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

Graham Cripps, Director 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 .

FMEA Flow 1 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).

FMEA Severity 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