Articles by Results

FMEA - Some frequently asked questions answered!

Posted by Graham Cripps on Fri, Jul 24, 2015 @ 01:30 PM

FMEA FAQ's (Frequently Asked Questions)

"When do I carry out an FMEA?"

FMEA is a predictive risk assessment tool and looks at the probablility of failure of a design or process.

As Machinery FMEA and System FMEA are also based upon  Design FMEA, you can assume the followingcomments apply accordingly.

For Design FMEA (DFMEA), the FMEA should be carried out prior to setting up the process and already have the output from the DFMEA. The FMEA concentrates on the probablity of failure of the process to perform in terms of the process purpose or outcome and requires the following information:

  • Design specification
  • Design verification test planning
  • Reliability test data

For a Process FMEA (PFMEA), the FMEA should be carried out prior to settng up the process and already have the output from the DFMEA. The FMEA concentrates on the probability of failure of the process to perform in terms of the process purpose or outcome and requires the following information:

  • Critical charateristics of the design
  • Process capability (if known)
  • Machinery FMEA if available
These are ideal conditions, however an FMEA performed on existing products or processes can also provide valuable data and information for both the design and process teams

Download your FREE FMEA template

 

"How do I ensure that our FMEA efforts are well managed?"

For FMEA to be a useful tool for any business it must be an integrated part of the business process. Therefore this process, like any other, requires management support.

In order to have a framework for managing FMEA's someone that has a vested interest in the outcome of the FMEA and in a management role would be tasked with assessing each stage of the process to ensure all process steps have been completed successfully.

There are a whole set of assessing questions that are asked at the end of each stage of the FMEA process. These questions are designed to assist the team and the manager to focus upon each stage of the FMEA process to ensure full completion.

The following is a small selection of the questions but a full copy can be accessed by selecting the button at the end of this article.

These assessing questions are those followinng the review of existing design controls of a DFMEA:

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

 Download your FMEA Assessing Questions!

 

"What is an FMEA Application Workshop?"

There are a lot of training courses, including the ones we offer, that tell you how to carry out an FMEA. However, the most effective way of learning is doing.

Our application workshops are run over two to four days and are a mixture of "show and tell" on your own design and process, not a case study. 

The purpose of the workshop is not to fully complete an FMEA but to provide the ooportunuty for the attendees to be able to practice all steps on a design or process they are familiar with and have an outcome that can be taken away and completed with the new knowledge and experience. 

This approach provides a feeling of relevance for the attendees and the opportunity to practice in a facilitated environment.

We also offer a review service, during the Workshop process,  whereby one of our experienced practitioners will review one or more of your FMEA's and provide constructive fedback on the content. We have found this to be a valuable addition to the Workshop format, providing guidance and support for our clients.

To find out more about our FMEA Application Workshops please click the button below.

FMEA Application Support

 

"How do I use RPN to prioritise actions to be taken?"

In its simplest form, the RPN figure is used to rank priority of actions to be taken by considering the highest RPN number first. It has long been recognised that this method does not discriminate between the severities of the effects of failure.

There are two available methods (RPN and SOD)and we would recommend the following method.

Known as S-O-D (Severity - Occurrence - Detection) it involves looking at the severity ratings of 10 first. To prioritise these, take S x D (product of severity and detection) to prioritise within this group. If the two are the same, then use the RPN number within that sub-group.

Then repeat for severities of 9 etc. See the following example: 

                 

PRIORITY

 

S

 

O

(s X o)

D

 

RPN

 

RPN Method

S-O-D Method

EX 1

10

 

1

10

1

 

10

 

9

3

EX 2

10

 

6

60

3

 

180

 

3

1

EX 3

10

 

3

30

2

 

60

 

5

2

EX 4

4

 

7

28

1

 

28

 

8

5

EX 5

4

 

3

12

3

 

36

 

7

4

EX 6

4

 

10

40

10

 

400

 

1

6

EX 7

2

 

10

20

10

 

200

 

2

7

EX 8

2

 

7

14

3

 

42

 

6

9

EX 9

2

 

7

14

7

 

98

 

4

8

 

If we use the RPN method then moderate severity becomes the first priority for action, whilst a failure mode with an effect of 10 and a relatively high probability of occurrence is third.

The S-O-D method is more logical and still provides a reliable structure for prioritisation.
 
Download your free FMEA ranking Table

 

Topics: Design FMEA, Process FMEA, FMEA Template, FMEA Form, FMEA Action Planning, FMEA ranking tables, DFMEA, How to construct a boundary diagram, Continuous Improvement, FMEA FAQ's

Boundary Diagram – How to construct an FMEA boundary diagram.

Posted by Graham Cripps on Mon, Sep 23, 2013 @ 01:00 PM

Boundary Diagram - How to construct an FMEA boundary diagram


"A boundary diagram provides a focus for the FMEA Team".

FMEA Boundary diagram - step 1

Constructing a boundary diagram for an FMEA is a relatively simple task.

However, in my experience so many people get it wrong. This can have a major impact on the complexity and quality output of the FMEA.

 

Let me explain why and for this I will use Design FMEA as an example.

Photo Copier

In an organisation, the FMEA’s conducted all form part of a large jigsawpuzzle. For example, if we were to consider a copier machine then to carry out a Design FMEA on the whole product would be a daunting, if not impossible task. At a macro level, a photo copier has a number of sub-systems that are made up of sub-assemblies that have individual controls and management protocols.

So, we need to be able to manage our FMEA activity in a way that considers a number of factors including: -

  • Customer critical functions
  • Reliability
  • System and sub-system levels
  • Critical systems, sub-systems and components

 

FMEA Jigsaw

 

As stated in other articles, the target FMEA then forms just one small part of the FMEA puzzle.

