then this test will rate 9 or 10. If the it does test for the failure mode or the cause of failure and is such that it includes for the most demanding customer profile and life cycle, then the rate could be 2 or 3 (ranking needs to consider the size of the tested population).

This process is repeated for all failure modes. In many cases, the causes will impact on more than one failure mode.

Assessing Questions (1)

• Have all the failure modes within the scope of this DFMEA been considered?

• Have all the controls been realistically considered and entered onto the FMEA record?

• Have the design verification specifications been used as part of the controls review?

• Have all outstanding questions on the question log been answered?

• Are there any new questions to be added to the log?

• Has the DFMEA been communicated to all involved/interested parties?

Introduction - Calculate the RPN and establish priority of required actions

FMEA is a risk management tool. This and the next stage are all about minimising the risk of potential failures. We now have a record of the current detection and prevention design controls rated in terms of likelihood of detecting or preventing each failure mode.

Armed with this information we can now consider which of the failure modes is of greatest concern to the business. To do this we need to consider how we are going to prioritise them. To do this we calculate the Risk priority Number.

The Risk Priority Number (RPN) is the product of: -

Severity X Occurrence X Detection

Where: -      

• Severity is the severity ranking of the effects of failure

• Occurrence is the likelihood of the failure mode occurring

• Detection is the ranking of the current design control on terms of their likelihood of detecting/preventing the failure mode or the causes of failure

However, the RPN used in isolation can be misleading. I can best demonstrate this with an example.

Example: I am the designer of a domestic stainless steel toaster. My FMEA team have identified a potential failure mode of “electrical insulation fails” and this has a severity of 10. However the likely occurrence is 1and the detection is 1. My RPN is therefore (10 X 1 X 1) 10.

My FMEA team have also identified a failure mode “appearance degrades” with a severity of 2. However the likely occurrence is 10 and the detection rating is also 10. My RPN in this case is (2 X 10 X 10) 200. Using just the RPN to determine priority of actions would put the second scenario as the higher priority when clearly the first is of greater priority.

So, Results’ recommend the following generic approach when considering the priority of any required actions: -

Level 1 – Effects with severity ratings of 9 or 10 are considered first.

Level 2 – Severity X Occurrence are then used to sort.

Level 3 – Severity X Occurrence x Detection (RPN) is used to sort all other actions.

Note: where there is conflict of rankings (i.e. several 10’s) go to the next level to sort.

Remember – you know your products and customers best, let this be your guide as you establish your action priorities.

At this stage, actions are recommended to reduce the risk and consequences of potential failures including agreeing actions and dates to be completed. Remember to make individuals responsible for actions, not departments or general areas.

• Have all the failure causes been considered and the right prioritisation process used in all cases?

• Has the agreed prioritisation process been used consistently throughout?

• Have all interested parties been involved in discussions around problem areas?

• Have actions been recommended for all problem areas?

• Have individuals been tasked with the actions, including an agreed completion date?

• Have all outstanding questions been cleared or new ones added to the question log?

• Has the FMEA report been updated and circulated?

Next Time

In the final article in this series I will describe how to ensure actions are taken and how the RPN is reduced as a result. In the meantime, please download your free FMEA Snap-Shot.

Graham Cripps

Results Consortium Ltd

www.resultsresults.co.uk