THE FMEA FORM


There are many forms to develop a typical FMEA. However, all of them are basically the same in that they are made up of two parts , whether they are for system, design, process, or machinery. A typical FMEA form consists of the header information and the main body.

There is no standard information that belongs in the header of the form, but there are specific requirements for the body of the form.

In the header, one may find the following information ” see Figure 6.7. However, one must remember that this information may be customized to reflect one's industry or even the organization:

  • Type of FMEA study

  • Subject description

  • Responsible engineer

  • FMEA team leader

  • FMEA core team members

  • Suppliers

  • Appropriate dates (original issue, revision, production start, etc.)

  • FMEA number

  • Assembly/part/detail number

  • Current dates (drawings, specifications, control plan, etc.)

The form may be expanded to include or to be used for such matters as:

  • Safety: Injury is the most serious of all failure effects. As a consequence, safety is handled either with an FMEA or a fault tree analysis (FTA) or critical analysis (FMCA). In the traditional FTA, the starting point is the list of hazard or undersized events for which the designer must provide some solution. Each hazard becomes a failure mode and thus it requires an analysis.

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    Figure 6.6: Scope for PFMEA ” printed circuit board screen printing process.

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    Figure 6.7: Typical FMEA header.

  • Effect of downtime: The FMEA may incorporate maintenance data to study the effects of downtime. It is an excellent tool to be used in conjunction with total preventive maintenance.

  • Repair planning: An FMEA may provide preventive data to support repair planning as well as predictive maintenance cycles.

  • Access: In the world of recycling and environmental conscience, the FMEA can provide data for tear downs as well as information about how to get at the failed component. It can be used with mistake proofing for some very unexpected positive results.

A typical body of an FMEA form may look like Figure 6.8. The details of this form will be discussed in the following pages. We begin with the first part of the form; that is the description in the form of:

Part name /process step and function (verb/noun)

In this area the actual description is written in concise , exact and simple language.

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Figure 6.8: Typical FMEA body.

DEVELOPING THE FUNCTION

A fundamental principle in writing functions is the notion that they must be written either in action verb format or as a measurable noun. Remember, a function is a task that a component, subsystem, or product must perform, described in language that everyone understands. Stay away from jargon. To identify appropriate functions, leading questions such as the following may help:

  • What does the product/process do?

  • How does the product/process do that?

  • If a product feature or process step is deleted, what functions disappear?

  • If you were this task, what are you supposed to accomplish? Why do you exist?

The priority of asking function questions for a system/part FMEA is:

  1. A system view

  2. A subsystem view

  3. A component view

Typical functions are:

  • Position

  • Support

  • Seal in, out

  • Retain

  • Lubricate

ORGANIZING PRODUCT FUNCTIONS

After the brainstorming is complete, a function tree ” see Figure 6.9 ” can be used to organize the functions. This is a simple tree structure to document and help organize the functions, as follows :

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Figure 6.9: Function tree process.

Purposes of the function tree

  1. To document all the functions

  2. To improve team communication

  3. To document complexity and improve team understanding of all the functions

Steps

  1. Brainstorm all the functions.

  2. Arrange functions into function tree.

  3. Test for completeness of function (how/why).

Building the function tree

  • Ask:

    • What does the product/process do?

    • Which component/process step does that?

    • How does it do that?

      • Primary functions provide a direct answer to this question without conditions or ambiguity.

      • Secondary functions explain how primary functions are performed.

      • Continue until the answer to "how" requires using a part name, labor operation, or activity.

      • Ask "why" in the reverse direction.

      • Add additional functions as needed.

The function tree process can be summarized as follows:

  1. Identify the task function.

    Place on the far left side of a chart pad.

  2. Identify the supporting functions.

    Place on the top half of the pad.

  3. Identify enhancing functions.

    Place on the bottom half of the pad.

  4. Build the function tree.

    Include the secondary/ tertiary functions.

    Place these to the right of the primary functions.

  5. Verify the diagram: Ask how and why.

For an example of a function tree for a ball point pen (tip), see Figure 6.10.

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Figure 6.10: Example of ballpoint pen.

FAILURE MODE ANALYSIS

The second portion of the FMEA body form deals with the failure mode analysis. A typical format is shown in Figure 6.11.

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Figure 6.11: FMEA body.

