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As we all know, in addition to radial vibration, axial vibration often occurs during the operation of rotating machinery. Radial vibration is vibration that occurs in the vertical and horizontal directions. Axial vibration refers to a simple structure in which the bearing body includes an upper ring body and a lower ring body, with rolling elements installed between the inner ring body and the outer ring body or between the upper ring body and the lower ring body. Compared with the radial vibration, the resulting vibration is more difficult to deal with, so we will talk about it in this article. So what is the main cause of axial vibration? The excitation force is too large and the bearing seat stiffness is poor. There are two situations in the analysis of axial vibration caused by excessive radial vibration.: One bearing seat contains one bearing bush and one bearing seat contains two bearing bushes. 1. A bearing seat is equipped with a bearing bush (b) Two-Medium Bending Mode Intuitively speaking, the axial vibration of the bearing seat should be caused by the axial movement of the rotating shaft. But in fact, there is an oil film between the rotor and the bearing. After the axial movement of the rotor is transmitted to the bearing bush through the oil film, it is difficult to cause axial vibration of the bearing seat. Most of the axial vibration that occurs in actual units is caused by radial excitation force. From a dynamic perspective, radial force and axial vibration are not in the same direction and should not cause axial vibration. However, after considering the bearing seat support elasticity, the radial force will indirectly stimulate axial vibration. Let’s take the above picture as an example for analysis. Assuming that there is a first-order unbalanced force on the rotor, the rotor deflection exhibits a first-order vibration shape, as shown in Figure (a). At time t1, the load-bearing center of the bearing seats on both sides of the rotor is biased toward the outside. ； At time t2, the load-bearing centers of the bearing seats on both sides of the rotor are tilted inward. At different times, the bearing center point of the bearing seat changes periodically, causing the bearing seat to swing axially. Under first-order deflection, the axial swing directions of the bearing seats on both sides are opposite. Under the action of second-order flexural vibration, as shown in Figure (b), the axial vibrations of the bearing seats on both sides are in the same phase. If the bearing seat is very rigid, the axial vibration caused by it is very small and can be ignored. If the bearing seat is elastic, radial vibration will indirectly stimulate axial vibration of the bearing seat. 2. A bearing seat is equipped with two bearing bushes. In the above figure, it is assumed that the vertical vibrations on both sides A and B are in the same phase. If the support stiffness on both sides A and B is also the same, then the radial vibration will not excite axial vibration. If the vertical vibrations on both sides of A and B are out of phase, or if the vertical vibrations on both sides of A and B are in phase, but the support stiffness on both sides is asymmetric, both cases will cause axial vibration of the bearing seat. The axial vibration that occurs in the above two situations is often accompanied by larger radial vibration. Therefore, when the axial vibration is large, if the radial vibration is also large, the best way to deal with it is to first reduce the radial vibration. Engineering practice shows that after the radial vibration is reduced by 10%, the axial vibration may be reduced by a higher percentage. In this case, reducing the excitation force requires much less work than increasing the stiffness of the bearing seat, and the effect is much more obvious. Analysis of axial vibration caused by insufficient stiffness of the bearing seat. Sometimes, after the vertical and horizontal vibrations of the bearing seat are eliminated, the axial vibration is still large. This situation is mostly caused by insufficient stiffness of the bearing seat itself. Axial vibration may be induced by loose connections between the bearing seat and the table plate and between the table plate and the foundation, uneven contact between the joint surfaces, loose secondary grouting, movement of the pads at the bottom of the table plate, etc. This type of fault must be handled in conjunction with unit maintenance. Analysis of axial vibration caused by other factors 1. The impact of excessive rotor center deviation on oil film pressure and axial vibration. When the rotor center deviation is large, after the coupling bolts are connected, a large shake will appear at the journal, as shown in the figure above. As the journal rotates, the oil film pressure will change periodically, which may cause axial vibration of the bearing seat in severe cases. 2. The tightening force of the spherical bearing is too large. The structure diagram of the spherical bearing is as shown in the figure above. The outer surface of the bearing bush of the spherical bearing is a convex spherical surface, and the supporting surfaces of the bearing cap and the bearing seat are a concave spherical surface. This type of bearing has a self-aligning function. When the journal is tilted, the bearing can automatically adjust the angle of the center line of the bearing bush so that good axial contact is always maintained between the black metal surface of the bearing bush and the journal. However, when the interference between the spherical pad and the pad seat is large, the pad pillow will press the bearing pad, causing it to lose its self-positioning function, resulting in greater axial vibration. 3. Analysis of axial vibration caused by axial resonance: Axial vibration with a frequency of twice the frequency often occurs on some generator bearings. Electromagnetic excitation force is the main factor inducing double frequency vibration. When the stiffness of the bearing seat is normal, the natural vibration frequency of the bearing seat is mostly higher than twice the operating speed, and the electromagnetic excitation force will not induce large-scale vibration of twice the frequency. However, when the connection stiffness between the bearing seat and the platform or the platform and the foundation is poor, the natural frequency of the bearing seat may fall near twice the rotational speed frequency. Under the action of the electromagnetic excitation force of the generator, a large amplitude double frequency axial resonance occurs in the bearing seat. After this situation occurs, it is necessary to check the connection stiffness of the bearing seat, reduce the rotor alignment deviation, check and adjust the uniformity of the air gap between the generator rotor and the stator, etc. This article has been explained. Let’s briefly review, under what circumstances will axial vibration be caused? Axial vibration caused by excessive radial vibration Axial vibration caused by insufficient bearing seat stiffness Excessive rotor center deviation Axial vibration caused by excessive spherical bearing tightening force Axial resonance