The five whys troubleshooting technique


THE FIVE WHYS TROUBLESHOOTING TECHNIQUE GT001_1

One of the Japanese automobile manufacturers has a unique method of troubleshooting any type of mechanical failure. The system is called the “five whys” and it is worth learning. It is a simple but powerful idea based on the concept that nothing has been solved until the question “why?” has been asked at least five times and a sensible answer has been given for each of the “why” questions asked. As an example, let’s look at some premature seal failures:

1. Why did the seal fail?

  • The lapped faces opened and solids penetrated between them. (solids can’t get in until the faces open)

2. Why did the faces open?

  • The set screws holding the rotary unit slipped due to a combination of vibration and system pressure.

3. Set screws are not supposed to slip. Why did the set screws slip?

  • The seal was installed on a hardened sleeve.

4. Why was the seal installed on a hardened sleeve?

  • This was a packing conversion and a stock standard sleeve was used.

5. Why couldn’t the mechanic tell the difference between a hardened sleeve and a soft one?

  • They were both stored in the same parts bin.

6. Why were they stored in the same parts bin?

  • Because they had the same part number.

7. Why did they have the same part number?

  • They should have had different part numbers. Once that problem is corrected, the failures will stop.

Now you get the idea! Needless to say you may have to go further than just five “whys”. In this case it took seven. Let’s try another example:

1. Why did the seal fail?

  • The pump was cavitating and the vibration caused the carbon face to crack.

2. Why was the pump cavitating?

  • It did not have enough suction head.

3. Why didn’t it have enough suction head?

  • The level in the tank got too low.

4. Why did the level in the tank get too low?

  • I don’t know.

You have not finished “five whys” so you better go find out why the level in the tank got too low, or the problem is going to repeat its self.

In this example I learned that the indicator float got stuck on a corroded rod giving an incorrect level indication.

One more example should do it. I ran into this one at an Opal factory in Germany.

1. Why did the seal start to leak?

  • The dynamic elastomer (O-ring) became hard and cracked.

2. Why did the elastomer get hard and crack?

  • It got too hot.

3. Why did it get too hot?

  • The pump stuffing box ran dry.

4. Why did the stuffing box run dry?

  • It was running under a vacuum and it was not supposed to.

5. Why was it running under a vacuum?

  • A Goulds pump impeller was adjusted backwards to the back plate and the impeller pump-out rings emptied the stuffing box.

6. Why was it adjusted backwards?

  • Most of the pumps in the facility are of the Duriron brand and they normally adjust to the back plate. The mechanic confused the impeller adjustment method. He has since been retrained

This is a powerful trouble shooting technique. I hope you make good use of it.

Category

Posted

  • On February 18, 2018