Understanding Failure Mode

Failure mode (a specific loss of a function) is the inability of a component/subsystem/system/process/part to perform to design intent. In other words, it may potentially fail to perform its function(s). Design failure mode is a technical description of how the system, subsystem, or part may not adequately perform its function. Process failure mode is a technical description of how the manufacturing process may not perform its function, or the reason the part may be rejected.

Failure Mode Questions

The process of brainstorming failure modes may include the following questions:

DFMEA

  • Considering the conditions in which the product will be used, how can it fail to perform its function?

  • How have similar products failed in the past?

PFMEA

  • Considering the conditions in which the process will be used, what could possibly go wrong with the process?

  • How have similar processes failed in the past?

  • What might happen that would cause a part to be rejected?

Determining Potential Failure Modes

Failure modes are when the function is not fulfilled in five major categories. Some of these categories may not apply. As a consequence, use these as "thought provokers" to begin the process and then adjust them as needed:

  1. Absence of function

  2. Incomplete, partial, or decayed function

  3. Related unwanted "surprise" failure mode

  4. Function occurs too soon or too late

  5. Excess or too much function

  6. Interfacing with other components , subsystems or systems. There are four possibilities of interfacing. They are (a) energy transfer, (b) information transfer, (c) proximity, and (d) material compatibility.

Failure mode examples using the above categories and applied to the pen case include:

  1. Absence of function:

    • DFMEA: Make marks

    • PFMEA: Inject plastic

  2. Incomplete, partial or decayed function:

    • DFMEA: Make marks

    • PFMEA: Inject plastic

  3. Related unwanted "surprise" failure mode

    • DFMEA: Make marks

    • PFMEA: Inject plastic

  4. Function occurs too soon or too late

    • DFMEA: Make marks

    • PFMEA: Inject plastic

  5. Excess or too much function

    • DFMEA: Make marks

    • PFMEA: Inject plastic

General examples of failure modes include:

Design FMEA:

No power

Failed to open

Water leaking

Partial insulation

Open circuit

Loss of air

Releases too early

No spark

Noise

Insufficient torque

Vibration

Paper jams

Does not cut

And so on

Process FMEA:

Four categories of process failures:

  1. Fabrication failures

  2. Assembly failures

  3. Testing failures

  4. Inspection failures

Typical examples of these categories are:

  • Warped

  • Too hot

  • RPM too slow

  • Rough surface

  • Loose part

  • Misaligned

  • Poor inspection

  • Hole too large

  • Leakage

  • Fracture

  • Fatigue

  • And so on

Note  

At this stage, you are ready to transfer the failure modes in the FMEA form ” see Figure 6.12.

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Figure 6.12: Transferring the failure modes to the FMEA form.

FAILURE MODE EFFECTS

A failure mode effect is a description of the consequence/ramification of a system, part, or manufacturing process failure. A typical failure mode may have several effects depending on which customer(s) are considered . Consider the effects/consequences on all the "customers," as they are applicable , as in the following FMEAs:

SFMEA

  • System

  • Other systems

  • Whole product

  • Government regulations

  • End user

DFMEA

  • Part

  • Higher assembly

  • Whole product

  • Government regulations

  • End user

PFMEA

  • Part

  • Next operation

  • Equipment

  • Government regulations

  • Operators

  • End user

Effects and Severity Rating

Effects and severity are very related items. As the effect increases , so does the severity. In essence, two fundamental questions have to be raised and answered :

  1. What will happen if this failure mode occurs?

  2. How will customers react if these failures happen?

    • Describe as specifically as possible what the customer(s) might notice once the failure occurs.

    • What are the effects of the failure mode?

    • How severe is the effect on the customers?

The progression of function, cause, failure mode, effect, and severity can be illustrated by the following series of questions:

  • In function: What is the individual task intended by design?

  • In failure mode: What can go wrong with this function?

  • In cause: What is the "root cause" of the failure mode?

  • In effect: What are the consequences of this failure mode?

  • In severity: What is the seriousness of the effect?

The following are some examples of DFMEA and PFMEA effects:

Customer gets wet

Loss of performance

System failure

Scrap

Loss of efficiency

Rework

Reduced life

Becomes loose

Degraded performance

Hard to load in next operation

Cannot assemble

Operator injury

Violate Gov. Reg. XYZ

Noise, rattles

Damaged equipment

And so on

Special Note:  

Please note that the effect remains the same for both DFMEA and PFMEA.

