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Air Tightness Testing of Retread Aircraft Tires

A modular acoustic air tightness testing platform has been developed for manufactured products such as valves, catheters, bags, vials, packing, bottles, syringes, cartridges and tanks. Bridgestone Aircraft Tire Europe uses it to check the air tightness of retread tires.

The defective area is positioned in front of a laser marker.

Bridgestone aircraft Tire Europe was seeking a reliable method to check the air tightness of retread tires. Their main objective was to find a measurement technique that would make the decision (taking away the responsibility of an operator to make the decision) as to whether a tire is air tight, or whether it should be discarded. Bridgestone has assessed several techniques, such as loss of air pressure or infrared thermography. They opted for ultrasound air tightness testing after confirming that this method was the best to address their issue.

The next step dealt with the validation of the measurement capability and repeatability of the ultrasonic technique. It consisted of testing the air tightness of all sizes of tire at different pressures, using an SDT portable instrument. Their aim was not only to check, on a large enough sample, that all leaking tires were detected as NOK, but also, that all airtight tires were rated as OK. Based on conclusive results, Bridgestone has entrusted SDT International with the development of an ultrasonic air tightness measurement system to be fitted on a new testing machine.

The Principle of Ultrasound Detection

After applying overpressure or depression to the part to be tested, the ultrasound technique consists of measuring the sound pressure level. This is achieved using sensors set around the part to be tested. If there is a leak, the air that is released through the leak hole generates an increase in sound pressure, thus raising the pitch of the sound. The technique therefore consists of the measurement of the sound level increase. The online system then issues a discard part alarm.

The measurement of the sound pressure level is carried out over a frequency band typically centred on 40 kHz, hence the ultrasonic measurement name. Of course, the leak needs to be noisy enough to be detected. It is easily confirmed using a portable instrument.

This technique shows many benefits. For instance, it is not sensitive to the temperature effect resulting from the part's pressurization. The stabilization time is then eliminated and detection occurs immediately after pressurization.

The tire is loaded into the control machine.
The 4 ultrasonic sensors of the control machine.
The maximum amplitude of the signal generated by the 4 sensors shows the presence of a leak and its position on the tire.

Cycle times are shorter and test rates are thus increased.

Bridgestone has developed a semi-automatic control machine. It includes an SDT Online4US system and 4 ultrasonic sensors set on the surface of the tire. The loading and unloading of the tire are the only manual operations. A mandrel system is used to clamp the tire, which is then pressurized. The PLC sends an order to the online system to start its measurement cycle. It also controls the tire rotation: It should perform at least one full revolution.

Throughout the entire test cycle, the online system processes, individually and in real time, the signals delivered by the 4 sensors. It measures the signals and sends them to the PLC through a serial line. When the test is completed, the online system decides on the tire air tightness and transfers the OK part/NOK part information through TTL outputs. The decision on air tightness is automatic. It is no longer made by the operator.

Detection and Localization of Leaks

One of the outstanding features of the ultrasound technology is its capacity to localize the leak origin, in addition to its detection function: the closer the sensor to the leak, and oriented in its direction, the higher the ultrasonic signal. We then call it a local method.

In Bridgestone's case, by saving the tire's angular coordinates when the ultrasound signal reaches its maximum, the PLC can determine the exact position of the defective area.

When an air tightness fault is detected by the SDT Online4US system, after the testing cycle, the tire is presented in such a way that the incriminated area is placed in front of a laser marker. The operator can then confirm or not the presence of a leak and discard the tire.

If the tire is in good condition, the operator can validate the test status. It allows the tire to proceed to the next steps of the retread process.

Bridgestone Aircraft Tire

Aircraft tires operate under extreme conditions during take-off and landing phases, withstanding loads of several dozen tons, typically 25 to 30 tons. After a predefined period of activity, airline companies return their aircraft tires to Bridgestone Aircraft Tire Europe, the leading aircraft tire retreading factory in the world, to have them refurbished. In addition to the layout of new gum, tires are subjected to a rigorous series of inspections. One of the tests consists of checking the air tightness criterion. The FAA (Federal Aviation Administration) recommends a maximum pressure loss of 5 percent over 24 hours in its decree AC20-97B.

Online Condition Monitoring of Most Critical Assets

The new SDT Online4US utilizes ultrasound and vibration to provide continuous feedback about the health of your factory. Designed for critical and remote access assets, its versatility is limited only by your imagination. This modular online monitoring system combines ultrasound, vibration, temperature and rotation speed measurements and provides a condition check-up of the tool and issues alarms in case of drifts. It monitors any type of rotating machine with 4 condition indicators and some electrical facilities. It also allows controlling the operating of valves, drain valves and steam traps and detecting leaks on gas fluid networks.

The system allows the coupling of many standard interface sensors. Measurements are performed continuously and simultaneously on all connected sensors (up to 32 sensors inputs). Its large colour screen offers all user information and programming at a glance and a touch. The IP65 rated enclosure and sensors mean worry-free operation in the harshest environments. Quick cable management makes installation simple.

The system offers a complete online asset condition management solution evolving according to needs. You can add measurement channels, inputs, outputs, and communication features. Pay for what you need today with the flexibility to scale the system for the future. By combining or adding required modules, the user can set up a customized control system to meet all requirements and production rates.

This article was published in Maintworld Magazine

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