The requirements of centrifugal compressors for bearings are safe and reliable, stable operation, good vibration resistance and long service life. At present, most of the bearings used in domestic centrifugal compressors are dynamic pressure sliding bearings lubricated by high-speed light-load fluids. There are two types of bearings commonly used in compressors: radial bearings and thrust bearings.
The role of the radial bearing is to bear the weight of the rotor and other additional radial forces, maintain the center of rotation of the rotor and the center of the compressor cylinder, and operate normally at a certain speed. The role of the thrust bearing is to bear the axial force of the rotor, limit the axial movement of the rotation f, and keep the axial position of the rotor in the compressor cylinder constant.
The rotational speed of centrifugal compressors is very high. The radial speed of their radial bearings is generally above 50mn/s, and the linear speed of thrust bearings is generally above 80n/s, both of which belong to high-speed sliding bearings. In order to ensure a high degree of reliability in operation, fluid-lubricated dynamic pressure bearings are used. Dynamic pressure bearing refers to relying on the rotation of its own journal (or thrust disk) to bring lubricating oil between the shaft (or thrust disk) and the bearing to establish oil pressure to support the shaft (or to bear the shaft of the rotor) Thrust) bearings.
The working principle of radial bearings:
During the operation of the radial bearing, a thin oil film will form between the bearing and the journal. This oil film can make the shaft float. During the operation of the radial bearing, due to the non-stop rotation of the journal, the journal brings the lubricant between the collar and the bearing, forming a thin oil film from the surface. Because the journal is not concentric with the bearing center, but has an eccentricity, this wedge-shaped oil film can float the heavy rotor.
Different types and use characteristics of radial bearings:
The radial bearings commonly used in centrifugal compressors are round pad bearings, elliptical pad bearings, multi-oil wedge fixed bearings and tilting pad bearings. Most of the current compressors use tilting pad bearings.
The advantage of the round pad bearing is its simple structure, but its high-speed stability is poor, and it is now rarely used.
Compared with the round bearing, the advantages of the oval mesh bearing are first of all, its good stability, if the shaft sways up and down during operation, such as upward shaking, the upper interspace becomes smaller, the oil film k force becomes larger, and the lower The gap becomes larger and the oil film pressure becomes smaller. The change of the combined force of the two components will push the journal back to the original position to stabilize the shaft operation. Secondly, due to the large side clearance, the amount of oil flowing out in the axial direction is large and the heat dissipation is good. The temperature of the bearing is low; but the bearing capacity of this bearing is low. Due to the generation of upper and lower oil films, the power consumption is large. The vibration resistance in the vertical direction is good, but the vibration resistance in the horizontal direction is poor.
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The advantages of multiple oil wedge fixed bearings are that the vibration resistance of the bearing is good in all directions, the temperature rise of the bearing is low, and oil film oscillation is not easy to occur, which is often used in old compressors.
Compared with other bearings, the tilting pad bearing has the advantage that each tile can swing freely and can form the best oil wedge under any circumstances. The high-speed stability is very good and the oil film oscillation is not easy to occur. In the centrifugal compressor Universal application. The tilting pad bearing is mainly composed of a bearing body, oil seals on both sides and pads (as shown in the figure below). The pads of this bearing generally use five pads, and each pad can swing freely. Five pads are evenly distributed along the periphery of the journal, each of which can swing around a fulcrum of itself. There is a normal bearing clearance between the pad and the journal. The clearance is generally 1.5% to 2% of the diameter. The outer diameter of each pad is smaller than the inner diameter of the bearing body, and the arc of the back of the pad is in line contact with the bore of the bearing body. It is equivalent to a fulcrum. When the running speed and load of the unit change, the pad can swing freely on the support surface of the bearing body, automatically adjust the position of the pad, and form the best lubricant wedge. In order to prevent the pad and the journal from rotating together in the circumferential direction, each pad is positioned with a screw mounted on the bearing body. In most cases, the positioning screw is located in the middle of the pad. In order to prevent the pads from moving in the axial and radial directions, the pads are installed in the T-shaped grooves in the bearing body. The pads are generally made of 20 steel or 25 copper, and the babbitt alloy is cast on top. The thickness of the alloy is generally 0.8-2.5mm. In order to ensure that the babbitt alloy closely fits the tile body, grooves can be pre-fabricated on the tile body. The bearing body is divided into upper and lower half horizontally and is installed in the bearing housing.
The bearing body and bearing seat are tightly matched by the positioning stop. In order to prevent the bearing body from rotating, a radial positioning pin is also installed to prevent rotation. The number of oil inlets of the bearings is different. Some bearings have only one oil inlet hole, and some bearing pads have oil inlet holes between the pads, but they are always arranged in a place that does not damage the oil film. Lubricating oil is generally discharged in the axial direction. There are grooves at both ends of the bearing body, and the oil discharge holes communicate with it. The lubricating oil concentrates in the grooves and flows back to the bearing box through the oil discharge holes.
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