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FMEA | Function Tree - Defining Product Design Functions

Posted by Graham Cripps on Wed, Aug 05, 2015 @ 10:46 AM

Function Tree - Defining Product Design Functions

Defining the product design functions (or design intent) is one key to the success of conducting a successful Design FMEA.

FMEA is a risk management tool that looks at and manages the risk of failure of the products function(s). Failure to identify these leads to the inability to manage the risk of failure of missed functions.

If we know all of the functions that a design has to achieve, then we can consider how those functions could potentially fail to meet the customers requirements. This is the key to understanding the exact functions that the product performs.

This often causes difficulties with FMEA's as there is a tendency to generalise functions as a combination of functions.  Let me explain, using a fountain pen as an example.

FMEA_FT_Blog_P1Lets consider the design of a fountain pen. What is a primary function of a pen? To write?

We cannot design this pen to write on its own!

We have to consider several design functions which might include: to retain ink, to dispense ink, to provide a comforatble grip, to allow for left and right handed use, to resist leaks, to provide smooth contact on a writing surface

The outcome that a user expects is that (in the hands of a capable person) the pen will write. However, this is not a single function of the pen's design (as seen above) but a combination of functions that deliver the customer requirement.

Developing A Function Tree

A robust method of defining a products' function is the application of the Function Tree. The function tree is a process used to determine prime functions and their subordinate functions for a given product. The process steps are as follows:

  • Brief the Team
  • Brainstorm the functions of the product
  • Agree primary functions
  • Organise the brainstorm outputs onto a function tree

Brief The Team

The Team is led by the designer owner and briefed on the process steps and methodology for defining the functions.

At this stage it is important to establish that all the team members are sufficiently familiar with the FMEA process to be able to fully contribute to the FMEA effort. The Team meeting would be arranged, with appropriate Team roles (leader, scribe, time manager and facilitator) and an agenda that sets time aside for briefing, brainstorming, organising the brainstorm outputs and reveiwing.

Brainstorm The Functions Of The Design

Follow the rules of brainstorming:

  • Keep the output visual
  • Keep to the alloted timescales (time managers role)
  • Volume is King!!
  • Do not discuss the ideas (this comes later)
  • Do not criticise
  • Involve all of the team (facilitators role)
  • Do not 'organise' the brainstorm (this comes later)

We use a VERB - NOUN sentencing technique to describe each function

FMEA_FT_Blog_P2

Organise The Brainstorm Outputs Onto A Function Tree

It is very seldom that design specifications are written in this way, so we use the Function Tree tool to capture and sort all the functions of a design from Primary functions to Tertiary functions.

Primary Functions - those that are identified as nearest to the customer usage intent

Secondary Functions - those unique functions that support the primary functions

Tertiary Functions - functions that support the secondary functions and typically suppport more than one

Measureable Level - sometimes referred to as Actionable Level, these functions have a specific measurable or set of measurables that can be specified.

Function Tree Construction

FMEA_FT_Blog_P3

 

First, organise the Brainstorm Output by selecting the primary functions, then secondary and so on. Remember there may be more or less than the four levels shown here, and it is okay to add or duplicate functions

 

 

FMEA_FT_Blog_P4

 

To check that the functions actually support one another you should ask 'How?' the primary function is achieved, through to the secondary and so on

 

 

 

FMEA_FT_Blog_P5

 

 

 

Then look at the tertiary functions (lowest level) and ask 'Why?', moving back up the tree and providing the answer at each level, making grammatical sense throughout.

 

 

 

Assessing Questions

  • Have all functions been transferred from the brainstorm to the function tree?
  • Have all primary functions been developed to an actionable level?
  • Is the sentencing technique obvious and correct?
  • Have all primary functions been recorded on the FMEA record?

 

graham_cripps_dec_2010_formal

Graham Cripps
ResultsResults

 

Topics: Design FMEA, Process FMEA, FMEA Form, FMEA Action Planning, DFMEA, FMEA FAQ's

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

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