1. What is the function of the main bearing?
Bearings are an important component of steam turbines, and main bearings are also known as radial bearings. Its function is to bear the entire weight of the rotor and the centrifugal force caused by the unbalanced mass of the rotor, and determine the correct radial position of the rotor in the cylinder. Due to the high load that each bearing must bear and the high speed of the shaft neck, the bearings of the steam turbine are based on the theory of liquid friction using bearing type sliding bearings. With the help of lubricating oil under certain pressure, an oil film is formed between the shaft neck and the bearing shell, establishing liquid friction and ensuring the safe and stable operation of the steam turbine.
2. How is the lubricating oil film formed on bearings?
The aperture of the bearing shell is slightly larger than that of the journal. When at rest, the journal is located at the lower part of the bearing shell and directly contacts the inner surface of the bearing shell, forming a wedge-shaped gap between the bearing shell and the journal. When the rotor starts to rotate, direct friction occurs between the journal and the bearing shell. However, as the journal rotates, the lubricating oil adheres to the surface of the shaft due to its viscosity and is carried into the wedge-shaped gap between the journal and the bearing shell. As the speed increases, the amount of oil carried in increases. Due to the decreasing outlet area of the oil flow in the wedge-shaped gap, the oil pressure continues to rise. When this pressure increases enough to balance all the forces of the rotor on the bearing shell, the journal is lifted by the oil film and suspended on it, thus avoiding direct metal friction and establishing liquid friction.
3. What are the main structural types of steam turbine main bearings?
There are four main types of main bearings for steam turbines:
(1) Cylindrical tile support bearing;
(2) Elliptical tile support bearing;
(3) Three oil wedge support bearings;
(4) Tilting pad support bearings.
4. What is the structure of a fixed cylindrical support bearing?
Fixed cylindrical support bearings are used in steam turbines with a capacity of 50~100MW. The outer shape of the bearing shell is cylindrical, consisting of upper and lower halves connected by bolts. The lower bearing is supported on three shim blocks, which are lined with shims. Adjusting the thickness of the shims can change the center position of the bearing in the bearing groove. The gasket on the top of the bearing shell can be used to adjust the tension between the bearing shell and the bearing cover. Lubricating oil is introduced from the center hole of the pad iron below the bearing shell side, passes through the oil circuit in the lower bearing shell body, and enters the bearing shell through the oil inlet hole on the horizontal joint surface. Due to the rotation of the shaft, the oil first passes through the top clearance of the bearing shell, then through the wedge-shaped clearance between the shaft neck and the lower bearing shell, and then leaks out from both ends of the bearing shell, returning to the oil tank through the bearing seat oil chamber. There is a throttle orifice plate at the oil inlet of the bearing shell to adjust the amount of oil entering. Both sides of the bearing shell are equipped with oil barriers to prevent oil from being thrown out. The locking pad at the horizontal joint surface of the bearing is used to prevent the bearing from rotating. The bearing shell is generally cast with high-quality cast iron, with dovetail grooves machined inside the bearing shell, and cast with tin based bearing alloy (i.e. Babbitt alloy), also known as Wujin.
5. What is a self-aligning bearing?
Cylindrical and elliptical support bearings can be divided into fixed and self-aligning (also known as spherical) types according to their support methods.
The only difference between self positioning and fixed type is that the bearing body has a spherical shape. When the center of the rotor changes and the shaft neck tilts, the bearing can automatically adjust its position with the rotation of the shaft neck, keeping the gap between the shaft neck and the bearing shell unchanged throughout the length of the bearing shell. However, the processing and adjustment of this type of bearing are more complicated.
6. What is the difference between elliptical bearings and cylindrical bearings?
The structure of elliptical support bearings is basically the same as that of cylindrical support bearings, except that the clearance on the side of the bearing is increased, usually twice the clearance on the top. The curvature radius of the bearing shell increases. Increasing the absolute eccentricity of the journal within the bearing shell enhances the stability of the bearing. Both the upper and lower parts of the bearing shell can form oil wedges (hence also known as double oil wedge bearings). Due to the downward force of the oil film on the upper oil wedge, the stability of the bearing operation is good, and this type of bearing is widely used in large and medium capacity steam turbine units.
