FMEA - Failure Mode and Effect Analysis

Guest Contribution : Sept-Oct 2000

F.M.E.A by SEAT

SEAT stands for the School of Engineering and Advanced Technology. The school was formed in 1992 when Staffordshire Polytechnic became Staffordshire University. Based in the Beacon Building at the Beaconside campus on the outskirts of Stafford it shares its facilities and lecture rooms with the schools of Computing, Business and Health. This adds to the excellent atmosphere of the university as different students all study in the same place.

The site offers a lot of useful information about design procedures & methods. It's worth your visit.

Pro-forma FMEA sheet.

1.0 Introduction

The purpose of this manual is to give an introduction to potential Failure Mode and Effects Analysis (FMEA) and also to give general guidance on the use of this technique, as applied to design projects.

Because of the rapidly changing customer expectations and increasing regulations, industry's need for a disciplined use of a technique to identify and prevent potential problems is more important than ever before.

In its most rigorous form, an FMEA is a summary of the Engineer's thoughts (including the analysis of every conceivable item that could go wrong based on experience and past problems) as he/she designs a component or system. This systematic approach parallels and formalises the mental discipline that an Engineer normally goes through in a design process.

Definitions

To provide for a uniform development of the Potential Failure Mode and Effects Analysis, common understanding of FMEA terminology is required.

Potential Failure Mode and Effects Analysis (FMEA)

A Potential Failure Mode and Effects Analysis is an analytical technique utilised as a means to assure that every conceivable potential failure of a design has been considered and addressed.

Design Project FMEA

The design project FMEA is completed by the Designer. It is then used as a quality check by the "checker" (in the teaching situation, the checker is the lecturer) to ensure that all the possible failure modes have been accommodated by the design.

2.0 Design Project FMEA

Design FMEA's should cover:-

all new components
carried over components in a new environment
modified components and is mandatory on control and load carrying parts.

Failure
A failure simply means a component or system not meeting or not functioning to design intent. The parameters of the design intent might be stated in lifetime/cycles linear dimensions/tolerances, load or deflection, coat thickness, etc. A failure to design intent might also include such static occurrences as sharp edges, too glossy a finish etc. Using this definition a failure does not need to be readily detectable by a customer to still be considered to be a failure.

Failure Mode

A failure mode is the manner in which a component or system failure occurs; it is the manner in which the part or system does not meet design intent. The failure mode is the answer to the question, "How could the component or system fail"?

Examples of failure modes are:-

        Fatigue                 Collapse
        Cracked                 Performance/Deterioration
        Deformed                Stripped
        Worn (prematurely)      Corroded
        Binding                 Seized
        Buckled                 Sag
        Loose                   Misaligned
        Leaking                 Falls off
        Vibrating               Burnt
        etc

It should be noted that the potential failure mode is expressed in "physical" terms and not as the symptoms the customer may experience.

1. Failure Mode

List out the potential failure modes for the particular part or function. The assumption is made that the failure could occur, but will not necessarily occur.

In the space provided, sketch a Free Body Diagram (if applicable) of the part, showing the in-coming and the reaction loads. Indicate the place where failure under this condition is likely.

2. Potential Cause(s) of Failure

List conceivable potential causes of failure assignable to each failure mode. The causes listed should be concise and as complete as possible. Typical causes of failure are:-

        Incorrect material used         Poor weld
        Corrosion                       Assembly error
        Error in dimension              Over stressing
        Too hot                         Too cold
        Bad maintenance                 Damage
        Error in heat treat             Material impure
        Forming of cracks               Out of balance
        Tooling marks                   Eccentric

3. Occurrence

Estimate the probability of occurrence on a scale "1" to "10". Any fail-safe controls intended to prevent the cause of failure should be considered in with this estimate.

When estimating the occurrence ranking, the following two probabilities should be considered:-

The probability that the potential cause of the failure will occur. For this probability all current fail safe and controls which are in place to prevent the cause of failure from occurring on the part, must be assessed.
The probability that once the cause of the failure has occurred, it will result in the indicated potential failure mode. For this estimate, it must be assumed that the cause of failure and the failure mode are not detected before the product reaches the customer.

The engineer should mentally combine these two probabilities when estimating the occurrence ranking.

The following occurrence ranking system should be used to ensure consistency:-

CRITERIA RANKING

Remote - probability of occurrence. Unreasonable to expect failure to occur. -1
Low - failure rate. Related to similar designs having low failure rates 2/3
Moderate - failure rate. Related to similar designs that have occasional failures of the moderate type. 4/5/6
High - failure rate. Relates to failures in similar designs that have failed in the past. 7/8
Very high - failure rate. Almost certain that failure will occur in major way. 10
To change the occurrence ranking for a particular design level, one of two actions would have to be taken:-
Change the design to reduce the probability that the cause of failure will result in the failure mode.
Increase or improve the fail-safe "control systems" which prevent the cause of failure from occurring.

4. Severity

Estimate the severity of the effects of failure to the customer on a "1" to "10" scale.

CRITERIA RANKING

Unreasonable - to expect the customer to notice the very minor failure 1
Low Severity - ranking. Only slight customer annoyance 2/3
Moderate - failure causing some customer dissatisfaction. Customer annoyed. 4/5/6
High - degree of failure resulting in the product not working and customer anger.7/8
Very High - degree of failure. This rank indicates that the customer is at risk. Safety regulations are being infringed. 10

5.0 Results of the Design Project FMEA

From the analysis done by the designer, two assessments can be made:

The Risk Priority Number (RPN) resulting from this analysis will be assessed by the "checker" and an agree/disagree evaluation will be made.
The failure modes that have been used in the design calculations can easily been seen and an.y omissions pointed out.

(This design project FMEA has been modified to suit the needs of design teaching. In it's full blown form, FMEA is a very powerful quality assurance tool which can be applied to both design and manufacturing as well as in other areas of "Idea Implementation")

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