Page 439 - Design for Six Sigma a Roadmap for Product Development

Failure Mode–Effect Analysis 401

(2) Environment and operator usage.
(3) Deterioration: wear over time.
The effect of a failure is a direct consequence of the failure
mode on the next higher hierarchical level: the customer and
regulations. Potential failure causes can be analyzed by tools
such as fault-tree analysis (FTA), cause-and-effect diagram, and
cause-and-effect matrix.
Two golden rules should be followed in cause identification:
the team should start with modes with the highest severity
rating and try to go beyond the first-, second-, or third-level
cause.
6. Ranking of potential failure modes using the RPN numbers so
that actions can be taken to address them. Each failure mode has
been considered in terms of severity of the consequences, detec-
tion, and occurrence of its causes.
7. Classifying any special DPs as “critical” or “significant” character-
istics that will require controls. When the failure mode is given a
severity rating greater than specific critical rating, then a poten-
tial “critical” characteristic,* a DP, may exist. Critical characteris-
tics usually affect safety or compliance to regulation. When a
failure mode–cause combination has a severity rating in the same
range below the “critical” threshold, then a potential “significant”
characteristic, a DP, may exist. “Significant” implies significant to
some CTSs in the QFD. Both types of classification are inputted to
the PFMEA and are called “special” characteristics. Special char-
acteristics require “special controls,” requiring additional effort
(administrative, measurement, overdesign, etc.) beyond the nor-
mal control. Robust design methodology is generally used to iden-
tify the “special” characteristics.
8. Deciding on “design controls” as the methods used to detect failure
modes or causes. There are two types of controls:
a. Those designed to prevent the occurrence of the cause or fail-
ure mechanism or failure mode and its effect. This control type
also addresses reduction of the occurrence.
b. Controls that address detection of cause, mechanism, or failure
mode, by either analytical or physical methods, before the item is
released to production.
9. Identification and management of corrective actions. On the basis
of the RPN numbers, the team moves to decide on the corrective
actions. The corrective-action strategy includes

*Manufacturing industry uses , inverted delta, to indicate “critical” characteristics.
The terms critical and significant were initially used in the automotive industry.