Summary and Introduction
Recently, you informed me that an unbalance in our demonstration test rig was causing it to experience high levels of vibration at the first critical speed. Because the test rig is to be used in presentations next week, you requested that I balance the unit. Using the single plane balancing procedure, my partner and I balanced the rotor at an operating speed of 3000 rpm. For the first critical speed of 2050 rpm and the balancing speed of 3000 rpm, the original and corrected vibration responses for vibration amplitude are summarized in Table 1. The next section presents the balancing procedure, and the section following that presents my recommendations on your questions about long-term storage of the test rig.
Table 1. Original and Corrected Amplitude Responses
| Response |
Operating Speed
N=3000 rpm |
Operating Speed
N=2050 rpm |
| Original |
14.8 mils p-p (300°) |
1.92 mils p-p (83°) |
| Corrected |
9.6 mils p-p (238°) |
1.70 mils p-p (29°) |
Procedures
As mentioned in the previous section, we balanced the test rig using the technique for single plane graphical balancing. Details of the balancing procedure are presented in the Balancing Worksheet, which is in the laboratory, and a summary of the procedure is provided here. In the first step of the balancing procedure, we spun the rotor in its previous unbalanced state and obtained the slow roll vector and original response vector (O) at the balancing speed of 3000 rpm. From the Bode plot of this original response, we found that the first critical speed occurred at approximately 3000 rpm. Next, a calibration weight was added as close to 90° from the heavy spot as the balancing holes allow. With the calibration weight in place, the rotor was again spun to 3000 rpm and the new response (O+C) was measured. By using the O and O+C vectors, the necessary size and position of the correction weight were added to the balance hole nearest the required weight position. Again, we spun the rotor to 3000 rpm to measure the corrected response (O').
Recommendations
In regards to your question of storing the test rig for long periods of time, I suggest that the test rig be stored vertically in a dry environment. If the test rig's rotor were stored horizontally, the weight of the rotor would more than likely cause a permanent bow in the shaft. Also, storage in a dry environment will protect the components from corrosion. When the test rig is put into use after a long period in storage, I recommend that oil be applied to each of the bushings. As was shown in Table 1, the balancing procedure has reduced, but not eliminated, the unbalance in the test rig. To further lower the vibration amplitudes, I suggest making a second run through the balancing procedure. If you would like me to perform this task or answer any further questions, please do not hesitate to contact me.
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