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How to calculate and analyze the sealing capacity of magnetic fluid

numerical calculation and analysis of sealing capacity of magnetic fluid:

the magnetic field and the saturation magnetization of magnetic fluid have a direct impact on the sealing of magnetic fluid. The change of magnetic fluid seal clearance, shaft eccentricity, amount of magnetic fluid, shaft diameter, centrifugal force, etc. have an impact on the magnetic field at the magnetic fluid seal clearance, and also have an impact on the seal pressure difference of magnetic fluid. In this paper, the effects of sealing clearance, shaft eccentricity, shaft diameter and centrifugal force on the sealing ability of magnetic fluid are quantitatively analyzed

there are many factors that affect the sealing ability of the rotating shaft of magnetic fluid. Among them, the material characteristics of the magnetic fluid and the strength of the magnetic field have a direct impact on the shaft sealing. For sealing devices with the same materials and similar structures, the eccentricity of the rotating shaft, the change of the sealing clearance, the diameter of the rotating shaft and the role of centrifugal force will all have an impact on the sealing ability. It is very important to analyze the influence of these factors for the design and use of magnetic fluid seals. In this paper, the influence of design and manufacturing factors on sealing capacity is analyzed by numerical calculation method

1. Theoretical basis of sealing

generally speaking, the applied magnetic field is strong, the magnetic fluid is in a saturated state, and its magnetization is approximately equal to its saturation magnetization Ms. regardless of the effect of centrifugal force during rotation, the pressure at any point in the magnetic fluid single-stage seal is:

where H - the distance from the reference point to the magnetic fluid micelle along the gravity direction, C - the integration constant, which is determined by the boundary conditions

when there are many magnetic fluids, it can be considered that in the limit state, the magnetic induction intensity at the low-pressure side of the fluid is 0, ignoring the gravity effect, and the limit sealing pressure difference of the single-stage shaft seal is approximately:

when the rotating shaft rotates at the angular speed W0, the magnetic fluid will also be affected by the centrifugal force. Assuming that the radius of the rotating shaft is R1 and the inner diameter of the magnetic pole is R2, the pressure in the rotating sealing magnetic fluid is:

where:

2. The effect of the applied magnetic field strength and the magnetization of the magnetic fluid on the sealing pressure difference

from formula (2), it can be seen that the sealing pressure difference of the magnetic fluid is directly proportional to the applied magnetic field strength and the saturation magnetization of the magnetic fluid. Therefore, in order to improve the sealing ability of magnetic fluid, we should improve the magnetic field strength and use magnetic fluid with strong magnetism

3. Influence of seal clearance on seal differential pressure

seal differential pressure depends on the strength of magnetic field. When the seal gap changes, the magnetic induction intensity at the seal gap will decrease with the increase of the seal gap, because the volume of the permanent magnet and the magnetic potential will not change. Figure 1 shows the axial distribution of magnetic induction intensity in the sealing gap. With the increase of the sealing gap, the maximum magnetic induction intensity in the sealing gap decreases gradually

figure 1

on the premise that the amount of magnetic fluid remains unchanged, because the sealing pressure difference is proportional to the magnetic induction intensity, the sealing pressure difference decreases with the increase of sealing clearance while a large number of orders flow into China. Figure 2 shows the relationship between the pressure difference of magnetic fluid seal and the seal clearance. On the premise of machining accuracy, the sealing clearance shall be smaller

figure 2

in addition to the design and manufacturing factors determining the sealing clearance, the influence of shaft eccentricity is also similar to that of clearance. When the rotating shaft produces eccentric action, the sealing clearance is unevenly distributed along the circumference. For the whole sealing device, the pressure difference corresponding to the position with large air gap is the pressure difference of the whole device when the amount of magnetic fluid remains unchanged. The larger the eccentricity, the smaller the seal pressure difference. Figure 3 shows the relationship between the differential pressure of magnetic fluid seal and the eccentricity of rotating shaft. The diameter of the rotating shaft is 10mm and the sealing clearance is 0.5mm

