# Experimental Study of See-Saw Mode Nano-Vibration on Orifice-Type Restrictors
## 4 Discussion  
From the results in Figure 9, it is clear that:  
* Normal nano-vibrations are generated at different measurement positions of the restrictor under different air-supply pressures with amplitudes ranging in picometer or nanometer order. For each position, the amplitudes of the normal nano-vibrations increase equally with the increase in air-supply pressure. This reveals the existence of normal vibration, and its magnitude is related to the air pressure. The greater the air pressure, the greater the vibration. * For the double-ring-grooved style restrictor under the same air-supply pressure, the measured positions of P1, P2, P3 and P4 had the largest similar vibration amplitudes. Those of positions P5 and P6 were reduced and close, and that at position P7 was the smallest. This shows that the normal vibration is a form of rotational vibration, which is consistent with the theoretical analysis (Section 2.2). This vibration form represents the see-saw mode discussed. For the one-orifice double-ring-grooved style restrictor, its vibration rotation axis should be in the restrictor's center at the orifice. More specifically, it should be located between the orifice and the center of gravity. * For the double-U-grooved style restrictor with two orifices, the measurement data show that the vibration rotation axis is not at the center of the restrictor. It can be imagined that from a microscopic perspective, the two restrictors have the same nonidentical effect on the vibration as the spring model. Thus, the axis of rotation appears between restrictor orifice 1 and the center of gravity or between restrictor hole 2 and the center of gravity at different times. Therefore, position P7 near the center has a very large vibrational amplitude. * The variety of air-supply pressures changes the corresponding maximum vibration amplitude among P1, P2, P3 and P4. Hence, we speculate that the different air-supply pressures change the air-film thickness, flow rate and pressure distribution, which cause slight changes in the rotation axis, including the position and angle. However, we must consider that this difference could be caused by the inaccuracy of the laser-spot positioning. Figure 10: Power spectral density of vibration signals under different air-supply pressures: (**a**) double-ring-grooved [25]; and (**b**) double-U-grooved. Although the overall size was the same, the vibration of the double-U-grooved-style restrictor was larger than that of the double-ring-grooved-style restrictor. If the influence of the equalizing groove were to be ignored, then it is most likely that the two orifices would magnify the vibration far greater than one orifice would. * The simulation results of Section 2.2 showed that even if the speed of the inlet air changes per air-supply pressure and gas film distribution, the restrictor maintains the same rotation amplitude. However, we did not observe this phenomenon. The rotation amplitude varied evidently with different inlet air pressures. This requires further exploration in a future study. * From the results in Figure 10, it is clear that for the double-ring-grooved-style restrictor, the normal nano-vibration generated at positions P1-P7 at 0.5 MPa carried large power near 9.4 and 10.6 kHz, even though P7 had no evident power concentration. Furthermore, different measurement locations carried certain amounts of power around 21.2 and 23.1 kHz. Because 21.2 and 23.1 kHz are approximately twice the frequencies of 9.4 and 10.6 kHz, respectively, it can be assumed that 9.4 and 10.6 kHz are the fundamental frequencies of the normal nano-vibration of the restrictor, and 21.2 and 23.1 kHz are their harmonics frequencies. * For the double-U-grooved-style restrictor, the normal nano-vibration generated at different positions at 0.5 MPa air-source pressures carried large power around 7.9, 17.7 and 25.3 kHz. However, there was no evident power concentration phenomenon at P7.