The Measurement of Cord Tensions in Tires

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Abstract
Tensions were measured in the reinforcing cords of tires during curing, post-inflation, and in service, using a miniature transducer employing resistance strain gauges. Cord tension varied throughout the cure cycle and was influenced by temperature, ply fabric width, cord modulus, and shrinkage. The cord tension reached a maximum (i.e., 2.2 lb) immediately after mold closing when the tire was forced out to mold dimensions. The tension was proportional to the fiber modulus and inversely proportional to the width of ply fabric. About two minutes after mold closing, the tension decreased sharply because the rubber became soft. The cord position shifted in the rubber and the tension was removed. Thereafter, thermal shrinkage of the fiber determined the cord tension as the tire temperature increased and the rubber cured around the cord. During post-inflation, tension was proportional to inflation pressure and reached 5.0 lb at 35 psi. Under these conditions, the cord stretched and tire size was related to cord extensibility. Tension in the cured tire increased in proportion to inflation pressure and was at a maximum at the tread center (3.0 lb at 24 psi), decreasing toward the bead to 2.3 psi. These findings were in agreement with calculations based on membrane theory. In straight running, cord tension in ply one opposite the center of the tread followed an essentially sawtooth pattern, decreasing below the inflation tension (from 3.0 lb to 1.4 lb) and returning to the inflation tension as the point of measurement moved into and out of the deflected region at the road surface. Increasing or decreasing inflation pressure shifted the sawtooth profile up or down without changing its basic shape. At constant pressure (i.e., 22 psi), increasing load from 700 to 1200 pounds increased the tension decrement from 1.1 to 1.5 lb. The principal effect of speed was to increase the overall tension because of higher operating pressure without significantly changing the sawtooth pattern. A condition of severe overloading, underinflation, or a combination of the two caused the cord in the sidewall to go into compression. This is believed to contribute to fatigue damage, based on previous work.

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