Fig. 3

4. Effect of change in shaft diameter on seal differential pressure

according to formula (1), when considering the effect of gravity, the pressure increases with the increase of H in the direction of gravity. Fig. 4 shows the magnetic fluid cross-sectional shape of a single-stage seal with a horizontal rotating shaft calculated by the numerical method. The dotted line is the section shape without considering the gravity, while the solid line is the section shape with considering the gravity. Because the amount of magnetic fluid is constant, the area surrounded by the cross-section shape is the same in both cases

with the increase of shaft diameter, the effect of gravity becomes more obvious. When the amount of magnetic fluid is constant, the amount of magnetic fluid at the gap at the lower part of the shaft increases, while the amount of magnetic fluid at the gap at the upper part of the shaft decreases. The total effect is equivalent to the decrease of the amount of magnetic fluid. Therefore, with the increase of the shaft diameter, the upper seal differential pressure of the shaft decreases and the lower seal differential pressure increases. As far as the whole sealing device is concerned, the sealing capacity is determined by the minimum sealing differential pressure. Therefore, with the increase of the shaft diameter, the sealing capacity will decrease

Fig. 4

Fig. 5

Fig. 5 shows the relationship between the pressure difference of magnetic fluid single-stage seal and the diameter of rotating shaft with the same other dimensions and the same sealing cross-sectional area. The sealing clearance is 1mm. It can be seen that when the diameter of the rotating shaft is 200mm, the effect of gravity will reduce the sealing capacity by about 6%. In order to overcome the influence of gravity, when the shaft diameter is large, more magnetic fluid should be added

5. Effect of magnetic fluid quantity on sealing differential pressure

Figure 6 shows the relationship between sealing differential pressure and magnetic fluid quantity. It can be seen from the figure that with the increase of the amount of magnetic fluid, at the beginning, due to the large change of the magnetic field at the pole tip, the area surrounded by the isobar is small, and the sealing pressure difference of magnetic fluid increases rapidly with the amount of magnetic fluid. At the place far away from the pole tip, the magnetic field changes slowly, and the area surrounded by the isobar increases, so the seal differential pressure increases slowly and tends to the limit value. When there is less magnetic fluid, the sealing band cannot be formed, so the starting point of the curve does not pass through the origin. In order to make full use of the sealing capacity of the sealing device, a sufficient amount of magnetic fluid must be added. However, when the amount of magnetic fluid reaches a certain level, the role of increasing the injection will gradually decrease

figure 6

6. Effect of centrifugal force on seal differential pressure

when the rotating shaft rotates, the magnetic fluid will be affected by centrifugal force. Under the action of centrifugal force, the magnetic fluid moves along the radial direction, and the cross-sectional area of the sealing ring changes. The axial length on the surface of the rotating shaft decreases, and the area surrounded by the corresponding isobar at the same pressure difference increases compared with that at rest. Therefore, when the amount of magnetic fluid is the same, the seal pressure difference decreases

5. Automatic optimization of graph curve scale in order to reduce the effect of centrifugal force, the magnetic pole can be placed on the rotating shaft and rotated at the same speed as the rotating shaft. When the magnetic fluid is displaced along the radial direction by centrifugal force, the axial length of the magnetic fluid cross section on the inner surface of the shell is increased. Therefore, only a few fabrics are completely suitable. The rotary seal differential pressure is greater than the static seal differential pressure

Figure 7 shows the relationship between magnetic pole rotation and shaft rotation, sealing pressure difference and magnetic fluid volume. It can be seen from the figure that under the premise of a certain volume of magnetic fluid, the pressure difference of magnetic pole rotating seal is greater than that of shaft rotating seal

in the actual air gap separation seal, the magnetic field is strong, the sealing gap is small, and the change of magnetic fluid cross-section shape is not obvious when the rotating shaft speed is low. At this time, the effect of centrifugal force can be ignored, and the analysis can be carried out according to the static seal

figure 7

7. Conclusion

for sealing devices with the same materials and similar structures, the sealing pressure difference of magnetic fluid is directly proportional to the strength of applied magnetic field and the saturation magnetization of magnetic fluid; With the increase of sealing clearance and shaft eccentricity, the sealing capacity will be weakened; With the increase of the amount of magnetic fluid, the sealing pressure difference of magnetic fluid will increase; Under the action of gravity, the increase of the shaft diameter is equivalent to the decrease of the amount of magnetic fluid, which weakens the sealing ability. The effect of centrifugal force makes the interface shape of magnetic fluid change. Some improvement measures can be taken to reduce the effect of centrifugal force on the sealing ability of magnetic fluid, or the effect of centrifugal force can be used to improve the sealing ability

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