The vast majority of faults in large rotating equipment supported by sliding bearings are manifested as 1-fold vibration caused by imbalance. The diagnosis of the cause of the fault should be based on the changes in vibration with speed, load, temperature, and time. The diagnosis of sliding bearing equipment mainly relies on eddy current sensors to measure the relative vibration between the shaft and the bearing shell, and to determine various problems related to the rotor. Determine the problem of the support system based on the absolute vibration of the bearing seat measured by the speed sensor or acceleration sensor.
The eddy current sensors installed on sliding bearings are symmetrically installed at positive and negative 45 degrees, and the most common problem is why the values measured by these two sensors are different. This question has been mentioned many times on various platforms, and it is still frequently asked. In fact, the answer is very simple because the stiffness is different.
The following is a schematic diagram of a sliding bearing, with the bearing shell and bearing seat drawn as one unit. Two eddy current sensors on the left and right sides are fixed on the bearing shell. When the shaft rotates counterclockwise, due to friction, the shaft carries lubricating oil to the lower left side. The speed is fast enough, and the pressure on the lower left side is greater than the weight of the rotor, causing the shaft to be lifted. The higher the speed, the more it lifts up.
Due to the high pressure on the left side and low pressure on the right side, the shaft will be lifted and offset to the right. When the reverse axis rotates clockwise, it will shift to the left. Due to the fact that the axis is always on one side and not in the middle position under normal circumstances, the oil film thickness in the measurement direction of the two sensors installed at a 45 ° angle is different. The support stiffness of the oil film is directly related to its thickness (the thinner the oil film, the greater the support stiffness, the specific principle and formula can be found in fluid mechanics data), so the support stiffness in the X and Y directions is different. Under the same centrifugal force, the vibration in the X and Y directions is different.
Of course, the horizontal and vertical vibrations of the rolling bearing equipment should not differ too much. If the vibration difference between X and Y is too large, it indicates that the stiffness difference between the two directions is too large, which is abnormal. For example, there is a case where the installation clearance of the bearing shell is too large, causing a vibration difference of three times in the X and Y directions.
The critical speed of Y is also different from X
Since the support stiffness in the X and Y directions is different and the rotor mass is the same, the natural frequencies in the two directions are different. The following figure shows the Bode plot of two measuring points X and Y on the same bearing shell, and there is a slight difference in the maximum vibration speed between the two measuring points.
Friction
Friction refers to the contact between the rotor and stationary parts such as the housing, steam seal, oil seal, etc. (axial friction of gears has also been seen). The vibration of the rotor in the contact direction is limited, resulting in clipping of the axis trajectory (the waveform may be clipped in one direction, but can be clipped in both directions at the same time), and the amplitude of the direct frequency and first harmonic in the two directions may be significantly different. Friction adds additional support to the rotor, resulting in a much higher support stiffness in that direction than the oil film support stiffness, thus reducing vibration in that direction. Friction, as a special fault, will be discussed in the future.
The critical speeds for shaft vibration and tile vibration are also different
The shaft has stiffness, the oil film has stiffness, and all levels of bolt connections have stiffness. The rotor is essentially supported on a foundation by a series of springs, with each spring supporting a different mass (not to mention two parallel springs).
Shaft vibration measurement refers to the vibration of the shaft relative to the bearing shell. The total mass of the rotor and shaft, as well as the total stiffness of the shaft and oil film, affect the natural frequency of shaft vibration. Tile vibration measures the vibration of the bearing seat, and the mass and stiffness that affect its natural frequency are different. Therefore, the critical speeds of shaft vibration and tile vibration are also different.
The issue of determining stiffness based on vibration distribution
The unbalanced force acting on this string of springs causes vibrations that are inversely proportional to the stiffness of each spring. The lower the stiffness of a certain part, the greater the vibration at that location.
For sliding bearings, the magnitude of shaft vibration and tile vibration is inversely proportional to the oil film stiffness and bearing seat support stiffness. Due to the high installation stiffness of the bearing seat and housing, the stiffness of the oil film is relatively low. Under normal circumstances, the shaft amplitude value is greater than the tile amplitude value, with three or four times being normal.
If the tile vibration is similar to the shaft vibration, or even significantly greater than the shaft vibration, it indicates that the stiffness of the series of springs connecting the bearing seat and the foundation is lower than that of the oil film. This brings us back to the problem of checking the connection stiffness, which can be found by measuring the vibration difference between the joint surfaces.
Axial vibration of sliding bearing seat
The bearing shell supports the rotor through lubricating oil, and even if the rotor has axial vibration, it is difficult for the axial force to be transmitted to the bearing seat through the friction of the lubricating oil. Therefore, it seems that the sliding bearing seat should not produce axial vibration. In fact, how is the measured axial first harmonic vibration generated?
Let's first look at a schematic diagram of the support of a bearing on a shaft. The bearing has a certain length in the axial direction and is supported by the stiffness of the oil film between the bearing and the shaft. The bearing has its own support stiffness.
Due to gasket installation and other reasons, the stiffness of the bearing support may be unevenly distributed in the axial direction. Unbalance causes the shaft to vibrate up and down at the rotational frequency, and the pressure exerted on the bearing shell fluctuates once per revolution. The pressure is highest when the shaft is at the bottom and lowest when it is at the top. If the support stiffness on the left side of the bearing shell in the above figure is lower, the left side of the bearing shell will drop more every time it is pressed down in the axial direction. During the rotation of the rotor, the bearing shell will perform a seesaw motion at the rotational frequency, driving the bearing seat to 'nod' at the rotational frequency, and a vibration of one frequency will be measured in the axial direction.
So when the axial double frequency vibration of the sliding bearing seat is large, the installation of the bearing shell should be checked to see if the axial support stiffness is uniform and consistent.
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