Installation method of tilting bearings

Please refer to the following article (Source: Pump and Valve Technology Information Network): Characteristics and Maintenance Techniques of Tilting Axle Bushes in Steam Turbines. Date: 2007-11-10 10:52:16. Author: Shi Weigang. Source: Hebei Electric Power Technology. Number of views: 92. Text size: [Large][Medium][Small]. Introduction: Characteristics and Maintenance of Tilting Axle Bushes in Steam Turbines. By Shi Weigang (Xibaipo Power Generation Co., Ltd., Pingshan, Hebei 053000). Abstract: This article discusses the issues related to tilting axle bushes in steam turbines at a certain power plant after major repairs or due to malfunctions. Through study and practice, the author has mastered the techniques for fitting, adjusting these bushings as well as performing maintenance by grinding shims, thereby ensuring proper operation of the turbine bushes and stable power generation by the unit. 　　Keywords：tilting pad ； Tile fitting ； Adjustment ； Grinding of shims? Abstract: This paper describes the process of treating the tilted axle bushes in a steam turbine at a power plant. Through experience and practice, it is possible to master the techniques of aligning and adjusting these tilted bushes as well as grinding the corresponding shims. This helps to ensure proper operation of these bushes and stable power generation. Keywords: tilted bush ； matching of bushes ； adjust ； Pad grinding? The steam turbines of Hebei Xibaipo Power Generation Co., Ltd. (hereinafter referred to as Xidian) are single-axis, double-cylinder, subcritical, once-intermediate-reheat, condensing type turbines manufactured by Harbin Steam Turbine Factory Co., Ltd. using American Westinghouse technology. Units #1 and #2 are of the N300-16.7/537/537 model, while units #3 and #4 are of the N300-16.7/538/538 model. The high-pressure rotors of these 4 turbines are equipped with tilting bearings at both ends. Tilting pads, also known as self-aligning tilting pad bearings or Cermic bearings, are a new type of bearing with high load-bearing capacity and good stability. ? 1 Structure and characteristics of tilting bearing shells Tilting bearing shells are usually composed of 3 to 5 or more arc-shaped babbitt shells that can tilt freely at the pivot points. During operation, the tiles can swing freely depending on the rotation speed, load, and bearing temperature, thereby creating multiple oil wedges around the shaft diameter. Each tile is supported in the bearing sleeve by a spherical pin and a gasket on its back side, allowing it to swing around its spherical support pin ； Springs are installed on the upper bearing shell and the back side of the bearing, respectively; these springs exert pressure on one end of the bearing shell to artificially create an oil wedge. Lubricating oil enters the bearing through the gaps between the bearing pads, and exits from the openings in the oil seal rings at both ends of the bearing. If the inertia of the bearing pads, as well as the frictional forces at the pivot points and the shear internal friction forces within the oil film, are ignored, the force exerted by the oil film on the shaft diameter due to each bearing pad always passes through the center of the shaft diameter; thus, no unstable forces that could cause vortexing in the shaft diameter are generated, resulting in high stability. In theory, it is possible to completely avoid the occurrence of oil film oscillations. Furthermore, since the tiles can swing freely, this increases the flexibility of the support, and they also have the ability to absorb vibration energy from the rotating shafts, meaning they offer excellent vibration damping properties. The cross-section of the tilting bearing shell is shown in Figure 1. 　　Tilting pads have many advantages, but their disadvantages include complex structure, difficulty in installation and maintenance, and high costs. However, as the requirements for bearings in high-power units in terms of stability, power consumption, and load-bearing capacity become increasingly stringent, tilting pad bearings are being adopted by more and more high-power units. Tilting pad bearings rank first among all types of supporting bearings in terms of stability, load-bearing capacity, and power consumption; three-oil- wedge bearings and elliptical bearings come next, while cylindrical bearings perform the worst. 2 Issues with tilting bearings and their solutions a. On January 28, 1999, during the overhaul of Unit #1, it was found that the convex surfaces of the shims used to adjust each bearing pad in #1 bearing were severely worn, having become flat ； #The adjustment shim on the B side of the lower bearing shell was shattered by vibration. Treatment: Replace all adjustment shims, measure, and fit the bearings. 　　b. On 1999-12-09, during the overhaul of Unit #2, it was found that the temperature probe for the lower bearing block of Unit #2 had penetrated through the tungsten carbide, and the adjustment shims were severely worn. Solution: Replace the tiles with new ones, adjust the shims, and re-arrange the tiles. 　　