3 minute 5 Why: Case2

The operators and maintainers collaborate in a 5 Why investigation into why the Mill does not restart.

They ask the question: Why did the mill shut down?

Answer: High current draw on mill motor.

They continue on with these questions and answers. The questions, answers and notes the operators use in their investigation could be recorded in a table like the following one.

Question	Answer	Notes
Why did the mill motor stop?	Because the current overload circuit breaker tripped.
Why did the current circuit breaker trip?	Because of high current draw by the motor.	Electricians identified that the circuit breaker was operating correctly and saw discoloration of the shroud around the drive end bearing.
Why was there a high current draw by the motor?	Because the drive end bearing had damage to the rollers and race.	Maintenance workers identified that the drive end bearing was seized.
Why was the Drive End Bearing damaged?	Because the bearing had been supplied with contaminated lubricant.	Inspection revealed signs of contamination in the lubricant circuit and parts of the bearing surfaces indicated scoring consistent with contaminants in the lubricant.
Why was the oil contaminated?	Because there was residual debris in the oil reservoir from a previous failure.	A similar problem had occurred in the past and there had not been enough time to properly clean the lubrication circuit so it was felt that there must have been some residual contamination which caused the problem.

The operators felt that their investigation was successful and had led to the real cause of the problem. They decided to enter the findings into the RCA Rt software to communicate their observations to the other shifts and as a way to store the details in case the problem happens again in the future. A screen print of their RCA Rt record is shown below. They did a quick check of the Risk Matrix to confirm that they had completed the job in accordance with company policy.

Click here to see their check on the Risk Matrix.