Care and Caution
Few Ways to Predict Zipper Rupture; Here’s How to Keep Techs Safe
A steel cord radial truck tire is an engineering marvel. It has to endure a wide range of temperatures in every imaginable environment. This flexible chamber enables large vehicles to ride on a virtual cushion of compressed air.
In reality, the compressed air inside the tire carries the load of the truck. A radial truck tire utilizes steel cord sidewall plies to maintain the strength and integrity of all the components which make up that chamber. The beads simply act as anchors for these body cords, and the belt package protects them from the road surface.
The steel cord used for body plies and belt packages is actually a cable comprised of numerous smaller wires. In some instances, the cable is held together by a continuously wrapped "fret" wire. This single strand of wire keeps the cable wound tightly and ensures uniformity with the other components.
As long as radial truck tires are properly inflated and loaded, the steel cords are not subjected to excessive flexing or heat. Over the course of time, there is an insignificant amount of variation if correct air pressures are the norm rather than the exception.
On the other hand, any tires that are underinflated or overloaded will experience an increase in flexing and heat, often resulting in problems. If these tires make it to the truck stop or tire dealer before they separate, they become prime candidates for a dangerous zipper rupture.
Inflation is Vital
I coined the term "boot-o-meter" because it represents one of the most common forms of tire inflation maintenance. Some drivers go high tech and use a club, but others "just give ’em a kick." Neither method can be effectively calibrated. The only way to determine the inflation pressure of any tire is to put a gauge to the valve stem.
When radial truck tires are properly inflated according to the load being carried, a minimal amount of stress is applied to the components. According to the manufacturers, "any tire known or suspected to have been run at 80% or less of normal operating inflation pressure and/or overloaded, could possibly have permanent structural damage (steel cord fatigue)."
You don’t want to experience "steel cord fatigue."
Like any machine, if a tire is worked too hard for too long it’s going to break down. Once the assembly reaches less than 80% of the recommended inflation pressure, the tire is forced to flex more which generates a lot more heat. Overloading produces the same effect and can result in the same thing Ð a zipper rupture.
Any All-Steel Tire
A zipper rupture is a spontaneous burst of compressed air that can occur in the sidewall/flex area of EVERY steel cord radial tire. I put an emphasis on the word "EVERY" because a number of radial light truck tires have a steel sidewall ply. Tossing an LT235/35R16 on the center post machine could be hazardous if it’s an "all-steel" construction. Rule of thumb: If the sidewall ply is made of steel, the tire can experience a zipper rupture, so it must be inflated in a safety cage without exception.
The problem with zipper ruptures is the unpredictability of "fatigued" or damaged sidewall cables. In some severe cases, the cables break and produce an audible "pop" and/or a visible bulge during inflation. Sometimes the innerliner will show discoloration or wrinkling. Any bead damage or sidewall irregularity could trigger a zipper rupture. On the surface, if you don’t see or hear the signs, there’s no steel cord fatigue. But sometimes a rupture happens without warning.
That’s right, a perfectly good new, used, repaired or retreaded steel radial tire can suddenly zipper rupture when a customer goes to install it on the vehicle a week after it was delivered. It won’t have any bulges and there won’t be any popping sound. All of a sudden, it just goes "BOOM!"
Technology has yet to produce an affordable and dependable inspection machine for detecting potential zipper ruptures. The available equipment can be justified in a high-volume retread plant, but the average tire dealer cannot afford it. When it comes to detecting zipper ruptures in steel cord radial truck tires, service personnel have one option, the Tire Information Service Bulletin Vol. 33, No. 2, published by the Rubber Manufacturers Association (RMA).
RMA established inspection procedures for identifying potential zipper ruptures in steel cord radial medium and light truck tires in March 1995. This publication is an integral component of TANA/ITRA’s OSHA 29 CFR 1910.177 Compliance Training Program. These inspection procedures must be used on every tire returning to service.
Low Pressure or Overload
The first step is to determine whether the tire has been operated at 80% or less of the recommended inflation pressure or overloaded. Technicians should always check the air pressure of any inflated tire before it is removed from the vehicle. If one tire of a dual assembly is less than 80% of the recommended pressure, then both tires must be completely deflated, demounted and inspected.
After the deflated tire is removed from the vehicle, it must be demounted from the rim and thoroughly inspected for any irregularities. If the tire passes inspection on the inside as well as the outside, it can be mounted and inflated in a safety cage to 20 psi with the valve core removed. If no signs of a zipper rupture are present, the tire should be inflated to 20 psi OVER the recommended operating pressure.
Over? Once again, a simple inspection procedure that’s been around since 1995 is known by virtually no one. How many people actually know that every used, retreaded or repaired tire should be overinflated by 20 psi? How many know a suspect tire should remain in the safety cage at 20 psi over operating pressure for 20 minutes?
Outside of a thorough high-tech electronic inspection, the only real protection against a potential zipper rupture is a detailed visual inspection and overinflating the tire by 20 psi in a safety cage with the valve core removed. Any tire suspected of being operated underinflated or overloaded should remain in the safety cage overinflated by 20 psi for 20 minutes. Obviously, you’ll want to stay outside the trajectory zone during those 20 minutes.
The additional stress the overinflation will place on the steel sidewall cables may show irregular bulges Ð a telltale zipper sign before it is deflated to the recommended operating pressure. If lightening happens to strike, the best defense is strict adherence to established industry inspection and inflation guidelines.
Notice how many times I’ve said, "with the valve core removed." Every tire should be inflated without the valve core in case it needs to be deflated rapidly. Imagine a bulging and popping radial truck tire inflated to 120 psi in a safety cage with the valve core installed. How do you choose someone to remove the valve core if the clip-on air chuck somehow dislodges and doesn’t allow remote inflation or deflation?
Once the appropriate practice of overinflating and waiting becomes the prevailing method for checking tires before they are returned to service, the number of injuries due to zipper ruptures should be reduced.
It is likely that a few tires will rupture while overinflated. As long as it happens in a safety cage, neither your technicians nor the truck driver will likely get hurt.
Kevin Rohlwing is the director of training for TANA/ITRA.