Vibration Response Modes and Severity Rating for Freight Car Appliances

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.