Understanding Simulation Professional's Frequency Analysis Support Issue
Every structure has the tendency to vibrate at certain frequencies, called natural or resonant frequencies. Each natural frequency is associated with a certain shape, called mode shape, that the model tends to assume when vibrating at that frequency. When a structure is properly excited by a dynamic load with a frequency that coincides with one of its natural frequencies, the structure undergoes large displacements and stresses. This phenomenon is known as resonance.
Recently, one issue came up for one of our customers where the natural frequency values (in Hz) were all very small (approximately zero.) The issue wasn’t with the Simulation software, but with the properties set for that type of analysis. If you right click on the study, and select Properties, you will get a box with all the properties for a frequency analysis. One of the properties is Number of Frequencies. This property lets you set the desired number of natural (resonant) frequencies to be calculated. The default is set to calculate the lowest five frequencies. Now the problem arises when there are no supports (fixtures) on the component being analyzed. Basically, a body without any restraints has six rigid body modes and these rigid body modes have zero frequencies (infinite period.) So, SolidWorks will assign to these six rigid body modes (the first six natural frequencies) a value approximately equal to zero. Therefore, if you are looking for the first natural frequency of your component that has no supports, you must input 7 for the Number of Frequencies property.