The FMEA boundary (what will be included and will not be included in this FMEA) describes the size and shape of this piece of the puzzle and will have an impact on the boundary of subsequent FMEA’s

  

There are some key foundations for a successful FMEA:

Adopt a team approach - the skills, knowledge and experience in the design team is unlikely to rest with one person. Also part of the process is to challenge and test all assumptions which need more than one person. Most FMEA's require expertise from multiple disciplines

Determine the prime functions that are to be the target of the FMEA - use of a Function Tree can help to identify these 

Clearly understand and describe the prime functions of the design. The whole purpose of an FMEA is to establish what could go wrong (design does not provide the intended function) and ensure that the risk of failure is minimised by having controls in place to prevent or detetct the failure mechanism

Determine the FMEA scope (use the boundary diagram). Keeping within the scope of each FMEA is key to getting good quality outcomes from the FMEA process
Be clear about what is a failure mode, an effect of the failure and a cause of the failure by:

Establishing what could possibly go wrong, the failure mode (potential failure modes are limited)
Establishing the effects of the failure of the design to perform the intended functions (use the effects list)
Determine the causes of potential failure modes

 

The constructing of the boundary diagram is a critical step in the management of the FMEA effort.

Constructing a Robust Boundary Diagram

In previous articles I have explained how to construct a boundary diagram. I want to now look at some of the considerations and understanding that underpin boundary diagram construction in our effort to achieve robust FMEA’s.

Identify the subject of the FMEA – make sure that the team clearly understand the subject of the FMEA, describing the system, sub-system, assembly or component.

Example: If the FMEA is to be carried out on a car braking system, make sure you determine each sub-assembly and concentrate on just one at a time. These sub-assemblies could include:-

  • Master cylinder sub-assembly
  • Front disc brake assembly
  • Slave cylinder sub-assembly
  • Hydraulic actuation assembly (including delivery and reservoir sub-system
  • Rear disc brake assembly
  • Hand brake sub-assembly
  • Sensor system (various depending upon complexity)

Understand the Robustness linkages – in considering our product design, we will have considered how it will perform in the hands of the customer and in all of the environments that it is likely to be subjected to. Remember that there are unintended functions that the customer will subject the product to as well.

In understanding the robustness linkages we also need to understand the influence of neighbouring systems or sub-systems that can also have a direct impact upon the operating environment of the target of the FMEA.

Example:  a car steering wheel has primary functions that generically include: providing housing for air bag, horn etc.; provide mechanical advantage (turning motion); provide comfortable grip; facilitate access to stick controls. The steering wheel is also used to get into and out of the car by some customers (unintended function) – we need to design for this.

Establish the interactions – how are the various components related to each other, directly or indirectly? Are they bolted together, welded, fixed to the same substrate, connected electrically, etc.?

The Process

If we now look at the process for constructing the Boundary Diagram, we can then start to think about the tools and techniques available to us to support this activity.

From previous articles the process was described as: 

  • Draw a block diagram 
  • Determine the interactions 
  • Establish and agree of this FMEA

For the purposes of this article I will use a the design of an automotive spark plug to demonstrate how to construct the Boundary Diagram

Step 1 - Draw the Block Diagram  

FMEA Boundary Diagram - Step 1

 

The first step is to draw a block diagram including all of the directly connecting parts, sub-assemblies or systems.  

 

 

 

 

Step 2 - Determine the interactions

The second step requires the FMEA Team to consider the relationships between all the items in the block diagram.

FMEA Boundary Diagram - Step 2

 

In this example I have suggested some linkages.

The relationship between the plug assembly and the gasket ring is the retaining fit.

The relationship between the spark plug assembly and the fuel mixture is the spark gap in the correct position.

I have kept this diagram fairly simple for demonstration purposes. In most cases at this point we would also consider the robustness issues as well.

Robustness - introduced by Genichi Taguchi and refers to the operation of a component, sub-assembly or system in the presence of the operating environment and all of the internal and external variations. These variations are often refered to as noise.

Step 3 - Agree the Boundary of this FMEA

Having agreed the block diagram and the connections or linkages between each of the blocks, the team now determine the scope of the FMEA. The scope of a design FMEA (DFMEA) is restricted to:-

  • Failures attributed to design only (i.e. does not include manufacturing , fitting or other induced errors unless through design issues) E.g. If a spark plug was fitted but cross threaded, this would not be considered unless the relationship between the cylinder head and the fitting tool induced the error.
 

Boundary Diagram - Step 3

 

The completed boundary diagram shows that this FMEA will include the spark plug assembly, the provision for retaining the spark plug connector, the provision for retaining the retained gasket ring, the retained gasket itself, positioning of the spark gap and provision of a hexagonal shape for plug fitting and removal

  

What the boundary diagram does for the FMEA Team is to provide a focus for the analysis. This does not mean that all the other linkages and interactions are not subjected to analysis, just not in this instance.

Perhaps simple is not the right word, but this is not a difficult concept. 

So, the inputs to the construction of the boundary diagram are:

  • Clear description of the subject of the FMEA
  • Robustness linkages
  • Design brief including prime functions (a function tree would be very useful)
  • Multi-disciplined team with the appropriate level of training in FMEA to be able to contribute

For further reading to see where the block diagram fits in the FMEA process go to Articles

 

Graham CrippsGraham Cripps - Director

Results Consortium Ltd

www.resultsresults.co.uk

Topics: Design FMEA, FMECA, DFMEA, Boundary Diagram, How to construct a boundary diagram

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