PFMEA – Cause of Failure Analysis

Bi-Monthly Informational Guide

MBC, Inc. Quality Tips

 

 Process Failure Mode Effects Analysis (PFMEA)

Identifying “Cause-of-Failure”

 

            Over the last three quality tips we have discusses the PFMEA as risk analysis, PFMEA as a Management Tool, and PFMEA as a customer complaint reduction tool.  Now perhaps it is time to look at the PFMEA as a root cause investigative tool.  We have previously discussed the three core or basic questions of the PFEMA.

 

1. 
   What is the requirement?
   Example – Secure the bolt to the proper torque specification.
2. What is the failure mode of the requirement?
   Example – Five Failure Modes on the Table.
3. What is the effect when the failure mode occurs?
   Example – Some Failure Modes have Two Effects.

 

These questions get the process started and get the analyst the basic information needed to begin to understand the risk issues of a given manufacturing or assembly process. (NOTE: the PFMEA can also be applied to service organizations).  The next step in the FMEA process is to identify “root cause” of a give “failure mode”.  See the table on the right to begin to understand that relationship between; requirement, failure mode, effect of failure.  Notice that a requirement can have multiple failure modes and failure modes can have multiple affects, thus the PFMEA grows in complexity as the process is analyzed.  The example shown here is typical and has some “Failure Modes” that may be questioned.  The last two, stripped could be the effect of “over torque”, and Cross threaded could be the effect of “improper assembly technique”.  It is probably more important to ensure the issues are in the PFMEA and addressed than to spend countless hours in debate as to which-line-the-item-of-question-is-on.  If the issue is addressed and the result is an effective, robust, repeatable process that produces high quality products for the customer the greater battle has been won.  If you strive for the purity or perfection of the document you will probably not address the failure modes or the effects of failure properly or effectively.

 

Now it is time to address the cause of the failure mode.  There are several critical pitfalls to avoid in this step.  As will be demonstrated as we continue th

e analyst must ensure they are truly addressing the root cause or causes of the “Failure Mode”, and they are not addressing the associated symptom of the failure mode.  There are usually two major stumbling blocks for this step of the process.  Many analysts want to identify the “Cause” as “Operator Error” the AIAG fourth edition Potential FMEA manual specifically calls out on page 91;

 

“Only specific errors or malfunctions (e.g., seal not installed or seal installed inverted) should be listed.  Ambiguous phrases (e.g., operator error or seal mis-installed, etc.) should not be used.”

 

Likewise another ambiguous phrase that is often used is “machine malfunction”.  Neither description, operator error or machine malfunction lends itself to corrective action.  The typical answer to operator error is ‘retrain’, for machine malfunction the response is ‘repair’ neither of these corrective actions are effective.  Only specific causes should be utilized. Operator installed component xyz in the wrong orientation, or the operator didn’t hold the torque gun perpendicular to the surface while securing the fastener, etc.  These causes c

 

an be specifically addressed with permanent corrective actions to eliminate or reduce future failures of the process.  As the analyst continues through the Process FMEA working on the “Causes” of “Failure Modes” actionable causes are always the goal.  What benefit is there to identifying a cause that is not correctable?