In this original post, A Mechanic’s Story: Basic Component Fatigue, we took a detailed journey through the physical side of a shaft failure RCA. We stopped at the physical side of that failure, parallel misalignment. However, stopping at the component level of failure does not constitute a credible and thorough RCA. Actually stopping at this level is more along the lines of a Shallow Cause Analysis (SCA). So let’s explore what makes the difference between a Shallow Cause Analysis and a Root Cause Analysis (RCA).

In the previous post we stopped at parallel misalignment. We will continue drilling from that point down. We ask ‘How could we have had parallel misalignment?’ Our team of subject matter experts (SME) hypothesizes 1) it was either misalignment at installation or 2) it became misaligned during operations. 

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As many of you know, I have been encouraging my LI contacts for years to take me up on my offer to review any pictures they have of failed parts, and we would try and provide them some preliminary feedback. Well, someone finally took us op on the offer and we wanted to share what was learned (we obtained permission to do so providing the company name was not used).

Here was the original inquiry via LI instant messenger along with the pictures:

“Dear Sir, As per your advise, I’m sending you a photo of failed flange bolts. I belief they were failed due to fatigue. Could you please review them and identify/ label their failure mode. After your comments on this photo, I’ll put up a recommendation on my RCA. Looking forward to hear from you. Regards.”

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“Assumption 1: Safety is increased by increasing system or component reliability. If components or systems do not fail, then accidents will not occur. (p. 7)

This assumption is one of the most pervasive in engineering and other fields. The problem is that it is not true.

Safety is a system property, not a component property, and must be controlled at the system level, not the component level. 

New Assumption 1: High reliability is neither necessary nor sufficient for safety. (p.13)”

These statements were excerpted from Nancy Leveson’s “Engineering a Safer World“. 

This contradicts the common belief there is a direct correlation between Safety and Reliability. I personally, being in the Reliability field for 30+ years, believe there is a correlation between Reliability and Safety. But I would assert that it is not a direct correlation. 

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