Page 15 - RCM - A Practical Guide

RCM - A Practical Guide

THE SIX PATTERNS OF FAILURE
Let’s explore the six patterns of failure that were identified by Nowlan and Heap, these patterns show
statistical representations that describe how a type of equipment wears on, on average.
Pattern A
This is often referred to as the bathtub curve and accounts for approximately 4% of all items.
It has three distinct phases:
• Infant Mortality
o A period of relatively high probability of failure immediately after manufacture or
overhaul
• Constant probability of failure
o A period where random failure is expected but the probability of failure typically
remains low and doesn’t change over time
• Wear out
o A period in which the probability of failure begins to increase rapidly with age

Figure 4 - Pattern A

Pattern B
This is often referred to as the wear out curve and accounts for approximately 2% of all items.
It has two distinct phases:
• Constant probability of failure

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