The Association of American Railroads (AAR) has sponsored an ongoing program to solicit new suspension technologies for commodity specific freight services. The process has involved assessing needs, publishing a performance specification, soliciting new designs, computer modeling, and evaluating prototypes by test and by economic models. Since 1999, this program has focused on better economic performance for bulk commodity service. Modest changes to familiar bulk commodity truck designs have resulted in significant economic benefits in railroad service.
For many years, the North American Railway industry has been interested in the shock and vibration environment of freight cars. Initially this interest was related to the potential damage to cargo. More recently the interest has expanded to enhanced car and component reliability. A recent important subtopic has been the installed resonance frequencies of pneumatic equipment for air brake control valves (CVs). Perhaps the most critical accessory feature of a railcar is the braking system — a complicated assembly of pneumatic controls and air reservoirs. Millions of pneumatic brake control valves are in service every day in North America. To work properly, these devices depend on spring-loaded parts moving within very small dimensional clearances. As pneumatic controls, they are challenged to receive and repeat subtle air difference signals down the length of any train they comprise. Further, they are expected to work in all weather and operating conditions, and for several years at a time without any maintenance or inspection. At this time, the North American rail industry does not have any standards for CV design (or mounting) that accounts for vibration. The CV builders themselves have put one proposal forward. The industry is currently discussing this proposed design limit — it would require installed natural frequencies to be above a minimum value. The valves themselves have been studied at length. However an examination of the current variations in both acceptable and “suspect” railcars has not been available thus far. In this paper, resonance variations for existing attachment methods will be presented, as well as common response modes. In addition, a preliminary method for predicting the severity of vibration levels for different freight cars will be presented.
The Interchange Rules of the Association of American Railroads (AAR) limit the life of freight cars to 50 years from the date originally built. Recently, however, the AAR has instituted a new provision under Interchange Rule 88 that permits cars to operate for up to 65 years since their built date. The procedure incorporates two basic portions; demonstrating that the carbody has the structural integrity to last for a total life of 65 years and upgrading specific components on each car. After applying to the AAR Equipment Engineering Committee (EEC) requesting that ILS be granted to a particular group of cars, the car owner has two optional methods to demonstrate the structural integrity of the selected cars. The first option is to perform structural inspections on a specified number of representative cars and to perform a full-scale fatigue test on a test car. In place of the fatigue test, the second option is to perform structural inspections on a larger number of cars and conduct follow-up inspections every five years after receipt of approval. The physical fatigue test incorporates modern engineering best practices by utilizing finite element modeling and full-scale accelerated fatigue testing (AFT). Following the creation of a representative model, several load conditions, both real and worst-case, are then applied to determine the high-stress locations. Using instrumentation at the high-stress locations, a full-scale test is conducted with the car operating in a typical service environment. The objective of full-scale testing is to obtain real strain data and input loads produced by typical environment conditions. AFT enables the required load cycles to be applied to the test car in a dynamic test fixture in weeks or months versus years of actual service. A rapid accumulation of fatigue-damaging cycles representative of the remaining years necessary to bring the total life of the test car to 65 years are applied to the car. The requirements for the components to be replaced or upgraded under Rule 88 are similar to those for new cars and for rebuilt cars. Some components, such as air brake control valves, are to be upgraded to more recent standards. Others are to be replaced in kind with reconditioned parts. Even though the carbody is permitted to operate beyond 50 years, components must still comply with existing AAR and Federal Railroad Administration (FRA) age limits. In addition to obtaining Increased Life Status (ILS) from the AAR, the car owner must also apply to the Federal Railroad Administration for authorization to operate the cars beyond the 50-year limit of the FRA Freight Car Safety Standards. This paper will demonstrate the approval process, including AFT testing, as applied to two groups of flat cars in auto rack service, and a group of 60-foot flat cars.
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