Severity Rating (Seriousness of the Effect)

Severity is a numerical rating ” see Table 6.1 for design and Table 6.2 for process ” of the impact on customers. When multiple effects exist for a given failure mode, enter the worst-case severity on the worksheet to calculate risk. (This is the excepted method for the automotive industry and for the SAE J1739 standard. In cases where severity varies depending on timing, use the worst-case scenario.

Table 6.1: DFMEA ” Severity Rating

Effect

Description

Rating

None

No effect noticed by customer; the failure will not have any perceptible effect on the customer

1

Very minor

Very minor effect, noticed by discriminating customers; the failure will have little perceptible effect on discriminating customers

2

Minor

Minor effect, noticed by average customers; the failure will have a minor perceptible effect on average customers

3

Very low

Very low effect, noticed by most customers; the failure will have some small perceptible effect on most customers

4

Low

Primary product function operational, however at a reduced level of performance; customer is somewhat dissatisfied

5

Moderate

Primary product function operational, however secondary functions inoperable; customer is moderately dissatisfied

6

High

Failure mode greatly affects product operation; product or portion of product is inoperable; customer is very dissatisfied

7

Very high

Primary product function is non-operational but safe; customer is very dissatisfied.

8

Hazard with warning

Failure mode affects safe product operation and/or involves nonconformance with government regulation with warning

9

Hazard with no warning

Failure mode affects safe product operation and/or involves nonconformance with government regulation without warning

10

Table 6.2: PFMEA ” Severity Rating

Effect

Description

Rating

None

No effect noticed by customer; the failure will not have any effect on the customer

1

Very minor

Very minor disruption to production line; a very small portion of the product may have to be reworked; defect noticed by discriminating customers

2

Minor

Minor disruption to production line; a small portion (much <5%) of product may have to be reworked on-line; process up but minor annoyances

3

Very low

Very low disruption to production line; a moderate portion (<10%) of product may have to be reworked on-line; process up but minor annoyances

4

Low

Low disruption to production line; a moderate portion (<15%) of product may have to be reworked on-line; process up but minor annoyances

5

Moderate

Moderate disruption to production line; a moderate portion (>20%) of product may have to be scrapped; process up but some inconveniences

6

High

Major disruption to production line; a portion (>30%) of product may have to be scrapped; process may be stopped ; customer dissatisfied

7

Very high

Major disruption to production line; close to 100% of product may have to be scrapped; process unreliable; customer very dissatisfied

8

Hazard with warning

May endanger operator or equipment; severely affects safe process operation and/or involves noncompliance with government regulation; failure will occur with warning

9

Hazard with no warning

May endanger operator or equipment; severely affects safe process operation and/or involves noncompliance with government regulation; failure occurs without warning

10

Note  

There is nothing special about these guidelines. They may be changed to reflect the industry, the organization, the product/design, or the process. For example, the automotive industry has its own version and one may want to review its guidelines in the AIAG (2001). To modify these guidelines, keep in mind:

  1. List the entire range of possible consequences (effects).

  2. Force rank the consequences from high to low.

  3. Resolve the extreme values (rating 10 and rating 1).

  4. Fill in the other ratings.

  5. Use consensus.

At this point the information should be transferred to the FMEA form ” see Figure 6.13. The column identifying the "class" is the location for the placement of the special characteristic. The appropriate response is only "Yes" or "No." A Yes in this column indicates that the characteristic is special, a No indicates that the characteristic is not special. In some industries, special characteristics are of two types: (a) critical and (b) significant. "Critical" refers to characteristics associated with safety and/or government regulations, and "significant" refers to those that affect the integrity of the product. In design, all special characteristics are potential. In process they become critical or significant depending on the numerical values of severity and occurrence combinations.

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Figure 6.13: Transferring severity and classification to the FMEA form.

FAILURE CAUSE AND OCCURRENCE

The analysis of the cause and occurrence is based on two questions:

  1. What design or process choices did we already make that may be responsible for the occurrence of a failure?

  2. How likely is the failure mode to occur because of this?

For each failure mode, the possible mechanism(s) and cause(s) of failure are listed. This is an important element of the FMEA since it points the way toward preventive/corrective action. It is, after all, a description of the design or process deficiency that results in the failure mode. That is why it is important to focus on the "global" or "root" cause. Root causes should be specific and in the form of a characteristic that may be controlled or corrected. Caution should be exerted not to overuse "operator error" or "equipment failure" as a root cause even though they are both tempting and make it easy to assign "blame."