7. What is a three oil wedge bearing?
In large capacity units, such as domestically produced 125MW, 200MW, and 300MW units, three oil wedge bearings are used. There are three oil wedges of different lengths on the bearing shell of the three oil wedge support bearing. Theoretically, the oil film established by the three oil wedges exerts a force that turns from three directions towards the center of the shaft neck, which can ensure stable operation of the shaft neck. But the inclination angle between the joint surface of the upper and lower bearing shells of this type of bearing and the horizontal plane is 35 degrees. Causing inconvenience to maintenance and installation. From the phenomenon of oil film oscillation in some units' three oil wedge support bearings, it can be seen that the bearing capacity and stability of this type of bearing are not very high.
8. What is a tilting pad support bearing?
Tilting pad support bearings are usually composed of 3-5 or more arc-shaped pads that can tilt freely on the pivot point, so they are also called movable multi pad support bearings or swing pad bearings. Due to its ability to freely swing with different speeds, loads, and bearing temperatures, multiple oil wedges are formed around the shaft neck. And the pressure of each oil film always points towards the center, with high stability. In addition, tilting pad support bearings also have the characteristics of high support flexibility, good absorption of vibration energy, high load-bearing capacity, low power consumption, and adaptability to forward and reverse rotation. However, the tilting pad structure is complex, installation and maintenance are difficult, and the cost is high.
9. What types of rotors are different types of supporting bearings suitable for?
Cylindrical support bearings are mainly suitable for low-speed heavy-duty rotors and are commonly used in small and medium-sized steam turbines and compressors. Three oil wedge support bearings and elliptical support bearings are suitable for high-speed light, medium, and heavy-duty rotors, respectively. Tilting tile support? Bearings are suitable for high-speed light load and heavy-duty rotors.
10. What is the function of thrust bearings?
The function of thrust bearings is to withstand the axial thrust of the rotor during operation, and to determine and maintain the axial mutual position between the turbine rotor and the cylinder.
11. What are the types of thrust bearings? What is the main structure?
The thrust bearing can be set as a separate type or combined with the support bearing as a whole, called a combined type (thrust support combined bearing). According to the structural shape, it can be divided into multi jaw type and fan-shaped tile type, with fan-shaped tile type being commonly used now. Is the main structure composed of working tiles or non working tiles? Composed of sheet, adjusting gasket, installation ring, etc. Install ten to twelve working tiles and non working tiles on both sides of the thrust plate. Each tile is installed on the mounting ring, with the working tile bearing the forward axial thrust of the rotor and the non working tile bearing the reverse axial thrust of the rotor.
12. What is thrust clearance?
The movement distance of the thrust disc between the working tile and the non working tile is called the thrust gap, which is generally not greater than 0.4 mm. The thickness of the black gold on the tiles is generally 1.5 mm, which is smaller than the minimum gap between the moving and stationary parts of the turbine flow passage to ensure that even in the event of black gold melting accidents, the moving and stationary parts of the turbine will not rub against each other.
13. What is the working process of turbine thrust bearings?
There are several thrust pads on both sides of the working and non working surfaces of the thrust disc installed on the spindle. There is a pin hole on the back of the pad, which is used to place the pad on the pin of the installation ring. The pad can rotate slightly around the pin. The pin hole on the tile is located 7.54 mm away from the center, so a wedge-shaped gap is formed between the working surface of the tile and the thrust plate. When the thrust disc rotates, the oil is compressed in the wedge-shaped gap, and the pressure increases, so this oil film has the ability to withstand the axial thrust of the rotor. The installation ring is placed on the spherical seat, and the oil is sent into the thrust bearing inlet chamber through the throttle hole. It is divided into two paths and enters the annular oil chamber around the main shaft through the inlet hole on the spherical seat of the thrust bearing, and flows radially between the pads. Leave ample space between the tiles to facilitate the circulation of oil within them. The spherical seat of the thrust bearing is equipped with a return oil baffle ring, which surrounds the outer circle of the thrust disc to form an annular return oil chamber. There are two oil return holes at the top of the oil return baffle rings on both the working and non working surfaces, and a needle valve can also be used to adjust the oil return volume. An oil blocking ring is also installed between the installation ring of the thrust pad and the thrust disc, which surrounds the thrust pad and forms an annular oil inlet chamber for the thrust bearing.
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Three-layer metal-polymer self-lubricating bearings:
Three-layer metal-polymer self-lubricating bearing is lighter and has better mechanical and load performance. In addition, the wear resistance and noise absorbing feature is improved while more variable torque range could be fitted.
https://www.marginalbearings.com/bimetallic-bushes/three-layer-metal-polymer-self-lubricating.html
