FMEA - Plan and implement improvement actions and establish revised RPN

We are now nearing the completion of the DFMEA process whereby the identified and agreed actions are to be planned and implemented. To this end, as well as throughout the FMEA process in general, we use the Deming P.D.C.A. Cycle.

fmea_snapshot

Introduction

In this final FMEA article I will discuss the process of planning and implementing the actions necessary to reduce RPN numbers in the order of: -

• Reduce the severity of the effect of failure – this is the most difficult to do as it will normally involve designing out the function. If this cannot be done: -

• Reduce the likely occurrence – this is the most cost effective in most cases and certainly so when compared with the above option

• Improve detection methods – this can be costly, particularly if involving detection rather than prevention

As already mentioned the planning and implementation is best served by using the Deming Plan-Do-Check-Act cycle. It is applied as follows: -

PLAN – plan the implementation of the corrective or improvement action. These may have wide ranging impacts across the design and design verification processes. All actions should be comprehensively considered in terms of the likelihood of success. This means that testing modifications may need to be modelled during the planning stage. Establish how success will be measured or assessed.

DO – Carry out the modifications as planned to the design, design processes or test processes. This includes documenting the changes in all associated processes and reviewing test data outcome requirements. It is important to ensure that the changes are made permanent to ensure “old or established” methods are not readopted (Ensure procedural documentation is updated and re-issued. Make the change sustainable).

CHECK – Carry out checks to ensure that the changes have taken place and are effective. This may not be feasible using practical methods, so modelling may be required, which should have been planned for as above. Have the changes had the planned effects, has the previous Severity, Occurrence or Detection levels been improved as planned?

ACT – Ensure that all data has been recorded, review the DFMEA and update the Severity, Occurrence and Detection ratings accordingly. This should result in a required reduction in the RPN. If outcomes are not as planned, take further actions to achieve the required improvements.

What Next?

What we have been through with these series of articles is just the beginning. FMEA fits within the processes of new product introduction and release and has direct linkages with other design tools including: -

• Product reliability

• Piece part approval

• Design verification process

• Adjoining FMEA efforts

• Problem Solving

• Robustness engineering

• To name but a few………………………..

The message therefore is quite clear - FMEA is only successful when and if it is an  integrated part of the business and quality processes. Failure to do so makes FMEA a “bolt-on” activity. This less effective.

Assessing Questions 

• Have all recommended actions been completed?

• Have all required process and system documentation been completed in accordance with company standards?

• Have any shortfalls in terms of expected outcomes been addressed through further considered action planning?

• Have all outstanding questions been closed, new ones added and the question log updated?

• Has the FMEA record been updated and communicated to all interested parties?

• Have all the team members been thanked for their contribution?

Next Time

Next time you are tasked with carrying out an FMEA, I hope you will feel better equipped. Remember to download the FMEA Snap-Shot and use this as a reminder of the steps that need to be taken to get the best from your FMEA efforts.

Finally, thanks for sticking with me through these articles. I hope they have helped. If you need any further assistance, please visit www.resustsresults.co.uk/fmea.

Graham Cripps

Results Consotium Ltd

www.resultsresults.co.uk

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

FMEA - Determine Causes of failure and Rank their likely occurrence

Having identified the failure modes and the effects of failure, we now need to determine the possible causes of failure. In this article we will be looking at establishing those causes and the processes used to ensure all causes are considered and their likely occurrence determined by the FMEA team.

Introduction

At this point we need to understand that the effect of the failure mode and the cause are not linked, other than through the failure mode itself. In other words, we are looking for the cause/s of the failure mode. The failure mode has effects that are experienced (sometimes called symptoms) as a result of that failure mode.

This is described in the sentencing model opposite.

To ensure that the best result is achieved the following should be noted: -

• Ensure that brainstorming is managed effectively (this a creative process and should have the basic rules applied)

• Do not constrain thoughts around current detection methods, this comes later

• Keep the boundary diagram and other visual aids in view to focus the brainstorming effort

• One cause can be the cause of more than one failure mode

• All failure modes are captured and recorded for discussion and analysis after the brainstorming session

• Causes are limited to those of the specific failure modes and cannot include errors outside of the boundary diagram

• Potential mistakes that could be made during manufacture of the design can only be considered if they are as a direct result of poor design (the principles of design for manufacture).

• A causal theory should be developed to root cause level for all failure modes with a severity of 9 or 10.

• Robustness linkages should be considered if they are within the agreed scope of the FMEA.

The team should brainstorm all the possible causes of failure. Failure of the design to perform as intended.

Once the causes of failure have been established, they are arranged in order against the failure mode (a continuation of the flow established during the last process step (see diagram below).