c. 2001-01-24, during the major overhaul of Unit #4, it was found that the bearing bush of #1 was severely worn, and the temperature probes for the 4 bearing blocks penetrated through the bushing. Solution: Replace the tiles with new ones, adjust the shims, and re-arrange the tiles. 　　d. On 2002-01-16, Unit #1 was shut down due to a fire caused by insulation failure in the generator carbon brush slip rings. This has caused accumulation of #2 waujin, severe crushing of the lower waujin, and accumulation of waujin between the mid-surface and the shaft diameter. Treatment: Repair and scrape the black metal, measure the thickness of the gasket, and replace it with a new one. 　　e. 10/01/2002: Major overhaul of Unit #1; poor contact in the bearing of #2, resulting in high bearing vibration. Solution: Replace 4 new tiles, adjust the shims and springs, and re-arrange the tiles. 　　Through major overhauls and emergency repairs, we learned from Harbin Turbine Factory the techniques for adjusting tilting bearings and fitting bearings, and applied these methods in subsequent maintenance work, thereby gaining considerable experience. 　　Based on experience, the main defects of tilting tiles include severe wear of the black iron components, which requires the replacement of those tiles ； The adjusting shims or pins are severely worn; wear on the convex surface or fragmentation of the shims requires replacement with new pins and shims ； During major overhauls of the engine unit, it is necessary to make significant adjustments to locate the central bearing shells; issues such as poor contact with the bearing shims and high vibration levels can occur. The first two of these involve the replacement of new components, and they are classified as bearing shells for tilting pads ； The latter falls under adjustments and lapping based on the existing foundation, and is classified as adjustments for tilting bearings. ? 3 Maintenance of tilting bearings 3.1 Bearing assembly The principle for assembling tilting bearings is to ensure good contact between the bearing elements ； The tiles must be properly aligned with the axis; the deviation in their alignment shall not exceed 0.02 mm, meaning that the distance between the blackened surfaces of the four tiles relative to the axis should not exceed 0.02 mm. 3.1.1 Preparation Work Whether it is replacing new tiles, pins, or adjusting shims, it is necessary to measure and calculate again, as well as re-arrange the tiles. 　　Tilting pad bearings are usually processed by manufacturers, so there isn’t much surface grinding required. After the simple scraping of the black oxide contact has been completed and found to be satisfactory, clean the new tiles, new gaskets, new pins, and the tile housing; pay special attention to adjusting the gaskets and the grooves of the pins between the tile housing and the tiles. All dirt must be removed; no burrs are allowed. Finally, wipe it clean with alcohol. 3.1.2 Measurement The measuring tool is a specialized device designed for a certain type of tilting pad: a depth gauge equipped with standard thickness shims, the size of which should allow them to fit comfortably into the recess ； A specialized micrometer, designed based on the thickness of the tilting pad to be measured, ensures a measurement range of up to 100 mm above the pad’s thickness; moreover, its opening size and probe length allow it to measure the total thickness of the tilting pad including any gaskets in between. The purpose of the measurement is to ensure that the distance between the 4-watt component and the axis is equal; the deviation is adjusted by changing the thickness of the flat shims. 3.1.3 Calculation The measurement and calculation process is illustrated using Xidian #1 bearing as an example; refer to Figure 2 for the calculation of bearing shells. 　　a. The following standard values can be obtained from the drawings for reference: the inner diameter ø of #1 vacuum chamber is (463.55±0.05) mm, so the radius r1 = 231.77 mm ； The inner diameter of the tile is ø (304.8+0.05) mm, so the radius r2 is 152.4 mm. 　　b. Use a marker to number the 4 tiles, the recesses in the tile housing, as well as the corresponding gaskets and pins, and keep a record of them; for example, label them as 1-1, 2-2, 3-3, 4-4 to avoid confusion. 　　c. Measure the depth of the recess in housing 1-1. Place the standard gasket (with a thickness of 8.8 mm) into the recess, and measuring with a depth gauge shows a depth of 9.91 mm. Then the depth of the shell recess is (9.91+8.8) mm, which should be recorded. 　　d. Insert the pin with protrusions into the recess of the tile, and use a micrometer to measure the total thickness of the pin and the tile. The measurement should be carried out by experienced personnel, and the value of the highest point represents the total thickness m. Denoted as m = thickness of the brick + thickness of the pin = 86.9 mm. 　　e. Calculate the thickness of the flat gasket. σ1-1 = depth of the shell’s recess + shell radius r1 – thickness of the washer and pin m – radius of the washer r2 = 9.91 + 8.8 + 231.77 – 86.9 – 152.4 = 11.18 mm. 　　f. Similarly, calculate σ2-2, σ3-3, and σ4-4. Adjust the thickness of the flat shims based on the data, ensuring that the difference in thickness among the 4 flat shims is within 0.02 mm. 3.1.4 Precautions a. The measurement method must be consistent; all 4 tiles should be measured by the same experienced person. 　　