5-Why Root Cause Analysis for Equipment Failures
The 5-Why method is a simple way to move beyond the immediate failure symptom and investigate the chain of conditions that allowed the event to occur. It works best when it is evidence-based rather than treated as a paperwork exercise.
Start with a precise problem statement
A good analysis begins with facts: what failed, where, when, under what operating condition and what consequence followed. “Machine breakdown” is too vague. “Dryer drive stopped after motor overload trip during normal production at 14:20” gives the investigation a useful starting point.
Ask why, then verify the answer
Each “why” should be supported by evidence such as inspection findings, alarms, trends, photographs, measurements, maintenance history or component examination. The method does not require exactly five questions. Stop when you reach causes that can be acted upon and for which the evidence is strong.
Example chain
- Why did the motor trip? Current exceeded the overload setting.
- Why was current high? The driven shaft required excessive torque.
- Why was torque excessive? A bearing had deteriorated and partially seized.
- Why was bearing deterioration not detected? Temperature and vibration were not part of the existing inspection routine.
- Why were they not included? The PPM checklist had never been updated after previous bearing failures.
The investigation now points to both the failed component and a maintenance-system weakness. Replacing the bearing alone restores the machine, but updating the inspection method helps prevent recurrence.
Separate root cause from contributing factors
Many failures have several contributors. Misalignment, contamination, excessive load and poor lubrication may all influence a bearing failure. Record these factors rather than forcing the event into a single simplistic explanation.
Turn findings into CAPA
Corrective action removes the immediate problem; preventive action reduces the chance of recurrence. Actions should have owners, deadlines and verification criteria. “Monitor machine” is weak. “Add monthly vibration reading at DE/NDE bearings, define alarm limit, train shift technicians and review trend monthly for three months” is measurable.
Validate the RCA later
An RCA is not complete when the report is signed. Review whether the corrective actions actually changed performance. If the same failure returns, reopen the analysis and challenge the original assumptions.