Application: Measuring the depth of steel and fiber ply cords or belting in all types of rubber tires, and in many cases total wall thickness as well.

Background: For quality control purposes, manufacturers of rubber tires need to know the position of the steel or fiber ply cords that are embedded in the walls of automobile, truck, and off-road vehicle tires. This is particularly important in the case of the large, expensive tires used for heavy trucks and construction equipment, where improper cord positioning can lead to costly failures. In some cases, manufacturers also want to measure total wall thickness. Because rubber is very attenuating to high frequency sound waves, and because of the complex pattern of echoes that are generated by the internal structure of most tires, these measurements require special consideration.

Equipment: There are two common approaches to rubber tire measurement, depending on the thickness of the tires to be measured. We recommend the following instruments for specific applications.

Model 35HP, 35DL-HP, or 38DL PLUS thickness gauge with an M1036 (2.25MHz) transducer. Used mostly for automobile and light truck tires, the 35HP or 35DL-HP gage with a low frequency transducer is generally used to measure depth of ply cords only. Typically the measurement range is approximately 3 mm (0.125") to 25 mm (1.0"), with a calibrated accuracy of +/- 0.25 mm (0.010") or better. The optional waveform display of the 35HP/35DLHP is useful for verifying detection of the first ply cord echo that represents the minimum rubber thickness.

EPOCH series flaw detectors with a square wave pulser such as the EPOCH XT, EPOCH LTC, EPOCH 600, or EPOCH 1000, used with selected low frequency transducers. Square wave flaw detectors with selectable bandpass filtering offer much greater penetration than a thickness gage and are recommended for applications involving larger tires such as those for trucks and off-road vehicles. Rubber thicknesses up to 200 mm (8") or more can usually be measured with 500 KHz transducers, sometimes in pairs used in pitch/catch mode. Typical measurement accuracy ranges from +/- 0.25 mm (0.010") or better on passenger car tires to +/- 1 mm (0.040") on very large truck and off-road vehicle tires.

Procedure: Sound attenuation and internal structure vary widely between various types of tires, so it is important to evaluate each application individually. Measurement of rubber always requires low frequency transducers, 2.25 MHz or lower. In some cases it is advantageous to use a transducer with an epoxy wear surface to improve sound coupling between the transducer and the rubber. The most commonly recommended transducers for tire testing are the Panametrics-NDT M1036 (2.25 MHz, 0.5" diameter, high penetration) and the V601-RB (500 KHz, 1" diameter).

In applications involving very thick tires (greater than 4" or 100 mm of rubber), pairs of V601-RB transducers are often used in pitch/catch mode. Separating the transmitter and receiver functions allows use of very high receiver gain without the potential noise problems associated with amplifying the excitation pulse recovery. Gel couplant is normally recommended for all rubber tire applications. To insure good wetting, work the couplant into the surface. As with any ultrasonic thickness measurement, it is necessary to calibrate the sound velocity of the rubber in question using a sample of known thickness. The velocity of rubber changes rapidly with temperature, so for best accuracy measure the sound velocity at a temperature that closely corresponds to actual measurement conditions.

Successful ultrasonic measurement of total tire thickness as opposed to the depth of the ply cords depends on the structure of the ply cords. In some types of tires where there are many layers of ply cords and/or the ply cords are very close together, not enough sound energy will reach the inside wall to permit measurement of the total thickness. In other tires where there is more separation between the individual ply cords, some sound energy will get through to the inside wall and measurement of total thickness will be possible. This must be evaluated on a case-by-case basis. In cases where an inside wall echo is visible, the instrument will be switched to as second stored measurement setup to read it.

Figure 1 shows the echo from steel ply cords, measured from the outside tread, in an automobile tire whose tread is approximately 12.5mm (0.5") thick. It is measured here with the Model 35DLHP gage and an M1036 transducer (2.25MHz). In this case the echo is measured to the first positive lobe.

In figure 2, the gage's gain and blanking settings have been readjusted for measurement of the total tire thickness of approximately 17 mm (0.67"), which is represented by the negative peak that follows the positive echoes from the ply cords.

Figure 3 shows a measurement to the fabric belting at depth of approximately 7.5" (190 mm) in a large construction vehicle tire, using an Epoch flaw detector with a 500 KHz V601-RB transducer. The measurement gate has been positioned so as to capture the rubber thickness represented by the negative peak.

Figure 3