Using the Design FMEA Ranking tables, each potential cause is ranked as to the likelihood of occurrence. The ranking is recorded alongside the cause in question.

Because the use of ranking tables is not an “exact science”, the introduction of past experiences and other data is useful.

The FMEA flow is imprtant. Ensure that the causes are arranged against each failure mode. You will now have an FMEA report that has the effects of the failure mode and the causes of the failure mode aligned to the individual failure modes (see graphic above left). This is critical at the next stage because we will want to be looking at how to reduce the likelyhood of failure and this is around the failure mechanism for each failure mode.

Assessing Questions

On completion of this and every stage of the FMEA, assessing questions need to be addressed. This will include: -

• Have all failure modes been considered fully?

• Have cause theories been developed for all causes (these must be what could actually happen)?

• Have causes met the correct sentencing criteria (see above figure)?

• Has the scope of the FMEA been adhered to for each possible cause?

• Has the FMEA report structure been completed correctly (this is important when calculating RPN)?

• Have all previous questions been closed out and any new ones entered on the question log?

• Has the FMEA report been communicated to all interested parties?

Next Time

In the next article I will explain how to assess current preventative and detection controls and their effectiveness. I will also describe how to calculate RPN as a method of establishing action priorities.

Graham Cripps

Results Consortium Ltd

www.resultsresults.co.uk

FMEA - Determine and Rank the Severity of the Effects of Failure               

At this stage of the FMEA process we have: -

• Established the FMEA Team members

• Defined the Scope of the FMEA

• Determined the functions of the design

• Determined the failure modes

The next step is to determine the effects of failure and to rank these failures in terms of their potential severity.

Introduction

A failure of a product or service to provide an intended function can have more than one effect. These effects will have varying levels of severity.

Example: one of the functions of a domestic toaster is to eject the toast at a determined level of browning. A failure mode could be “fails to eject the toast”. This failure mode could results in the toast continuing to heat. The effects could therefore be: -

• On the toaster – elements and platform contaminated

• The toast – burnt

• The user – dissatisfied customer

• The environment – smoke or possible fire

The first three effects will be less serious than a fire.

In this article we will look at the process for determining the effects of the failure modes and rank them in terms of their serverity (seriousness).

I will refer to 2 tools that are used, the Design Effects List and Ranking Tables.

Determine the Effects of Failure

Again, it is important to keep visible the boundary diagram, the FMEA record, question log and Function Tree for the team to refer to.

The effects of the failure modes are considered by the team in the following 7 categories: -

•  Part (subject of the design FMEA)

• Assembly (the next level assembly that the part fits)

• System - the system that this part contributes to

• Product - the overall product

• Customer - the user of the product

• Regulations - current legislation applicable in the country of use

• Other - any other category that may be industry or market specific

NOTE: the terms used in this list are generic and should be replaced with your industry specific terms. For help with this please consider and FMEA Application Workshop. To find out more visit our web site at www.resultsresults.co.uk or e-mail for more information on info@resultsresults.co.uk .

The effects are recorded on the Design FMEA effects list (optional) and the FMEA record against the failure mode being considered. This will give a natural left to right flow across the FMEA record. The importance of this will become apparent later in the process (see model opposite)

The use of visual management tools are important during the current process steps to remind the team of the focus of this FMEA.

Once recorded on the FMEA form, each effect is ranked in terms of its severity using the Design FMEA ranking tables. The highest severity is the one carried forward and recorded against the failure mode being considered.

Ranking (or rating) tables are available through a number of sources. However, Results have their own versions that are available as a free download (click the on button at the end of this article).

This section of the FMEA form shows 4 effects with severities of 3 to 9. As 9 is the highest, this is carried forward.

The Classification used depends upon your specific industry so will not be covered in this article. If you require further assistance consider an FMEA Application Workshop.

This activity is carried out for all failure modes on the FMEA form.

Complete the FMEA Form

Ensure that all ratings are completed for failure modes and complete the FMEA form.

If there are any outstanding questions not answered, complete the question log and assign someone to provide the answer for the next meeting (or before if urgent).

Assessing Questions

There are a number of assessing questions that should be asked at the end of this stage and include: -

•  Have all questions from Question Log been actioned?

• Have all failure modes been considered?

• Have all failure modes been listed and considered against the effects list criteria?

• Have all effects for each failure mode been recorded on the FMEA record?

• Have all failure modes been rated using the correct rating table?

• Have each set of effects been assessed in terms of overall criticality?

Next Time

In the next article we will look at determining causes of failure and looking at the likely occurrence of those failures.

Graham Cripps

Results Consortium Ltd

www.resultsresults.co.uk