Question 3.10: A manufacturer produces an integrated ventilating system for...
A manufacturer produces an integrated ventilating system for customers, and Fig. 3.44 shows one of its products with two blowers. In mounting the blowers, a misalignment of the transmission plane of a blower will cause an excitation to the system. Conduct a modal analysis to confirm that two blowers operate at the frequencies that are below to the natural frequencies of the integrated system.
The SolidWorks Simulation is used to create an FEA model for frequencies analysis of the ventilating system. Firstly, the assembly model is imported as shown in Fig. 3.45a, and it is simplified to (1) suppress insignificant components such as the control system, the piping system, and panels, (2) the interfaces at the contacts of two components are checked to eliminate possible interferences for appropriate contact conditions, (3) thematerial properties are defined for all of the inclusive components.
Secondly, a modal analysis is defined, the conditions of contact surfaces are specified, the support components are defined as fixed geometries, the applied controls are defined for some critical parts or areas, and the mesh is created for the assembly model as shown in Fig. 3.45b. Finally, the number of the natural frequencies of interest is specified, the FEAmodel is solved, the list of calculated natural frequencies is exported in Table 3.13, and the corresponding modal shapes are shown in Fig. 3.46.
Table 3.13 The first six natural frequencies of the integrated ventilating system
| Period (s) | Frequency (Hertz) | Frequency (Rad/s) | Mode No |
| 0.086013 | 11.626 | 73.049 | 1 |
| 0.068318 | 14.637 | 91.969 | 2 |
| 0.046127 | 21.679 | 136.22 | 3 |
| 0.043557 | 22.958 | 144.25 | 4 |
| 0.030336 | 32.964 | 207.12 | 5 |
| 0.024534 | 40.76 | 256.1 | 6 |
Since the excitations are from the blower motors that are operated at the speed of 1750 resolutions per minute (RPM). Therefore, the excitations have the frequency of 29.167 (Hz) or 183.25 (rad/s). This operating frequency lies between the fourth (22.958 Hz) and the fifth (32.964 Hz) natural frequencies of the system in Table 3.13. It implies that the mounting misalignment may lead to an amplified vibration in operation and a premature failure of some fastening parts such as brackets and screws.
