Rigid Body Modes

Rigid Body Modes If you analyse an engine then the first 6 modes are rigid body modes, in which the engine itself is rigid, but it moves in various ways on the engine mounts, typically at frequencies between 6 and 35 Hz. At a frequency of (say) 80 Hz we get the first flexural mode of the engine, where it bends in its own right. So in our terminology a RBM is merely a non-flexural mode whereas a zero frequency mode is an unconstrained motion. The same terminology is used with smaller components on the engine, so the alternator, for example, will have 6 rigid body modes where its mounting brackets are flexing, but the alternator itself is just a mass, and then at say 1000- 1500Hz it will have its first flexural mode, where the casing starts to flex. Typically the alternator's rbms are important for engine noise, and the flexural modes are responsible for whines and so on.