b. Calibrate the micrometer and depth gauge before measurement ； Before use, the measuring surfaces of the micrometer and depth gauge should be cleaned; a ratchet must be used once the measuring surfaces come into contact with the workpiece ； After use, wipe the measuring surfaces clean; the two measuring surfaces should not touch each other. 　　c. Be sure to clean all areas thoroughly before measurement; there must be no burrs. 　　d. Keep accurate measurement records for each tile and gasket; avoid any confusion. 3.2 Adjustment of tilting bearings and lapping of shims If it is found during maintenance that the shims are not in proper contact, lapping must be carried out. If, when centering the shafting, the adjustment amount of the tilting bearing shells up and down or left and right is large, in addition to adjusting the shims, grinding is also required. 3.2.1 Adjustment of tilting pads Still taking the tilting pad shown in Figure 1 as an example. There are 3 adjustment shims at the lower part, and 2 upper shims, distributed at a 45° angle. The position of the shaft bearing can be adjusted by changing the thickness of the shims on the 3 adjusting shims located on the lower half of the shaft bearing. The angle between the center line of the shims on both sides and their vertical center line is 45°, as shown in Figure 3. 　　a. When moving vertically in the y direction, the reduction value for the lower shims is equal to y; the thickness of the shims on both sides changes by ycosα, with each of the upper shims having its thickness reduced by ycosα as well. 　　b. When moving horizontally by x, the lower shims remain stationary, while the shims on both sides are adjusted by +xsinα or –xsinα. When moving to the right, subtract xsinα from the shim on the lower right side, and add xsinα to the shim on the lower left side. The same for the upper half. 3.2.2 Grinding of shims This tilting pad is equipped with 5 shims; the grinding process is relatively labor-intensive and requires a lot of time. It is necessary to improve the quality of this grinding work and pay attention to the techniques used in order to enhance efficiency. 　　When the adjustment amount of the bearing bush is too large, it has a significant impact on the contact with the shims. Therefore, the scraping of the shims should be carried out simultaneously with the centering work. The center can be measured first; based on the requirements for alignment with that center, the adjustment amounts for the shims inside the 3 shims located beneath the bearing shell can be calculated. Based on the calculation results, along with the contact conditions, the gasket thickness is pre-adjusted using the estimated amount of grinding. The reserved grinding amount should not be too large; around 0.10 mm is sufficient. 　　Since the shims are in three-point contact, raising the lower shim slightly can create a gap between the two side shims; as a result, the bearing shell will swing and the contact points will not be accurate. Therefore, when grinding the shims, the shims on both sides should be in contact, while there can be a slight gap in the lower shim. First, grind the side shims until they are basically satisfactory, and then focus on grinding the lower shims. 　　Additionally, when grinding the shims to the final stage, attention should be paid to the amount of grinding required for the bottom shims and the side shims; generally, the number of times the bottom shims are ground should be increased to ensure that both the bottom shims and the side shims meet the requirements regarding the contact surface. ? Acceptance criteria for grinding shims: The contact marks should cover more than 70% of the total area, with the contact points being evenly distributed in a dot-like pattern ； When the rotor is not installed, 0.03 mm cannot be inserted into the shims on both sides, and 0.05 mm cannot be inserted into the shim at the bottom ； After installing the rotor, 0.03 mm of shims cannot be inserted. ? 4 Conclusion 　Through multiple repairs of tilting pads, certain experience in maintenance has been accumulated over time. At present, the tilting bearings of the 4 units at Xidian are operating normally, ensuring stable power generation by the units. ? References ? [1] Hong Zao. Turbine Equipment and Operation [M]. Beijing: Water Resources and Electric Power Press, 1988. [2] Electrical Engineering Societies of Six Provinces and One Municipality in East China. Turbine Equipment and Its Systems [M]. Beijing: China Electric Power Press, 2000. [3] Hao Kaiguo. Turbine Equipment Maintenance Technology [M]. Beijing: Water Resources and Electric Power Press, 1985.

Note: The thickness difference between tiles in the same group should not exceed 0.02 mm; those that exceed this limit must be replaced