You must look for causes, not symptoms of the failure. Most failure modes have more than one potential cause. An easy way to probe into the causes is to ask:

  • What design choices, process variables , or circumstances could result in the failure mode(s)?

DFMEA failure causes are typically specific system, design, or material characteristics.

PFMEA failure causes are typically process parameters, equipment characteristics, or environmental or incoming material characteristics.

Popular Ways (Techniques) to Determine Causes

Ways to determine failure causes include the following:

  • Brainstorm

  • Whys

  • Fishbone diagram

  • Fault Tree Analysis (FTA; a model that uses a tree to show the cause-and-effect relationship between a failure mode and the various contributing causes. The tree illustrates the logical hierarchy branches from the failure at the top to the root causes at the bottom.)

  • Classic five-step problem-solving process

    1. What is the problem?

    2. What can I do about it?

    3. Put a star on the "best" plan.

    4. Do the plan.

    5. Did your plan work?

  • Kepner Trego (What is, what is not analysis)

  • Discipline GPS - see Volume II

  • Experience

    • Knowledge of physics and other sciences

    • Knowledge of similar products

  • Experiments ” When many causes are suspect or specific cause is unknown

    • Classical

    • Taguchi methods

Occurrence Rating

The occurrence rating is an estimated number of frequencies or cumulative number of failures (based on experience) that will occur in our design concepts for a given cause over the intended life of the design. For example: cause of staples falling out = soft wood. The likelihood of occurrence is a 9 if we pick balsa wood but a 2 if we choose oak.

Just as with severity, there are standard tables for occurrence ” see Table 6.3 for design and Table 6.4 for process ” for each type of FMEA. The ratings on these tables are estimates based on experience or similar products or processes. Non-standard occurrence tables may also be used, based on specific characteristics. However, reliability expertise is needed to construct occurrence tables. (Typical characteristics may be historical failure frequencies, C pks , theoretical distributions, and reliability statistics.)

Table 6.3: DFMEA ” Occurrence Rating

Occurrence

Description

Frequency

Rating

Remote

Failure is very unlikely

< 1 in 1,500,000

1

Low

Relatively few failures

1 in 150,000

2

   

1 in 15,000

3

Moderate

Occasional failures

1 in 2000

4

   

1 in 400

5

   

1 in 80

6

High

Repeated failures

1 in 20

7

   

1 in 8

8

Very high

Failure is almost inevitable

1 in 3

9

   

>1 in 2

10

Table 6.4: PFMEA ” Occurrence Rating

Occurrence

Description

Frequency

C pk

Rating

Remote

Failure is very unlikely; no failures associated with similar processes

<1 in 1,500,000

>1.67

1

Low

Few failures; isolated failures associated with like processes

1 in 150,000

1 in 15,000

1.50

1.33

2

3

Moderate

Occasional failures associated with similar processes, but not in major proportions

1 in 2,000

1 in 400

1 in 80

1.17

1.00

0.83

4

5

6

High

Repeated failures; similar processes have often failed

1 in 20

1 in 8

0.67

7

8

Very high

Process failure is almost inevitable

1 in 3

>1 in 2

0.51

0.33

9

10

At this point the data for causes and their ratings should be transferred to the FMEA form ” see Figure 6.14.

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Figure 6.14: Transferring causes and occurrences to the FMEA form.

Current Controls and Detection Ratings

Design and process controls are the mechanisms, methods, tests, procedures, or controls that we have in place to prevent the cause of the failure mode or detect the failure mode or cause should it occur. (The controls currently exist.)

  • Design controls prevent or detect the failure mode prior to engineering release.

  • Process controls prevent or detect the failure mode prior to the part or assembly leaving the area.

A good control prevents or detects causes or failure modes.

  • As early as possible ( ideally before production or prototypes )

  • As early as possible

  • Using proven methods

So, the next step in the FMEA process is to:

  • Analyze planned controls for your system, part, or manufacturing process

  • Understand the effectiveness of these controls to detect causes or failure modes

Detection Rating

Detection rating ” see Table 6.5 for design and Table 6.6 for process ” is a numerical rating of the probability that a given set of controls will discover a specific cause or failure mode to prevent bad parts from leaving the operation/facility or getting to the ultimate customer. Assuming that the cause of the failure did occur, assess the capabilities of the controls to find the design flaw or prevent the bad part from leaving the operation/facility. In the first case, the DFMEA is at issue. In the second case, the PFMEA is of concern.

