Look before you leap: Casing inspection can make or break a retread, says Bozarth
A retreaded tire is only as good as the casing foundation it's applied to during the retreading process.
However, each casing is just a little different in that it may have different types of injuries. The history of past use is usually unknown. And each casing may have been manufactured at a different time and location. This may have an effect on the retread's ability to safely deliver additional tread life.
A good casing inspector will need to be familiar with all the possible things that can affect the performance of a casing and know how to make the proper decisions about a casing's retreadability.
Good eyesight and proper lighting, including a hand-held light, are critical for proper casing inspection.
The inspector must establish a routine that should be followed with each casing. This routine will assure that all areas of the casing are thoroughly inspected.
All nail hole injuries must be probed thoroughly to determine if they have spread from rust or flexing and may need a section repair or need to be scrapped.
The inspector must determine if any injuries are repairable and also that repairs meet industry requirements and the specifications of the tire owner or fleet.
Finding hidden damage is always a challenge. The inspector will have to call upon his or her experience.
Radial casings are less likely than bias-ply tires to show a clear indication of having been run low or flat. A bias tire that has been run flat will usually show a crisscross flex pattern in the inside sidewall. A radial tire often will show little or no indication on the inside sidewall of being run flat.
Wear patterns in the tread like the rounding off of each tread rib and excessive rim flange wear in the bead area are all indications of a tire that has been run while severely under-inflated.
If a high-pressure tester is available, broken cords usually can be detected by inflating the casing to as little as 30 psi. Broken cords almost always will show up as different sized bulges that are randomly spaced around the outside sidewall flex area. Be aware that these bulges may show up only on one side of the casing. All plant personnel should watch for these bulges when the tire is inflated throughout the retread process.
In addition, weather checking can be difficult to see when a tire is under-inflated. It's a good idea to stress the tire sidewalls by hand or with an inspection spreader. The inspector needs to be aware that cracking from ozone or exposure to chemicals may only show up in just one sidewall or in a small area.
Play by the rules
The inspector needs to be aware that certain brands or types of tires may not perform well in a specific application and should see that the proper tread design is selected.
Tires that have had the DOT number or serial number intentionally removed should not be retreaded as they may contain hidden damage. The only situation in which it's acceptable to retread these casings is if you have documentation from the manufacturer that the adjustment of the tire was for fast tread wear or some other non-safety issue.
The inspector should always be aware of state and federal regulations that apply to the tire being retreaded and also verify compliance.
It also is important that the specifications of the production plant, the individual customer and the fleet are followed closely. These may apply to the number and location of repairs, tread design and width, the maximum age of the casing and/or the number of times a casing can be retreaded.
Failure to follow these specifications can be costly if the customer has an accident.
Another important part of casing inspection is to examine any in-plant failure and adjustments that are returned from the field. This helps establish a data base of failures that could be eliminated through a more critical inspection procedure.
This also can help identify certain types of casings that may not be acceptable for retreading, Many times these inspections may identify the need to buy newer inspection equipment.
Today we have a lot of available inspection equipment to help the inspector. It is extremely important to understand the information and images these machines transmit. It also is important to know the limitations of these machines.
The most commonly used inspection machine is the Hawkinson NDT. This device is commonly called a non-destructive tester or nail-hole detector. It uses an electrical charge to find penetrations.
However, the operator must still determine the size and location of the puncture and also if and how it is to be repaired.
High-pressure testers subject casings to a range of pressures, usually starting at 25 to 30 psi in the first stage, 60 to 70 psi in the second stage, and up to 100 psi in the third stage.
This test is primarily used to look for broken cords that have been caused by the tire being run flat abused in some other manner. High-pressure testers can be used at initial inspection, final inspection, or both.
X-rays are primarily used to look for breaks of frayed steel cords, but are time-consuming and can be expensive.
Ultra-sound is used in a few plants, mainly to look for separations between various casing components. A monitor shows the area where the separation or air pocket is located. This machine also can be used to inspect the finished product and will detect a loose repair patch or porosity at the buff line.
Shearography, though expensive, does an excellent job in finding separations in casings. Shearography machines take a series of photos of the inside of a tire while it's in a relaxed position and then repeats the same photos while the tire is subject to a vacuum in an enclosed dome or other type of sealed container.
The two different images are then fed into a computer, which detects any difference in the tire when it's in the vacuum chamber. Any movement means there is a void in that area of the tire. The information appears on a computer screen, giving a clear indication of where the separation is located as well as the size of the void. Some units are programmed to give acceptable or non-acceptable conditions.
Any separation that is open to the atmosphere from a cut or a nail hole will not be detected by this technology since the vacuum will not cause a movement in the separated area.
Not so fast
I am concerned that lots of good radial truck tire casings are being scrapped because some inspectors are misinterpreting what they are seeing during the inspection process.
Casings with a slight bulge due to a nail hole or spread cable are often scrapped in the belief that the tire is separated.
I encourage shop managers to buff into a few of these casings to illustrate that they are not separated and will indeed perform safely.
The same thing is true of pinch shock, which is caused when a radial truck tire sidewall is pinched between the rim flange and the curb. This causes a slight kink in the sidewall body ply. If the kink is severe, the tire must be rejected. If the kink is not at a sharp angle, the casing is not damaged.
Sometimes the only way to know if pinch-shocked tires are damaged is to buff into the distorted area. You will destroy some casings, but if you're scrapping them anyway, what have you got to lose if you can demonstrate that many tires that are thrown away are, in fact, retreadable?
Tires will continue to change, technology will change and operating conditions will change. What will not change is the importance of quality casing inspection.
Marvin Bozarth is the founder of Louisville, Ky.-based Bozarth Tire Industry Consultants LLC. The company provides tire and wheel failure analysis, expert witness testimony, equipment evaluation, plant surveys and analysis, fleet surveys and general troubleshooting. A member of the Tire Industry Hall of Fame, Bozarth also serves as senior technical consultant for the Tire Industry Association.