February 15, 2011
Tire restoration
New type of shoulder repair could save money and time
By: Kevin Rohlwing
When a 20 repair unit is installed to repair a puncture in the shoulder, the edge of the patch is positioned in the middle of the flex area on the sidewall.
As it stands today, there are four types of repairs in the commercial truck tire industry: puncture, spot, reinforcement and section.
A puncture repair can be installed in the field and must be located in the crown area of a truck tire, which is the center of the tread approximately 1 to 1.5 inches in from the edge of tread. Puncture repairs are also limited to a maximum of 3/8 of an inch in diameter. A spot repair is a rubber-only repair on the outside of the tire where an area of missing rubber is replaced without any damage to the body cables or belt wires. When a piece of rubber is missing and the casing is exposed or damaged without actually penetrating the innerliner, a reinforcement repair may be installed. Finally, when the damage exceeds 3/8 of an inch or the injury is located in the shoulder or sidewall area, a section repair is required.
A repair, by definition, restores the original condition of the tire. In order to accomplish this, there are several criteria that must be met.
First of all, the damage must be completely removed. Truck tires are a complex blend of rubber and steel cords, so when an object penetrates the belt package and body plies, the area surrounding the injury is initially weakened to some degree. Since each cord is actually wound like a cable, when it breaks, the cable will continue to unwind. As the cables unwind, the area becomes even more unstable, which often leads to a separation.
Another key step to restoring the original condition with a proper tire repair is to fill the void with rubber. In the case of puncture repairs, this is typically accomplished by installing a cured rubber stem. The stem is coated in cushion gum, which reacts with the vulcanizing cement to chemically cure, or bond, the stem with the tire.
When puncture repairs are installed in a retread plant, un-cured rubber may be used to fill the void with an extruder gun. In these instances, the tire must be placed in a chamber to cure the rubber or the main components of vulcanization (time, temperature and pressure) can be applied with a portable spotter.
In either case, the end result is identical. The void left by the penetrating object is filled with rubber so water and moisture cannot enter the casing which will lead to corrosion and, you guessed it, a separation.
Finally, the innerliner of the tire must be sealed with a repair unit to prevent any further air loss. On puncture repairs, this is accomplished by a universal or reinforced repair unit.
Since the goal is to restore the original condition of the tire following a repair, there is universal agreement on the basic steps and guidelines for all types of repair. However, one particular type of injury continues to be a problem for the truck tire service providers.
Puncture repairs are restricted to the crown area which means simple “nail holes” in the shoulder must be treated as section repairs. This results in a large number of tires and casings being permanently removed from service for a variety of reasons.
First and foremost is the fact that a section repair cannot be installed in the field or on the side of the road. The tools and curing equipment are not readily available in a typical commercial tire service facility, so these tires usually are sent to a retread plant. Then there is the issue of price. A section repair will cost the tire dealer anywhere from $30 to $50. After the markup, the fleet can pay up to $70, which in many cases is almost the price of the casing. These two factors alone are often sufficient to scrap the tire.
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