Table 6.5: DFMEA Detection Table

Detection

Description

Rating

Almost certain

Design control will almost certainly detect the potential cause of subsequent failure modes

1

Very high

Very high chance the design control will detect the potential cause of subsequent failure mode

2

High

High chance the design control will detect the potential cause of subsequent failure mode

3

Moderately high

Moderately high chance the design control will detect the potential cause of subsequent failure mode

4

Moderate

Moderate chance the design control will detect the potential cause of subsequent failure mode

5

Low

Low chance the design control will detect the potential cause of subsequent failure mode

6

Very low

Very low chance the design control will detect the potential cause of subsequent failure mode

7

Remote

Remote chance the design control will detect the potential cause of subsequent failure mode

8

Very remote

Very remote chance the design control will detect the potential cause of subsequent failure mode

9

Very uncertain

There is no design control or control will not or cannot detect the potential cause of subsequent failure mode

10

Table 6.6: PFMEA Detection Table

Detection

Description

Rating

Almost certain

Process control will almost certainly detect or prevent the potential cause of subsequent failure mode

1

Very high

Very high chance process control will detect or prevent the cause of subsequent failure mode

2

High

High chance the process control will detect or prevent the potential cause of subsequent failure mode.

3

Moderately high

Moderately high chance the process control will detect or prevent the potential cause of subsequent failure mode

4

Moderate

Moderate chance the process control will detect or prevent the potential cause of subsequent failure mode

5

Low

Low chance the process control will detect or prevent the potential cause of subsequent failure mode

6

Very low

Very low chance the process control will detect or prevent the potential cause of subsequent failure mode

7

Remote

Remote chance the process control will detect or prevent the potential cause of subsequent failure mode

8

Very remote

Very remote chance the process control will detect or prevent the potential cause of subsequent failure mode

9

Very uncertain

There is no process control or control will not or cannot detect the potential cause of subsequent failure mode

10

When multiple controls exist for a given failure mode, record the best ( lowest ) to calculate risk. In order to evaluate detection, there are appropriate tables for both design and process. Just as before, however, if there is a need to alter them, remember that the change and approval must be made by the FMEA team with consensus.

At this point, the data for current controls and their ratings should be transferred to the FMEA form ” see Figure 6.15. There should be a current control for every cause. If there is not, that is a good indication that a problem might exist.

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Figure 6.15: Transferring current controls and detection to the FMEA form.

UNDERSTANDING AND CALCULATING RISK

Without risk, there is very little progress. Risk is inevitable in any system, design, or manufacturing process. The FMEA process aids in identifying significant risks, then helps to minimize the potential impact of risk. It does that through the risk priority number or as it is commonly known, the RPN index. In the analysis of the RPN, make sure to look at risk patterns rather than just a high RPN. The RPN is the product of severity, occurrence, and detection or:

Risk = RPN = S — O — D

Obviously the higher the RPN the more the concern. A good rule-of-thumb analysis to follow is the 95% rule. That means that you will address all failure modes with a 95% confidence. It turns out the magic number is 50, as indicated in this equation: [(S = 10 — O = 10 — D = 10) - (1000 — .95)]. This number of course is only relative to what the total FMEA is all about, and it may change as the risk increases in all categories and in all causes.

Special risk priority patterns require special attention, through specific action plans that will reduce or eliminate the high risk factor. They are identified through:

  1. High RPN

  2. Any RPN with a severity of 9 or 10 and occurrence > 2

  3. Area chart

The area chart ” Figure 6.16 ” uses only severity and occurrence and therefore is a more conservative approach than the priority risk pattern mentioned previously. At this stage, let us look at our FMEA project and calculate and enter the RPN ” see Figure 6.17. It must be noted here that this is only one approach to evaluating risk. Another possibility is to evaluate the risk based on the degree of severity first, in which case the engineer tries to eliminate the failure; evaluate the risk based on a combination of severity (values of 5 “8) and occurrence (>3) second, in which case the engineer tries to minimize the occurrence of the failure through a redundant system; and to evaluate the risk through the detection of the RPN third, in which case the engineer tries to control the failure before the customer receives it.

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Figure 6.16: Area chart.
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Figure 6.17: Transferring the RPN to the FMEA form.



Six Sigma and Beyond. Design for Six Sigma (Vol. 6)
Six Sigma and Beyond: Design for Six Sigma, Volume VI
ISBN: 1574443151
EAN: 2147483647
Year: 2003
Pages: 235

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