Compressed Air Best Practices - May 2008 - (Page 22)

| 05/08 Compressed Air Industry AUTOMOTIVE ASSEMBLY | HOW TO AVOID THE MOST COMMON BUS/ COACH KNEELING FAILURES Test Results — At room temperature, all of the modules tested were leak free when solenoids were de-energized — normal ride mode for the bus. When filling airbags, all modules were also leak free. During the airbag exhausting phase, one 4-station module had excessive leakage due to an o-ring failure at the 800,000 cycle point (approx. 8 years). Upon completion of the cold and hot sticking tests, with the restoration of warmer temperature, all modules were leak free. All modules continued functioning during low temperature testing, but developed excessive leaks. As the test progressed, most of the low temperature leakage stopped when temperatures rose. Wear on the over-molded poppet (Figure 6), along with the poppet o-rings was minimal. Upon further examination after the completion of the test, it was discovered that the failed o-ring was due to a molding defect in the product and not as a result of the cycle testing. Figure 6: Over-molded poppet removed from kneeling module after 2 million cycles Failures Caused by Contamination While not as severe a factor as temperature, contamination is also a leading cause of failures of kneeling systems on busses and coaches. Contamination from weather and environmental conditions causes damage and potential failure to both the external and internal components of a kneeling module. External contamination to the module can eventually lead to leakage failures. Dirt and dust, water, salt, rocks and other debris can cause extensive damage to a kneeling module. For this reason, it is absolutely critical to design a system that will protect the module against these harsh elements. Internal corrosion is usually caused by contamination from water and/or oil from the compressor. This type of contamination can often lead to more severe life cycle failures. As mentioned above, if the system does not use a dryer to control moisture build up in module components and air lines, it can cause damage if the water either freezes or washes away lubricants needed to ensure that the module functions properly. Oil from other components of the air system can also cause damage to the module. This occurs when the oil has a reaction with different internal components. Seals suffer the most significant damage. A chemical reaction can damage seals significantly, thus reducing the life of the kneeling module. Testing Method — For this series of tests, four kneeling modules were used to measure the module’s performance in environments that cause contamination and corrosion — both externally and internally. The specific tests performed on these modules were Salt Spray Testing and Oil & Moisture Injection Tests. During the Salt Spray Test, four kneeling modules were placed in a salt fog chamber. The chamber was filled with a 5% salt and water solution. Humidity and heat were added throughout the test. The modules were soaked in the salt mixture for 200 hours and, following completion of the test, were function tested one final time. A second salt spray test was also performed under the same test conditions, but specifically on the kneeling module solenoid. In order to isolate the solenoid from the rest of the module during the testing, the second test used transportation-rated inline solenoid valves that utilize the same solenoid technology. To perform the Oil & Moisture Injection Test, four kneeling modules were connected downstream from a lubricator. An oil/moisture mixture was injected into each module at a rate of 3 cc per hour. The modules were then cycled at 18 cycles per minute continuously for 48 hours. Following the initial 48 hours, the modules were then tested for full function capability, as well as, all potential full system leaks using an upstream flow meter. Figure 7: Kneeling module after Salt Spray Testing Figure 8: Salt Spray Testing performed on pilot operator and coil 22 www.airbestpractices.com http://www.airbestpractices.com

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Compressed Air Best Practices - May 2008 Contents From the Editor Utility-Air News The Compressed Air Audit of the Month Pneumatics Real World Best Practices Delta Industries: Doing What is Right for Customers Delta Industries: Six Truths You Need to Know about Really Putting the Customer First Power Supply Industries Resources for Energy Engineers Wall Street Watch Advertiser Index Job Market

Compressed Air Best Practices - May 2008

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