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.

Lets 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

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

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

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

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?