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Sunday, January 22, 2012

Things to know before we get to failure analysis

For quite some time I’ve wanted to start a series on things to look for when examining a tire that has failed.

Why is it important to have an idea about why a tire failed based on an educated thought process rather than just a guess based on inaccurate or incorrect hearsay from an RV forum post? If you don’t know why something happened, how can you be sure the “corrective actions” you are taking such as increasing size or load capacity or changing manufacturer or moving your bowling ball collection from one side of the RV to the other, will prevent another failure?

Failed tire analysis is a combination of science and art coupled with decades of on-hands experience under the guidance of experienced engineers. When you undertake the process, even at the most basic level, obtaining reasonable results is better than a guess based on incorrect training or no actual knowledge of what a tire is. Without even the most basic understanding of how a tire is manufactured your guess would most likely be no better than that of a medieval healer suggesting that bleeding a patient would prevent a heart attack.

I have no expectations that this blog can substitute having a degree in chemistry or engineering and working decades in the tire industry, but I do think that I can provide some basic concepts to help you know what clues to look for so you can have a reasonable expectation of identifying the most common causes of tire failure.

Think for a moment about taking a class in CPR. You would not be able to do open heart surgery but you might be able to save someone’s life because you knew much more than that medieval healer did about how the cardiovascular system worked.

OK lets get started.

First off let’s talk about tire manufacturing. Experience has taught me that most people know that tire manufacturing involves putting rubber in a mold and cooking or more accurately “curing” it. Some even understand that before the curing process, rubber is mixed and different parts such textiles and steel components are assembled, like mixing cake ingredients before baking it. If you think about making an involved main course such as Boeuf à la Bourguignonne you know that there are a number of ingredients that are combined in different amounts and specific sequences before cooking.

I think that if we gave the same recipe for the beef dish to the great Chef Julia Child and to Gary Bunzer, The RV Doctor , we might end up with two edible meals but I think Julia’s might be better. Sorry Gary, I do not intend to disparage your skill as a cook but I believe Julia was a bit better cook than you might be.

Let’s first look at how a tire is built or assembled. I will cover chemistry and other items in another post.

This video is from 1934. You can see a four ply bias tire being manufactured. As you will see in the other videos many of the same steps are still performed in today’s tire plants with most of the steps now automated. Even though this is an old video I think it will help you understand the faster process you will see in other videos.

This 1951 video, also of a bias tire, is shorter but it shows the significant advances in manufacturing especially in the curing part of the process with the elimination of the water bag and all the attendant manual handling of hot metal and rubber.

A more modern video from Michelin mentions the dozens of components that were made from hundreds of chemicals and raw materials that go into manufacturing a radial tire. Notice another significant advance in the curing process with multi-piece molds allowing the curing of the stiff steel belted tread area

This video, I believe from the 1990s, shows a radial tractor tire being built. The tire size may be different and this is much less automated than current high volume passenger, light truck or TBR tires, but again a similar process is involved.

In this video from Pirelli, you will still recognize the steps we saw in 1934 but computer controls allow for an almost fully automated building process. Even if you don’t speak Italian, I think you can understand the process as we build your knowledge base.

Finally here is a short video from Hankook from 2010 that shows the overall flow through a tire plant. By now you should recognize all the steps.

I hope you enjoyed the videos and now have a little better appreciation of the process.

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