METHODS AND MEANS FOR DETERMINING THE NATURAL MODES OF VIBRATION OF MECHANICAL STRUCTURES

Cyclic strains in vibrating mechanical structures often lead to failures because of the resulting fatigue of the material. Both theory and practice indicate that the breaks or other types of failure occur at a node of one of the numerous natural modes of vibration of the structure. These natural modes of vibration can be determined theoretically if the structure is simple and the parts are of geometrical form but usually the problem is too complicated for exact solution and the best that can be hoped for from theory is a rough approximation of the resonant frequencies and a still rougher approximation of the location of the nodes of the vibrating system. The present paper describes a simple method and means for experimentally determining the several natural resonant frequencies of any mechanical system and the location of the nodes of their respective standing wave systems. Its effectiveness is demonstrated by analyzing three modes of vibration of a ship's propeller.