Mitigation of Underslung Engine Induced Rail Coach Floor Vibration

The underslung arrangement consists of a 500kVA (kilovolt amperes) diesel generator set mounted below a railway passenger coach. To meet stringent vibration requirement in passenger compartment, dual vibration isolation arrangement is proposed for underslung arrangement. Such type of arrangement provides a high degree of isolation from engine induced vibration. Performance of dual stage isolation depends on stiffness and mass ratio of genset mass to intermediate bracketry mass. High vibration excitations are observed on rail coach floor during testing of baseline design. These vibration levels are more than rail prescribed limits and passenger can easily feel these vibrations. Study of dual stage (2 degree of freedom) isolation system frequencies and transmissibility is performed. Root cause for existing system is assessed through vibration transmissibility analysis. Modal analysis was performed to understand mode shapes and interaction between the mounting structure and isolators. Assessment shows that existing isolation system bracket modes are within engine exciting frequency range hence it amplifies vibrations. Vibration transmissibility sensitivity with respect to mass and stiffness is studied to get combination of mass and stiffness such that frequencies will be away from the engine operating frequencies. Based on analysis, overall design was improved to meet vibration requirements. Proposed solution to mitigate the vibration issue was validated with experimental measurements. Paper highlights a common mistake that most of the studies consider foundation as rigid body; in actual application detailed system level study and foundation stiffness plays important role. This paper gives analytical lead design guidelines to implement dual level isolation system for rail, marine, industrial etc. applications.